Association of pesticide exposure with neurologic dysfunction and disease.

Poisoning by acute high-level exposure to certain pesticides has well-known neurotoxic effects, but whether chronic exposure to moderate levels of pesticides is also neurotoxic is more controversial. Most studies of moderate pesticide exposure have found increased prevalence of neurologic symptoms and changes in neurobehavioral performance, reflecting cognitive and psychomotor dysfunction. There is less evidence that moderate exposure is related to deficits in sensory or motor function or peripheral nerve conduction, but fewer studies have considered these outcomes. It is possible that the most sensitive manifestation of pesticide neurotoxicity is a general malaise lacking in specificity and related to mild cognitive dysfunction, similar to that described for Gulf War syndrome. Most studies have focused on organophosphate insecticides, but some found neurotoxic effects from other pesticides, including fungicides, fumigants, and organochlorine and carbamate insecticides. Pesticide exposure may also be associated with increased risk of Parkinson disease; several classes of pesticides, including insecticides, herbicides, and fungicides, have been implicated. Studies of other neurodegenerative diseases are limited and inconclusive. Future studies will need to improve assessment of pesticide exposure in individuals and consider the role of genetic susceptibility. More studies of pesticides other than organophosphates are needed. Major unresolved issues include the relative importance of acute and chronic exposure, the effect of moderate exposure in the absence of poisoning, and the relationship of pesticide-related neurotoxicity to neurodegenerative disease. Key words: fumigant, fungicide, insecticide, neurobehavioral performance, neurodegenerative disease, neurologic symptoms, organophosphate, Parkinson disease, pesticide. Environ Health Perspect 112:950-958 (2004). doi:10.1289/ehp.7135 available via http://dx.doi.org/[Online 20 May 2004]

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Pesticides are used extensively throughout the world. In the United States, more than 18,000 products are licensed for use, and each year > 2 billion pounds of pesticides are applied to crops, homes, schools, parks, and forests [U.S. Environmental Protection Agency (EPA) Office of Pesticide Programs 2002]. Such widespread use results in pervasive human exposure.

Evidence continues to accumulate that pesticide exposure is associated with impaired health. Occupational exposure is known to result in an annual incidence of 18 cases of pesticide-related illness for every 100,000 workers in the United States (Calvert et al. 2004). The best-documented health effects involve the nervous system. The neurotoxic consequences of acute high-level pesticide exposure are well established: Exposure is associated with a range of symptoms as well as deficits in neurobehavioral performance and abnormalities in nerve function (Keifer and Mahurin 1997). Whether exposure to more moderate levels of pesticides is also neurotoxic is more controversial. Pesticide exposure may also be associated with increased risk of neurodegenerative disease, particularly Parkinson disease (Le Couteur et al. 1999).

In this review, we summarize briefly what is known about the neurotoxic effects of high-level exposure, describe in more detail the existing data on neurotoxic effects of chronic exposure at lower levels, and then discuss the relationship of pesticide exposure to neurologic disease. Although pesticide exposure may have significant effects on neurodevelopment (Eskenazi et al. 1999), this review focuses on effects in adults [greater than or equal to] 18 years of age. Since differences in approach to evaluating pesticide exposure may play a crucial role in creating inconsistencies among studies, we first consider pesticide exposure assessment.

Pesticide Exposure

Pesticides are a broad range of substances most commonly used to control insects, weeds, and fungi (plant diseases). They are frequently classified by target organism or mode of use as insecticides, herbicides, fungicides, or fumigants. Insecticides are often subclassified by chemical type as organophosphates (OPs), organochlorines, carbamates, and pyrethroids. Individuals are frequently exposed to many different pesticides or mixtures of pesticides, either simultaneously or serially. These exposures are often highly correlated, particularly within functional or chemical groups, making it difficult to identify effects of particular agents.

Studies of pesticide neurotoxicity have typically evaluated either the long-term sequelae of pesticide poisoning or the effects of occupational exposure (Table 1). Pesticide poisoning may go undiagnosed, especially among farmworkers with poor access to medical care (Moses et al. 1993) and particularly among women (London et al. 2002). Thus, workers who have never been diagnosed with pesticide poisoning may still have sustained high exposures or experienced pesticide-related illness; therefore using diagnosed poisoning as a criterion for inclusion in an exposed group or exclusion from a comparison group may incorrectly classify individuals.

Some studies of occupational pesticide exposure have classified as exposed all members of an occupational group---typically farmers or farmworkers--sometimes also considering job duration. The potential for misclassification with this approach is high. Farm owners who employ others to apply pesticides may have limited personal exposure to pesticides. Even among pesticide applicators, exposure can vary widely. For example, farmworkers with little access to information about safety practices or protective equipment (Gomes et al. 1999) may sustain far more exposure than well-trained and equipped commercial applicators (Maizlish et al. 1987). Further, farmworkers who do not apply pesticides as part of their job may still be exposed, and even family members with no direct occupational exposure may be exposed at home or elsewhere (Fenske 1997; Gladen et al. 1998), so neither of these may be an appropriate comparison group.

Factors such as application method, use of personal protective equipment, work practices related to hygiene, spills, and attitudes toward risk may all influence the degree of pesticide exposure and can be incorporated into exposure estimates (Alavanja et al. 2004; Buchanan et al. 2001; Dosemeci et al. 2002; Gomes et al. 1999; Hernandez-Valero et al. 2001; London and Myers 1998; Ohayo-Mitoko et al. 1999; Stewart et al. 2001). The relationship of these factors to exposure can be complex. For example, wearing gloves can increase exposure under some circumstances (Hines et al. 2001), perhaps because fabric (as opposed to chemically impervious) gloves can become impregnated with pesticide and serve as a reservoir of exposure. The same may be true of other types of protective clothing (Ohayo-Mitoko et al. 1999). In developing countries, use of closed pesticide mixing and loading systems may increase exposure when the equipment is used to speed up work and increase productivity rather than to protect workers (McConnell et al. 1992). Additional factors may be crucial for evaluating exposure in farmworkers, such as availability of washing and drinking water, interval between application of pesticides to a field and re-entry of workers, and housing conditions (Arcury and Quandt 1998; Gomes et al. 1999; Hernandez-Valero et al. 2001; Tielemans et al. 1999). Studies of neurotoxicity have used all these kinds of information to evaluate pesticide exposure (Gomes et al. 1999; Ohayo-Mitoko et al. 1999). The most sophisticated approaches were employed by London and Myers (1998), who used a cropand job-specific job exposure matrix to evaluate exposure in a study of the neurotoxicity of chronic OP exposure among South African farmworkers, and by Buchanan et al. (2001), who developed an exposure algorithm to predict diazinon exposure for a study of chronic neurologic effects among sheep dippers in the United Kingdom.

Both historic and current exposures may be relevant to neurotoxicity and need to be characterized. Even among people who remain in the same occupation, current exposure may not reflect past exposure patterns because both available products and methods of use change over time. The need to evaluate past as well as current exposure has limited the utility of biomarkers; most modern pesticides are not persistent, so studies of chronic exposure rely primarily on questionnaire-based methods. Biomarkers are, however, useful in some situations. For example, organochlorines have a long half-life, so serum levels can be used as a marker of exposure to these pesticides. OP inhibition of erythrocyte acetylcholinesterase (ACHE) can also be used as an exposure marker. The effect lasts 3-4 months, so AChE activity in whole blood or erythrocytes can be used to evaluate subchronic exposure, although interpretation can be complicated by acute exposure. Although the clinical utility of this biomarker in individuals may be limited by variability in baseline levels, in populations chronic OP exposure is associated with small but reliable decreases in erythrocyte AChE activity (Karr et al. 1992; Ohayo-Mitoko et al. 1997). OPs also inhibit plasma butylcholinesterase, but the effect lasts at most a few weeks and is therefore not useful for evaluating chronic exposure. Cholinesterase inhibition by carbamates lasts only minutes, so it is not a useful marker of chronic exposure to these pesticides.

Estimating lifetime pesticide exposure quantitatively is difficult because it is affected by many factors, including the multiple chemicals involved, uncertainty regarding the degree of exposure related to specific job tasks or other events, and contributions from multiple sources of exposure, including sources unrelated to occupation. Further, the biologically relevant exposure measure is not known: Peak or average exposure intensity might be more important than cumulative exposure. Thus, attempts to assess quantitative dose-response relationships may be problematic. The goal of exposure assessment in epidemiologic studies is not, however, to assign quantitative dose estimates but rather to rank individuals by relative exposure level. Assignment of either exposed or unexposed individuals to the wrong category can be a significant problem, as can combining individuals with low and high levels of exposure into one group. Random misclassification of exposure, unrelated to health outcome, will typically weaken studies by making associations more difficult to detect, although it will not undermine the validity of any association that is observed. As discussed above, assuming that all farmers or even all pesticide applicators are equally exposed is likely to entail significant misclassification, as is assuming that all farmworkers who are not applicators are not exposed. Further, studies that identify only a single highly exposed group for study cannot evaluate the neurotoxicity of moderate exposure, which may have great significance to public health. Methods described above can correctly categorize study participants with respect to their relative exposure levels, and using such methods to increase precision of exposure assessment may help minimize inconsistencies among studies.

Neurotoxicity of High-Level Exposure

Most types of pesticides, including OP, carbamate, and organochlorine insecticides as well as fungicides and fumigants, can be neurotoxic, but only OPs have been studied in detail (Keller and Mahurin 1997). The response to OPs can occur within minutes. Less severe cases of OP poisoning display symptoms including headache, dizziness, nausea, vomiting, pupillary constriction, and excessive sweating, tearing, and salivation. More severe cases develop muscle weakness and twitches, bronchospasm, and changes in heart rate and can progress to convulsions and coma. The mechanism of OP neurotoxicity in most cases involves overstimulation of postsynaptic cholinergic receptors after inhibition of AChE (Keifer and Mahurin, 1997), although other macromolecular targets may also be involved (Pope 1999). An intermediate syndrome, occurring 1-4 days after exposure, is characterized by muscle weakness and can be fatal if respiratory muscles are affected. Two to five weeks after exposure, some patients develop OP-induced delayed polyneuropathy, a well-characterized syndrome involving sensory abnormalities, muscle cramps, weakness, and even paralysis, primarily in the legs. These symptoms are a consequence of axonal death following OP inhibition of a neural enzyme called neuropathy target esterase and may be irreversible (Keifer and Mahurin 1997).

Several studies have shown that OP poisoning has additional long-term sequelae. Studies of individuals with a history of pesticide poisoning--farmworkers (London et al. 1998; McConnell et al. 1994; Rosenstock et al. 1991; Wesseling et al. 2002), farmers (Stallones and Beseler 2002), rescue workers (Nishiwaki et al. 2001), or individuals identified from hospitals or pesticide registries (Miranda et al. 2002; Savage et al. 1988; Steenland et al. 1994)--have found that increased symptom prevalence, deficits in cognitive and psychomotor function, decreased vibration sensitivity, and motor dysfunction can occur long after the immediate episode is resolved. In some cases, effects were observed [greater than or equal to] 10 years after poisoning (Savage et al. 1988), suggesting that the residual damage is permanent. Even less severe poisoning can have long-term consequences: Banana farm workers who had been treated for intoxication with OPs or carbamates but did not require hospitalization performed worse on tests of cognitive and psychomotor function than did nonpoisoned workers when tested > 2 years later (Wesseling et al. 2002).

Neurotoxicity of Low-Level Exposure

Findings from studies of acute exposure to moderate levels of pesticides are inconsistent. Some studies of well-trained and -equipped pesticide applicators in the United States reported that exposure to OPs sustained during a single work shift (Maizlish et al. 1987) or assessed using a short-lived urinary biomarker (Dick et al. 2001) was associated with little neurotoxicity. However, several studies in developing countries, where exposures may have been higher, found that acute exposure to OPs was associated with increased symptom prevalence in commercial applicators (Misra et al. 1985) and farmworkers (London et al. 1998; Ohayo-Mitoko et al. 2000). Acute and chronic exposures are often correlated, sometimes making it difficult to separate their effects. The following discussion focuses on the effects of chronic exposure to moderate levels of pesticides, although in many studies acute exposure may also have occurred. Several types of neurologic end points are considered, including symptom prevalence, neurobehavioral performance, sensory and motor dysfunction, and direct measures of nerve function. Studies are summarized in Table 2.

Symptom Prevalence

Studies of symptom prevalence are often based on variations of an established checklist (Lundberg et al. 1997) and evaluate a broad range of symptoms, including headache, dizziness, fatigue, insomnia, nausea, chest tightness, and difficulty breathing as well as symptoms suggesting cognitive (confusion, difficulty concentrating), motor (weakness, tremor), and sensory (numbness, tingling, visual disturbance) dysfunction. Pesticide exposure is associated with increases in prevalence of many symptoms, with little evidence for specificity. Most studies have focused on OPs; most of these found an association of exposure with increased symptom prevalence. Farmworkers (Gomes et al. 1998), greenhouse workers (Bazylewicz-Walczak et al. 1999), and factory workers (Bellin and Chow 1974) exposed to OPs reported increased symptom prevalence compared to unexposed workers. In particular, farmers and farmworkers who applied OPs had higher symptom prevalence than nonapplicators (London et al. 1998; Ohayo-Mitoko et al. 2000; Smit et al. 2003), as did commercial applicators (Misra et al. 1985; Steenland et al. 2000) and sheep dippers (Pilkington et al. 2001). Pesticides other than OPs also affect symptom prevalence: one study found that exposure to dichlorodiphenyltrichloroethane (DDT) was associated with increased symptom prevalence (van Wendel de Joode et al. 2001), as did one study of fumigants (Anger et al. 1986) although not another (Calvert et al. 1998). Additional studies have evaluated changes in mood and affect, using either self-report or validated scales. Workers exposed to OPs (Bazylewicz-Walczak et al. 1999; Steenland et al. 2000; Stokes et al. 1995) or DDT (van Wendel de Joode et al. 2001) reported higher levels of tension, anger, or depression on standard symptom questionnaires, and OP applicators showed elevated levels of anxiety on personality tests (Levin et al. 1976). Three studies found no association of OPs with symptom prevalence or affect (Ames et al. 1995; Fiedler et al. 1997; Korsak and Sato 1977).

Increased symptom prevalence was correlated with inhibition of erythrocyte AChE in four studies of OP exposure (Bdlin and Chow 1974; Gomes et al. 1998; Leng and Lewalter 1999; Ohayo-Mitoko et al. 2000) and with inhibition of both erythrocyte AChE and plasma cholinesterase in two of these (Bellin and Chow 1974; Leng and Lewaher 1999). Another study found no relationship of symptom prevalence to inhibition of either erythrocyte or plasma cholinesterase (Lee et al. 2003). One study found that increased symptom prevalence was associated with self-reported pesticide exposure but not with depressed erythrocyte AChE activity (Ciesielski et al. 1994). Effects of OP exposure may not necessarily be caused by AChE inhibition (Pope 1999). Further, farmworkers have complex work histories and are likely to be exposed to pesticides other than OPs that may affect symptom prevalence without affecting ACHE.

Neurobehavioral Performance

Neurobehavioral test batteries, including the World Health Organization Neurobehavioral Core Test Battery (Anger et al. 2000), the Neurobehavioral Evaluation System (Letz et al. 1996), and portions of other batteries, have been used to evaluate pesticide effects on cognitive and psychomotor function. Tests included in these batteries assess memory, attention, visuospatial processing, and other aspects of cognitive function; commonly used tests include symbol digit, digit span, visual retention, pattern memory, trail making, and others. Most studies indicate that pesticide exposure is associated with deficits in cognitive function. Sheep dippers (Stephens et al. 1995), nursery workers (Bazylewicz-Walczak et al. 1999), and other workers (Korsak and Sato 1977) exposed to OPs, malaria-control workers who sprayed DDT (van Wendel de Joode et al. 2001), vineyard workers exposed to fungicides (Baldi et al. 2001), fumigators exposed to sulfuryl fluoride but not those exposed to methyl bromide (Anger et al. 1986; Calvert et al. 1998), and farmers (Cole et al. 1997), farmworkers (Gomes et al. 1998; Kamel et al. 2003), and pesticide applicators (Farahat et al. 2003) exposed to multiple pesticides all performed worse on tests of cognitive function. There are some inconsistencies among these studies. Although most studies found deficits on one or more tests of cognitive function, different tests were affected in different studies, and a few studies found no relationship of OP exposure to any test (Ames et al. 1995; Daniell et al. 1992; Fiedler et al. 1997; Rodnitzky et al. 1975; Steenland et al. 2000).

Deficits in psychomotor function could be caused by impairment of sensory input, motor output, or associative delays; tests used include reaction time, tapping, pursuit aiming, Santa Ana and other pegboard tests, and others. Most studies indicate that pesticide exposure is associated with deficits in psychomotor function. Farmworkers (Daniell et al. 1992; London et al. 1997), farmers (Fiedler et al. 1997) and termiticide applicators (Steenland et al. 2000) exposed to OPs, malaria-control workers who sprayed DDT (van Wendel de Joode et al. 2001), vineyard workers exposed to fungicides (Baldi et al. 2001), fumigators exposed to methyl bromide or sulfuryl fluoride (Anger et al. 1986; Calvert et al. 1998), and farmworkers with multiple exposures (Gomes et al. 1998; Kamel et al. 2003) all showed worse performance on tests of psychomotor function. Again, results for individual tests were not fully consistent within or among studies, and no change in psychomotor function was evident in two studies of OP exposure (Ames et al. 1995; Cole et al. 1997).

Sensory and Motor Dysfunction

Neurobehavioral test batteries are often supplemented with tests of sensory or motor function. One frequently used test is vibration sensitivity, which evaluates peripheral somatosensory function. Most available evidence suggests this is not affected by moderate pesticide exposure. One study of farmers exposed to OPs found decreased sensitivity (Stokes et al. 1995), and another of farmers exposed to multiple pesticides found both decreased sensitivity and other signs of peripheral neuropathy (Cole et al. 1998). However, other studies of individuals exposed to OPs (Ames et al. 1995; London et al. 1998; Pilkington et al. 2001; Steenland et al. 2000), DDT (van Wendel de Joode et al. 2001), fumigants (Anger et al. 1986; Calvert et al. 1998), or multiple pesticides (Kamel et al. 2003) found no relationship of exposure to vibration sensitivity or other measures of somatosensory function.

Few studies have evaluated other aspects of sensory function. One study suggested that the sense of smell was not affected by OPs (Steenland et al. 2000); another study suggested a relationship with fumigants (Calvert et al. 1998). Visual contrast sensitivity was not affected by exposure to OPs (Steenland et al. 2000; van Wendel de Joode et al. 2001) or multiple pesticides (Kamel et al. 2003), but color vision was (Steenland et al. 2000). Retinal degeneration was associated with fungicide exposure in a case-control study of licensed pesticide applicators (Kamel et al. 2000). In general, these data are too limited to draw conclusions about the relationship to pesticide exposure to sensory function.

Similarly, few studies have considered motor function, and few inferences can be made about its relationship to pesticide exposure. Tremor was related to exposure to multiple pesticides in one study (Davignon et al. 1965) but not to OPs in two others (London et al. 1998; Steenland et al. 2000). Grip strength was not related to exposure to fumigants (Anger et al. 1986), DDT (van Wendel de Joode et al. 2001), or multiple pesticides (Kamel et al. 2003).

Balance is an integrated sensorimotor function. An early study found deficits in balance in apple farmers exposed to multiple pesticides (Davignon et al. 1965). In modern studies, balance is commonly evaluated by a test of postural sway; varying the conditions of the test may indicate whether impaired balance is related to deficits in visual, proprioceptive, or vestibular input. Three studies of individuals exposed to OPs (Steenland et al. 2000) or to multiple pesticides (Kamel et al. 2003; Sack et al. 1993) found that impaired postural sway was associated with exposure, but effects were small and another study found no relationship of OP exposure to postural sway (Ames et al. 1995). Effects were most evident when both visual and proprioceptive inputs were removed, suggesting that vestibular function may be affected (Kamel et al. 2003; Sack et al. 1993).

Nerve Function

Studies that have evaluated peripheral nerve conduction have produced largely negative results. Several studies of OPs found little evidence of impaired nerve conduction (Ames et al. 1995; Engel et al. 1998; Steenland et al. 2000). One study of fumigators found deficits in nerve conduction (Calvert et al. 1998), but another did not (Anger et al. 1986). In contrast, fungicide exposure was related to impaired nerve conduction in a study of bulb farmers, which also found deficits in autonomic nerve function (Ruijten et al. 1994). One study found changes in dectroencephalogram (EEG) associated with OP exposure (Korsak and Sato 1977).

Three studies have performed clinical neurologic examinations in a subset of individuals identified by field studies as having deficits related to OP exposure. Beach et al. (1996) studied sheep dippers with increased symptom prevalence (Stephens et al. 1995); Horowitz et al. (1999) studied apple farmers with decreased vibration sensitivity (Stokes et al. 1995); and Jamal et al. (2002) studied sheep dippers with peripheral neuropathy (Pilkington et al. 2001). In general, clinical examination confirmed the results of the field studies, although clinically recognizable neurologic abnormalities were minor and not present in all individuals identified by the field studies.

Genetic Susceptibility to Pesticide Neurotoxicity

Individual response to pesticide exposure may be affected by polymorphisms in genes affecting pesticide metabolism. The best-known example is paraoxonase, an enzyme that hydrolyzes active metabolites of OPs (Costa et al. 2003). Animal studies suggest that changes in serum paraoxonase activity alter susceptibility to OP toxicity (Costa et al. 2003). In humans, paraoxonase polymorphisms affect the relationship of OP exposure to both erythrocyte AChE inhibition and symptom prevalence (Lee et al. 2003; Leng and Lewalter 1999; Mackness et al. 2003; Sozmen et al. 2002). Although Costa et al. (2003) have suggested that adequate evaluation of susceptibility requires measuring serum paraoxonase activity as well as genotype, recent population-based studies have suggested that the discrepancy between genotype and phenotype is relatively small and that nongenetic factors contribute relatively little to variation in serum activity (Ferre et al. 2003; Vincent-Viry et al. 2003).

Neurodegenerative Disease

Parkinson Disease

An extensive literature suggests that pesticide exposure may increase risk of Parkinson disease (Le Couteur et al. 1999). Many studies have found an association of Parkinson disease risk with living in rural areas, drinking well water, and farming as an occupation (Priyadarshi et al. 2001). More specifically, case--control studies have observed that pesticide exposure is associated with increased Parkinson disease risk, although results are not fully consistent. Studies published before 1999 were reviewed by Le Couteur et al. (1999), who noted that 12 of 20 studies found a positive association, with 1.6- to 7-fold increases in risk. Some of these studies evaluated risks associated with ever exposure to any pesticide. This broad definition of exposure permits significant misdassification, which could minimize the magnitude of any association observed.

Recent studies with more detailed exposure assessment have generally found an association of pesticide exposure with Parkinson disease, with 1.5- to 7-fold increases in risk. Case--control studies found increased risk associated with possession of a pesticide use license (Baldereschi et al. 2003), cumulative pesticide exposure based on complete occupational histories (Baldi et al. 2003a; Fall et al. 1999), or occupational or other pesticide use (Herishanu et al. 2001). A cross-sectional study found an association of parkinsonism with exposure to any pesticide, although not with specific pesticides or pesticide classes (Engel et al. 2001), and an ecologic study found that Parkinson disease mortality was higher in California counties where pesticides were used than in counties where they were not (Ritz and Yu 2000). Two cohort studies with detailed exposure information confirmed these findings: Risk was related to years of plantation work and to self-reported pesticide exposure in men enrolled in the Honolulu Heart Program cohort (Petrovitch et al. 2002), and occupational exposure to pesticides assessed with a job-exposure matrix was strongly associated with Parkinson disease risk (5.6-fold increase in risk) in an older cohort living in a vineyard-growing region of France (Baldi et al. 2003b). Three case-control studies found no association of pesticide exposure with Parkinson disease (Behari et al. 2001; Kuopio et al. 1999; Taylor et al. 1999).

Most studies of pesticide exposure and Parkinson disease risk have been unable to implicate specific pesticides. Several studies found increased risk associated with exposure to either insecticides or herbicides (Butterfield et al. 1993; Gorell et al. 1998; Semchuk et al. 1992), and one study indicated that risk was elevated by exposure to organochlorines, OPs, or carbamates (Seidler et al. 1996). Several studies have implicated the herbicide paraquat (Hertzman et al. 1990; Liou et al. 1997), which produces selective degeneration of neurons involved in Parkinson disease (McCormack et al. 2002). Case reports have described Parkinson disease in individuals exposed to OPs (Bhatt et al. 1999; Davis et al. 1978); to herbicides including glyphosate (Barbosa et al. 2001), paraquat (Sanchez-Ramon et al. 1987), and diquat (Sechi et al. 1992); and to fungicides including maneb (Meco et al. 1994) and other dithiocarbamates (Hoogenraad 1988). Higher concentrations of organochlorines, particularly dieldrin, have been found in postmortem brains of Parkinson disease patients compared to patients with other neurologic diseases (Corrigan et al. 2000; Fleming et al. 1994).

Animal models have also implicated pesticide exposure in the etiology of Parkinson disease. In rats, systemic administration of rotenone has been shown to produce highly selective neural degeneration similar to that found in Parkinson disease as well as a parkinsonian behavioral disorder (Betarbet et al. 2000). Treatment of mice with both paraquat and maneb reduced motor activity and striatal tyrosine hydroxylase activity, at doses at which neither compound was effective alone (Thiruchelvam et al. 2000).

Other Neurodegenerative Diseases

Information on pesticide exposure and other neurologic diseases is more limited. Several studies have suggested that risk of amyotrophic lateral sclerosis (ALS) is related to farming as an occupation, although not necessarily to living in rural areas (Nelson 1995-1996). Pesticide exposure has been considered in six case--control studies; three found some evidence for an association (Deapen and Henderson 1986; McGuire et al. 1997; Savettieri et al. 1991), whereas three others found none (Chancellor et al. 1993; Granieri et al. 1988; Gunnarsson et al. 1992). Only one study presented detailed exposure information (McGuire et al. 1997): Based on an industrial hygiene assessment of a complete occupational history, pesticide exposure was associated with > 2-fold increase in ALS risk, with greater risk at higher levels of exposure. This study did not implicate specific pesticides in ALS etiology. However, a cohort study found increased risk of ALS among workers exposed to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) compared to other company employees, although this result was based on only three deaths (Burns et al. 2001). Case reports have described ALS after exposure to OPs (Bidstrup et al. 1953) and organochlorines (Fonseca et al. 1993).

Dementia has also been related to pesticide exposure. Occupational exposure to unspecified pesticides and fertilizers was associated with risk of Alzheimer disease in a large case--control study (McDowell et al. 1994), although another smaller study of environmental exposure in the general population found no relationship to herbicides, insecticides, or pesticides (Gauthier et al. 2001). Occupational exposure to any pesticide assessed with a job-exposure matrix was associated with 2-fold increase in risk of Alzheimer disease in a cohort of older individuals living in a vineyard-growing region of France and exposed primarily to dithiocarbamate fungicides (Baldi et al. 2003b). Occupational pesticide exposure was also associated with mild cognitive dysfunction in a population-based prospective study (Bosma et al. 2000), with vascular dementia (Lindsay et al. 1997), and with risk of dementia among Parkinson disease patients (Hubble et al. 1998). Understanding the relationship of pesticide exposure to Alzheimer disease may be complicated by the fact that the basic neurochemical defect in Alzheimer disease is loss of cholinergic neurons, and that to increase cholinergic tone Alzheimer disease is sometimes treated with OP cholinesterase inhibitors (Ringman and Cummings 1999).

Conclusion

Most studies of neurotoxicity have documented an increase in symptom prevalence and changes in neurobehavioral performance reflecting cognitive and psychomotor dysfunction, but many found little effect of pesticide exposure on sensory or motor function or direct measures of nerve function. There are several potential explanations for these findings. Except for vibrotactile sensitivity, information on sensory and motor function is limited, and further study may reveal associations with pesticide exposure. Another possibility is that the increase in symptom prevalence is due to bias: Most studies were cross-sectional in design, and individuals with greater exposure or a history of poisoning may have been more motivated to recall or report symptoms. Confounding by head injury or neurologic disease, either of which might be related to both pesticide exposure and increased symptom prevalence, could also create the appearance of an association. Consistency of findings across many studies argues against these explanations, as do the positive findings of some studies that used more quantitative exposure measures. Further, bias and confounding are less likely to account for changes in neurobehavioral performance, which is assessed using objective test batteries. Thus, moderate pesticide exposure may in fact have greater effects on symptom prevalence and neurobehavioral performance than on sensory or motor function. The lack of specificity of the symptomatic response is also interesting. It is possible that the earliest or most general response to pesticide neurotoxicity is a general malaise lacking in specificity and related to mild cognitive dysfunction, similar to that described for Gulf War syndrome (White et al. 2001).

Although the weight of the evidence suggests that pesticide use is associated with increased symptom prevalence and deficits in neurobehavioral performance, there were some inconsistencies that future studies should attempt to resolve. It may be that certain functional domains are more sensitive to pesticides than others, but the current literature is too limited to resolve this question. Some of the inconsistencies among studies are likely due to methodologic differences. A critical concern is exposure assessment. Qualitative and quantitative aspects of the exposure under consideration differed among studies, as did the ability of the studies to assess exposure. Exposure measures ranged from job title to detailed assessment of cumulative exposure based on work history. There was, however, no clear-cut relationship between the quality of exposure assessment and the results of the studies.

The choice of comparison group may also influence results. Responses to symptom questionnaires and neurobehavioral performance are influenced by age, education, and cultural background (Anger et al. 1997), so it is important for comparison groups to be demographically similar to exposed populations. However, using a comparison group from the same community or workplace as the exposed participants can create problems. Although the former may have no documented exposure, they may nevertheless not be truly unexposed, limiting the power of the study to detect effects. There may be no one best solution to this problem.

Other aspects of study design, such as size, neurologic end points considered, and data analytic strategies including control for confounding, are likely to influence results. More than half of the studies considered were small, with < 100 exposed participants, and therefore had limited power to detect associations. Poor response rates in some studies may have biased results. Symptom questionnaires, neurobehavioral test batteries, and other methods for evaluating neurologic outcomes also varied among studies. In particular, different neurobehavioral batteries employ different tests of cognitive and psychomotor function. However, results were variable even for tests used in many studies. Implementation of a given test may vary between batteries; for example, a computerized version may differ from a paper-and-pencil model, but even this consideration may not explain all differences. A study of styrene found that grouping results of neurobehavioral tests provided increased power to detect effects of exposure, compared to evaluating individual tests (Heyer et al 1996). Use of similar analytic strategies might reduce inconsistencies among studies of pesticides.

Pesticide exposure may be associated with increased risk of Parkinson disease. Inconsistencies among studies are again likely to be caused by variations in study methodology, particularly lack of detailed exposure assessment in some earlier studies. The positive results from recent studies with more comprehensive exposure assessment, together with support from animal models, reinforces the hypothesis of an association. Results for ALS and Alzheimer disease are suggestive but too sparse to support firm conclusions. Whether the subtle signs of neurotoxicity found in studies of poisoning and occupational exposure are related to the later development of neurodegenerative disease is a question not adequately addressed by the literature, although one study showed that short- and long-term responses to moderate exposure are not necessarily related (Stephens et al. 1996).

Historically, most studies have focused on OPs, first to document sequelae of acute poisoning and then to explore the effects of chronic moderate exposure. There is also evidence suggesting that other types of pesticides, including organochlorines, carbamates, fungicides, and fumigants, are neurotoxic. No study has evaluated the association of herbicides with symptom prevalence or neurobehavioral performance, but these chemicals have been implicated as risk factors for Parkinson disease. Although it is important to identify classes of pesticides and even specific chemicals associated with neurotoxicity, it is also important to recognize that most workers are exposed to complex mixtures of pesticides, which may contribute synergistically to neurotoxicity.

Other aspects of the relationship of pesticide exposure to neurotoxicity remain to be clarified. Participants in most studies have sustained both chronic and acute exposures; because these are often correlated, the studies have not been able to disentangle their effects. It is also possible that studies of chronic moderate exposure have been influenced by inclusion of individuals with a history of pesticide poisoning in the exposed population. Several studies in which such individuals were excluded found no relationship of chronic exposure to neurobehavioral performance or nerve function (Ames et al. 1995; Engel et al. 1998; Fiedler et al. 1997), but other studies of nonpoisoned individuals have found associations (Kamel et al. 2003; Stephens et al. 1995; van Wendel de Joode et al. 2001), suggesting that moderate as well as high-level pesticide exposure is neurotoxic. An issue receiving increasing attention is genetic susceptibility to pesticide neurotoxicity. In particular, genetic variation in paraoxonase has been related to OP neurotoxicity.

In conclusion, there is mounting evidence that chronic moderate pesticide exposure is neurotoxic and increases risk of Parkinson disease. To substantiate these findings, future studies must employ more detailed assessment of exposure in individuals and consider the role of genetic susceptibility. More studies of pesticides other than OPs and greater attention to disentangling the effects of different types of pesticides are also needed. Better information is required to clarify the relative importance of acute and chronic exposure and the role of moderate exposure in the absence of poisoning. Finally, it will be important to clarify the relationship of pesticide-related neurotoxicity to neurodegenerative disease.

REFERENCES

Alavanja M, Hoppin J, Kamel F. 2004. Health effects of chronic pesticide exposure: cancer and neurotoxicity. Annu Rev Public Health 25:155-197.

Ames R, Steenland K, Jenkins B, Chrislip D, Russo J. 1995. Chronic neurologic sequelae to cholinesterase inhibition among agricultural pesticide applicators. Arch Environ Health 50:440-444.

Anger W, Moody L, Burg J, Brightwell WS, Taylor B J, Russo JM, et al. 1986. Neurobehavioral evaluation of soil and structural fumigators using methyl bromide and sulfuryl fluoride. Neurotexicology 7:137-156.

Anger WK, Liang YX, Nell V, Kang SK, Cole D, Bazylewicz-Walczak B, et al. 2000. Lessons learned--15 years of the WHO-NCTB: a review. Neurotoxicology 21:837-846.

Anger WK, Sizemore 0J, Grossmann SJ, Glasser JA, Letz R, Bowler R. 1997. Human neurobehavioral research methods: impact of subject variables. Environ Res 73:18-41.

Arcury T, Quandt S. 1998. Chronic agricultural chemical exposure among migrant and seasonal farmworkers. Soc Nat Resour 11:829-843.

Baldereschi M, Di CA, Vanni P, Ghetti A, Carbenin P, Amaducci L, et al. 2003. Lifestyle-related risk factors for Parkinson's disease: a population-based study. Acta Neurol Scand 108:239-244.

Baldi I, Cantagrel A, Lebailly P, Tison F, Dubroca B, Chrysostome V, et al. 2003a. Association between Parkinson's disease and exposure to pesticides in southwestern France. Neuroepidemiology 22:305-310.

Baldi I, Filleul L, Mohammed-Brahim B, Fabrigoule C, Dartigues J, Schwall S, et al. 2001. Neuropsychologic effects of long-term exposure to pesticides: results from the French Phytoner study. Environ Health Perspect 109:839-844.

Baldi I, Lebailly P, Mehammed-Brahim B, Letenneur L, Dartigues JF, Brochard P. 2003b. Neurodegenerative diseases and exposure to pesticides in the elderly. Am J Epiderniol 157:409-414.

Barbosa ER, Leiros da Costa MD, Bacheschi LA, Scarf M, Leite CC. 2001. Parkinsonism after glycine-derivate exposure. Mov Disord 16:565-568.

Bazylewicz-Walczak B, Majczakowa W, Szymczak M. 1999. Behavioral effects of occupational exposure to organephospherous pesticides in female greenhouse planting workers. Neurotoxicology 20:819-826.

Beach JR, Spurgeon A, Stephens R, Heafield T, Calvert IA, Levy LS, et al. 1996. Abnormalities on neurological examination among sheep farmers exposed to organophosphorous pesticides. Occup Environ Med 53:520-525.

Behari M, Srivastava AK, Das RR, Pandey RM. 2001. Risk factors of Parkinson's disease in Indian patients. J Neurol Sci 190:49-55.

Bellin JS, Chow I. 1974. Biochemical effects of chronic moderate exposure to pesticides. Res Comm Chem Pathol Pharmacol 9:325-337.

Betarbet R, Sherer TB, MacKenzie G, Garcia-Osuna M, Panov AV, Greenamyre JT. 2000. Chronic systemic pesticide exposure reproduces features of Parkinson's disease. Nat Neurosci 3:1301-1306.

Bhatt MH, Elias MA, Mankodi AK. 1999. Acute and reversible parkinsonism due to organophosphate pesticide intoxication: five cases. Neurology 52:1467-1471.

Bidstrup P, Bonnel JA, Beckett AG. 1953. Paralysis following poisoning by a new organic phosphorus insecticide (Mipafox): report on two cases. Br Med J 1:1068-1072.

Bosma H, van Boxtel MP, Ponds RW, Houx PJ, Jolles J. 2000. Pesticide exposure and risk of mild cognitive dysfunction. Lancet 356:912-913.

Buchanan D, Pilkington A, Sewell C, Tannahill SN, Kidd MW, Cherrie B, et al. 2001. Estimation of cumulative exposure to organophosphate sheep dips in a study of chronic neurological health effects among United Kingdom sheep dippers. Occup Environ Med 58:694-701.

Burns CJ, Beard KK, Cartmill JB. 2001. Mortality in chemical workers potentially exposed to 2,4-dichlorophenoxyacetic acid (2,4-D) 1945-94: an update. Occup Environ Med 58:24-30.

Butterfield PG, Valanis BG, Spencer PS, Lindeman CA, Nutt JG. 1993. Environmental antecedents of young-onset Parkinson's disease. Neurology 43:1150-1158.

Calvert GM, Mueller CA, Fajen JM, Chrislip DW, Russo J, Briggle T, et al. 1998. Health effects associated with sulfuryl fluoride and methyl bromide exposure among structural fumigation workers. Am J Public Health 88:1774-1780.

Calvert GM, Plate DK, Das R, Rosales R, Shafey O, Thomsen C, et al. 2004. Acute occupational pesticide-related illness in the US, 1998-1999: surveillance findings from the SENSOR-pesticides program. Am J Ind Med 45:14-23.

Chancellor AM, Slattery JM, Fraser H, Warlow CP. 1993. Risk factors for motor neuron disease--a case-control study based on patients from the Scottish motor neuron disease register. J Neurol Neurosurg Psychiatry 56:1200-1206.

Ciesielski S, Loomis DP, Mires SR, Auer A. 1994. Pesticide exposures, cholinesterase depression, and symptoms among North Carolina migrant farmworkers. Am J Publ Health 84:446-51.

Cole DC, Carpio F, Julian J, Leon N. 1998. Assessment of peripheral nerve function in an Ecuadorian rural population exposed to pesticides. J Toxicol Environ Health A 55:77-91.

Cole DC, Carpio F, Julian J, Leon N, Carbotte R, De Almeida H. 1997. Neurobehavioral outcomes among farm and nonfarm rural Ecuadorians. Neurotoxicol Teratol 19:277-286.

Corrigan FM, Wienburg CL, Shore RF, Daniel SE, Mann D. 2000. Organochlorine insecticides in substantia nigra in Parkinson's disease. J Toxicol Environ Health 59:229-234.

Costa LG, Cole TB, Furlong CE. 2003. Polyrnorphisms of paraoxonase (PON1) and their significance in clinical toxicology of organophesphates. J Toxicol Clin Toxicol 41:37-45.

Daniell W, Barnhart S, Demers P, Costa LG, Eaton DL, Miller M, et al. 1992. Neuropsychological performance among agricultural pesticide applicators. Environ Res 59:217-228.

Davignon LF, St Pierre J, Charest G, Tourangeau FJ. 1965. A study of the chronic effects of insecticides in man. Can Med Assoc J 92:597-602.

Davis K, Yesavage J, Berger P. 1978. Single case study. Possible organophosphate induced parkinsonism. J Nerv Ment Dis 166:222-225.

Deepen D, Henderson BE. 1986. A case-control study of amyotrophic lateral sclerosis. Am J Epidemiol 123:790-799.

Dick R, Steenland K, Krieg E, Hines C. 2001. Evaluation of acute sensory-motor effects and test sensitivity using termiticide workers exposed to chlorpyrifos. Neurotoxicol Teratol 23:381-393.

Dosemeci M, Alavanja MC, Rowland AS, Mage D, Zahm SH, Rothman N, et al. 2002. A quantitative approach for estimating exposure to pesticides in the Agricultural Health Study. Ann Occup Hyg 46:245-260.

Engel LS, Checkoway H, Keifer MC, Seixas NS, Longstreth WT Jr, Scott KC, et al. 2001. Parkinsonism and occupational exposure to pesticides. Occup Environ Med 58:582-589.

Engel LS, Keifer MC, Checkoway H, Robinson LR, Vaughan TL. 1998. Neurophysiolegical function in farm workers exposed to organophosphate pesticides. Arch Environ Health 53:7-14.

Eskenazi B, Bradman A, Castorina R. 1999. Exposures of children to organophosphate pesticides and their potential adverse health effects. Environ Health Perspect 107(suppl 3):409-419.

Fall PA, Fredrikson M, Axelson O, Granerus AK. 1999. Nutritional and occupational factors influencing the risk of Parkinson's disease: a case-control study in southeastern Sweden. Mov Disord 14:28-37.

Farahat T, Abdelrasoul GM, Amr MM, Shebl MM, Farahat FM, Anger WK. 2003. Neurobehavioral effects among workers occupationally exposed to organophosphorous pesticides. Occup Environ Med 60:279-286.

Fenske RA. 1997. Pesticide exposure assessment of workers and their families. Occup Med 12:221-237.

Ferre N, Camps J, Fernandez-Ballart J, Arija V, Murphy MM, Ceruelo S, et al. 2003. Regulation of serum paraoxonase activity by genetic, nutritional, and lifestyle factors in the general population. Clin Chem 49:1491-1497.

Fiedler N, Kipen H, Kelly-McNeil K, Fenske R. 1997. Long-term use of erganophosphates and neuropsychological performance. Am J Ind Med 32:487-496.

Fleming L, Mann JB, Bean J, Briggle T, Sanchez Ramos JR. 1994. Parkinson's disease and brain levels of organochlorine pesticides. Ann Neuro 136:100-103.

Fonseca RG, Resende LAL, Silva MD, Camargo A. 1993. Chronic motor neuron disease possibly related to intoxication with organochlorine insecticides. Acta Neurol Scand 88:56-58.

Gauthier E, Fortier I, Courchesne F, Pepin P, Mortimer J, Gauvreau D. 2001. Environmental pesticide exposure as a risk factor for Alzheimer's disease: a case-control study. Environ Res 86:37-45.

Gladen BC, Sandier DP, Zahm SH, Kamel F, Rowland AS, Alavanja MC. 1998. Exposure opportunities of families of farmer pesticide applicators. Am J Ind Med 34:581-587.

Gomes J, Lloyd O, Revitt MD, Basha M. 1998. Morbidity among farm workers in a desert country in relation to long-term exposure to pesticides. Scand J Work Environ Health 24:213-219.

Gomes J, Lloyd OL, Revitt DM. 1999. The influence of personal protection, environmental hygiene and exposure to pesticides on the health of immigrant farm workers in a desert country. Int Arch Occup Environ Health 72:40-45.

Gorell JM, Johnson CC, Rybicki BA, Peterson EL, Richardson RJ. 1998. The risk of Parkinson's disease with exposure to pesticides, farming, well water, and rural living. Neurology 50:1346-1350.

Granieri E, Carreras M, Tola R. 1988. Motor neuron disease in the province of Ferrara, Italy, in 1964-1982. Neurology 38:1604-1608.

Gunnarsson LG, Bodin L, Soderfeldt B, Axelson O. 1992. A case-control study of motor neurone disease--Its relation to heritability, and occupational exposures, particularly to solvents. Br J Ind Med 49:781-798.

Herishanu YO, Medvedovski M, Goldsmith JR, Kordysh E. 2001. A case-control study of Parkinson's disease in urban population of southern Israel. Can J Neurol Sci 28:144-147.

Hernandez-Valero MA, Bondy ML, Spitz MR, Zahm SH. 2001. Evaluation of Mexican American migrant farmworker work practices and organochlorine pesticide metabolites. Am J Ind Med 40:554-560.

Hertzman C, Wiens M, Bowering D, Snow B, Calne D. 1990. Parkinson's disease: a case-control study of occupational and environmental risk factors. Am J Ind Med 17:349-355.

Heyer NJ, Bittner AC Jr, Eeheverria D. 1996. Analyzing multivariate neurobehavioral outcomes in occupational studies: a comparison of approaches. Neurotoxicol Teratol 18:401-406.

Hines C, Deddens J, Tucker S, Hornung R. 2001. Distributions and determinants of pre-emergent herbicide exposures among custom applicators. Ann Occup Hyg 45:227-239.

Hoogenraad T. 1988. Dithiocarbamates and Parkinson's disease [Letter]. Lancet 1:767.

Horowitz SH, Stark A, Marshall E, Mauer MP. 1999. A multimodality assessment of peripheral nerve function in organophosphate-pesticide applicators. J Occup Environ Med 41:405-408.

Hubble JP, Kurth JH, Glair SL, Kurth MC, Schellenberg GD, Hassanein RE, et al. 1998. Gene-toxin interaction as a putative risk factor for Parkinson's disease with dementia. Neuroepidemiology 17:96-104.

Jamal GA, Hansen S, Pilkington A, Buchanan D, Gillham RA, Abdel-Azis M, et al. 2002. A clinical neurological, neurophysiological, and neuropsychological study of sheep farmers and dippers exposed to organophosphate pesticides. Occup Environ Med 59:434-441.

Kamel F, Boyes WK, Gladen BC, Rowland AS, Alavanja MC, Blair A, et al. 2000. Retinal degeneration in licensed pesticide applicators. Am J Ind Med 37:618-628.

Kernel F, Rowland A, Park L, Anger W, Baird D, Gladen B, et al. 2003. Neurobehavioral performance and work experience in Florida farmworkers. Environ Health Perspect 111:1765-1772.

Karr C, Demers P, Costa L, Daniell W, Barnhart S, Miller M, et al. 1992. Organophosphate pesticide exposure in a group of Washington State orchard applicators. Environ Res 59:229-237.

Keifer M, Mahurin R. 1997. Chronic neurologic effects of pesticide overexposure. Occup Med 12:291-304.

Korsak RJ, Sate MM. 1977. Effects of chronic organophosphate pesticide exposure on the central nervous system. Clin Toxicol 11:83-95.

Kuopio AM, Marttila R J, Helenius H, Rinne UK. 1999. Environmental risk factors in Parkinson's disease. Mov Disord 14:928-939.

Le Couteur BG, McLean AJ, Taylor MC, Woodham BL, Board PG. 1999. Pesticides and Parkinson's disease. Biomed Pharmacother 53:122-130.

Lee BW, London L, Paulauskis J, Myers J, Chdstiani DC. 2003. Association between human paraoxonase gene polymorphism and chronic symptoms in pesticide-exposed workers. J Occup Environ Mad 45:118-122.

Leng G, Lewalter J. 1999. Role of individual susceptibility in risk assessment of pesticides. Occup Environ Med 56:449-453.

Letz R, Green RC, Woodard JL. 1996. Development of a computer-based battery designed to screen adults for neurepsychological impairment. Neurotoxicol Teratol 18:365-370.

Levin HS, Rodnitzky RL, Mick DL. 1976. Anxiety associated with exposure to organophosphate compounds. Arch Gen Psychiatry 33:225-228.

Lindsay J, Hebert R, Rockwood K. 1997. The Canadian Study of Health and Aging: risk factors for vascular dementia. Stroke 28:526-530.

Liou HH, Tsai MC, Chen CJ, Jeng JS, Chang YC, Chen SY, et al. 1997. Environmental risk factors and Parkinson's disease: a case-control study in Taiwan. Neurology 48:1583-1588.

London L, De Grosbois S, Wesseling C, Kisting S, Rother H, Mergler D. 2002. Pesticide usage and health consequences for women in developing countries: out of sight, out of mind? Int J Occup Environ Health 8:46-59.

London L, Myers JE. 1998, Use of a crop and job specific exposure matrix for retrospective assessment of long-term exposure in studies of chronic neurotoxic effects of agrichemicals. Occup Environ Med 55:194-201.

London L, Myers JE, Nell V, Taylor T, Thompson ML 1997. An investigation into neurologic and neurobehavioral effects of long-term agrichemical use among deciduous fruit farm workers in the western cape, South Africa. Environ Res 73:132-145.

London L, Nell V, Thompson M, Myers J. 1998. Effects of long-term organophosphate exposures on neurological symptoms, vibration sense and tremor among South African farm workers. Scand J Work Environ Health 24:18-29.

Lundberg I, Hogberg M, Michelsen H, Nise G, Hogstedt C. 1997. Evaluation of the Q16 questionnaire on neurotoxic symptoms and a review of its use. Occup Environ Med 54:343-350.

Mackness B, Durrington P, Povey A, Thomson S, Dippnall M, Mackness M, et al. 2003. Paraoxonase and susceptibility to organophosphorus poisoning in farmers dipping sheep. Pharmecogenetics 13:81-88.

Maizlish N, Schenker M, Weisskoph C, Seiber J, Samuets S. 1987. A behavioral evaluation of pest control workers with short-term, low-level exposure to the organophosphate diazinon. Am J Ind Med 12:153-172.

McConnell R, Cordon M, Murray DL, Magnotti R. 1992. Hazards of closed pesticide mixing and loading systems--the paradox of protective pechnology in the phird world. Br J Ind Med 49:615-619.

McConnell R, Keifer M, Rosenstock L. 1994. Elevated quantitative vibrotactile threshold among workers previously poisoned with methamidophos and other organophosphate pesticides. Am J Ind Mad 25:325-334.

McCormack AL, Thiruchelvam M, Manning-Bog AB, Thiffault C, Langston JW, Cory-Slechta DA, et al. 2002. Environmental risk factors and Psrkinson's disease: selective degeneration of nigral dopaminergic neurons caused by the herbicide paraquat. Neurobiol Dis 10:119-127.

McDowell I, Hill G, Lindsay J, Helliwell B, Costa L, Beattie L, et al. 1994. The Canadian study of health and aging: risk factors for Alzheimer's disease in Canada. Neurology 44:2073-2080.

McGuire V, Longstreth W Jr, Nelson L, Koepsell T, Checkoway H, Morgan M, et al. 1997. Occupational exposures and amyotrophic lateral sclerosis: a population-based case-control study. Am J Epidemiol 145:1076-1088.

Meco G, Bonifati V, Vanacore N, Fabrizio E. 1994. Parkinsonism after chronic exposure to the fungicide maneb (manganese ethylene-bis-dithiocarbamate). Scand J Work Environ Health 20:301-805.

Miranda J, Lundberg I, McConnell R, Delgado E, Cuadra R, Torres E, et al. 2002. Onset of grip- and pinch-strength impairment after acute poisonings with organophosphate insecticides. Int J Occup Environ Health 8:19-26.

Misra U, Nag D, Bushan V, Ray PK. 1985. Clinical and biochemical changes in chronically exposed organophosphate workers. Toxicol Lett 24:187-193.

Moses M, Johnson ES, Anger WK, Burse WV, Horstman SW, Jackson RJ, et al. 1993. Environmental equity and pesticide exposure. Toxicol Ind Health 9:913-959.

Nelson LM. 1995-1996. Epidemiology of ALS. Clin Neurosci 3:327-331.

Nishiwaki Y, Maekawa K, Ogawa Y, Asukai N, Minami M, Omai K, et al. 2001. Effects of satin on the nervous system in rescue team staff members and police officers 3 years after the Tokyo subway sarin attack. Environ Health Perspect 109:1169-1173.

Ohayo-Mitoko GJ, Kromhout H, Karumba PN, Boleij JS. 1999. Identification of determinants of pesticide exposure among Kenyan agricultural workers using empirical modelling. Ann Occup Hyg 43:519-525.

Ohayo-Mitoko GJ, Kromhout H, Simwa JM, Boleij JS, Heederik D. 2000. Self reported symptoms and inhibition of acetylcholinesterase activity among Kenyan agricultural workers. Occup Environ Meal 57:195-200.

Ohayo-Mitoko GJA, Heederik DJJ, Kromhout H, Omondi BEg, Boleij JSM. 1997. Acetylcholinesterase inhibition as an indicator of organophosphate and carbamate poisoning in Kenyan agricultural workers. Int J Occup Environ Health 3:210-220.

Petrovitch H, Ross GW, Abbott RD, Sanderson WT, Sharp DS, Tanner CM, et al. 2002. Plantation work and risk of Parkinson disease in a population-based longitudinal study. Arch Neurol 59:1787-1792.

Pilkington A, Buchanan D, Jamal GA, Gillham R, Hansen S, Kidd M, et al. 2001. An epidemiological study of the relations between exposure to organophosphate pesticides and indices of chronic peripheral neuropathy and neuropsychological abnormalities in sheep farmers and dippers. Occup Environ Med 58:702-710.

Pope CN. 1999. Organophosphorus pesticides: do they all have the same mechanism of toxicity? J Toxicol Environ Health B Crit Rev 2:161-181.

Priyadarshi A, Khuder SA, Schaub EA, Priyadarshi SS. 2001. Environmental risk factors and Parkinson's disease: a metaanalysis. Environ Res 86:122-127.

Ringman JM, Cummings JL. 1999. Metrifonate: update on a new antidementia agent. J Clin Psychiatry 60:776-782.

Ritz B, Yu F. 2000. Parkinson's disease mortality and pesticide exposure in California 1984-1994. Int J Epidemiol 292:323-329.

Rodnitzky RL, Levin HS, Mick DL. 1975. Occupational exposure to organophosphate pesticides: a neurobehavioral study. Arch Environ Health 30:98-103.

Rosenstock L, Keifer M, Daniell WE, McConnell R, Claypoole K, the Pesticide Health Effects Study Group. 1991. Chronic central nervous system effects of acute organophosphate pesticide intoxication. Lancet 338:223-227.

Ruijten M, Salle HJA, Verberk MM, Smink M. 1994. Effect of chronic mixed pesticide exposure on peripheral and autonomic nerve function. Arch Environ Health 49:186-195.

Sack D, Linz D, Shukla R, Rice C, Bhattacharya A, Suskind R. 1993. Health status of pesticide applicators: postural stability assessments. J Occup Med 35:1196-1202.

Sanchez-Ramon J, Hefti F, Weiner WI. 1987. Paraquat and Parkinson disease [Letter]. Neurology 37:728.

Savage E, Keefe TJ, Mounce LM, Heaton RK, Lewis JA, Burcar PJ. 1988. Chronic neurological sequelae of acute organophosphate pesticide poisoning. Arch Environ Health 43:38-45.

Savettieri G, Salemi G, Arcara A, Cassata M, Castiglione MG, Fierro B. 1991. A case-control study of amyotrophic lateral sclerosis. Neuroepidemiology 10:242-245.

Sechi G, Agnetti V, Piredda M, Canu M, Deserra F, Omar HA, et al. 1992. Acute and persistent parkinsonism after use of diquat. Neurology 42:261-263.

Seidler A, Hellenbrand W, Robra BP, Vieregge P, Nischan P, Joerg J, et al. 1996. Possible environmental, occupational, and other etiologic factors for Parkinson's disease: a case-control study in Germany. Neurology 46:1275-1284.

Semchuk K, Love E J, Lee RG. 1992. Parkinson's disease and exposure to agricultural work and pesticide chemicals. Neurology 42:1329-1335.

Smit LAM, van Wendel de Joode BN, Heederik D, Peiris-John RJ, van der Hoek W. 2003. Neurological symptoms among

Sri Lankan farmers occupationally exposed to acetylcholinesterase-inhibiting insecticides. Am J Ind Med 44:254-264.

Sozmen EY, Mackness B, Sozmen B, Durrington P, Girgin FK, Asian L, et al. 2002. Effect of organephosphate intoxication on human serum paraoxonase. Hum Exp Toxicol 21:247-252.

Stallones L, Beseler C. 2002. Pesticide illness, farm practices, and neurological symptoms among farm residents in Colorado. Environ Res 90:89-97.

Steenland K, Dick RB, Howell R J, Chrislip DW, Hines C J, Reid TM, et al. 2000. Neurologic function among termiticide applicators exposed to chlorpyrifos. Environ Health Perspect 108:293-300.

Steenland K, Jenkins B, Ames RG, O'Mailey M, Chrislip D, Russo J. 1994. Chronic neurological sequelae to organophosphate pesticide poisoning. Am J Public Health 84:731-736. Stephens R, Spurgeon A, Berry H. 1996. Organophosphates: the relationship between chronic and acute exposure effects. Neurotoxicol Teratol 18:449-453.

Stephens R, Spurgeon A, Calvert IA, Beach J, Levy LS, Berry H, et al. 1995. Neuropsychological effects of long-term exposure to organophesphates in sheep dip. Lancet 345:1135-1139.

Stewart PA, Prince JK, Colt JS, Ward MH. 2001. A method for assessing occupational pesticide exposures of farmworkers. Am J Ind Med 40:561-570.

Stokes L, Stark A, Marshall E, Narang A. 1995. Neurotoxicity among pesticide applicators exposed to organophosphates. Occup Environ Med 52:648-653.

Taylor CA, Saint-Hilaire MH, Cupples LA, Thomas CA, Burchard AE, Feldman RG, et al. 1999. Environmental, medical, and family history risk factors for Parkinson's disease: a New England-based case control study. Am J Med Genet 88:742-749.

Thiruchelvam M, Richfield EK, Baggs RB, Tank AW, Cory-Slechta DA. 2000. The nigrostriatal dopaminergic system as a preferential target of repeated exposures to combined paraquat and maneb: implications for Parkinson's disease. J Neurosci 20:9207-9214.

Tielemans E, Louwerse E, de Cock J, Brouwer D, Zielhuis G, Heederik D. 1999. Exposure to fungicides in fruit growing: re-entry time as a predictor for dermal exposure. Am Ind Hyg Assoc J 60:789-793.

U. S. EPA Office of Pesticide Programs. 2002. FY 2002 Annual Report. Washington, DC: U.S. Environmental Protection Agency. Available: http://www.epa.gov/oppfeadl/annual/2002/2002annualreport.pdf [accessed 25 March 2004].

van Wendel de Joode B, Wesseling C, Kremhout H, Monge P, Garcia M, Mergler D. 2001. Chronic nervous-system effects of long-term occupational exposure to DOT. Lancet 357:1014-1016.

Vincent-Viry M, Sass C, Bastien S, Aguillon D, Siest G, Visvikis S. 2003. PON1-192 phenotype and genotypu assessments in 918 subjects of the Stanislas cohort study. Clin Chem Lab Med 41:535-540.

Wesseling C, Keifer M, Ahlbom A, McConnell R, Moon J, Rosenstock L, et al. 2002. Long-term neurobehevioral effects of mild poisonings with organophosphate and n-methyl carbamate pesticides among banana workers. Int J Occup Environ Health 8:27-34.

White RF, Proctor SP, Heeren T, Wolfe J, Krengel M, Vasterling J, et al. 2001. Neuropsychological function in Gulf War Veterans: relationships to self-reported toxicant exposures. Am J Ind Med 40:42-54.

Freya Kamel and Jane A. Hoppin

National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, USA

Address correspondence to F. Kamel, Epidemiology Branch, MD A3-05, NIEHS Box 12233, Research Triangle Park, NC 27709. Telephone: [phone]. Fax: [phone]. E-mail: [e-mail]

We appreciate the thoughtful comments of D. Baird and M. Longnecker on an earlier version of this paper.

This work was supported by internal funding to the Epidemiology Branch, NIEHS.

The authors declare they have no competing financial interests.

Received 30 March 2004; accepted 19 May 2004.

Table 1. Studies of chronic pesticide exposure and neurotoxicity:
exposure measurement. (a)

Reference                         Exposed population       Chemical (b)

Ames et al. 1995                  Pesticide registry       OP
Anger et al. 1986                 Fumigators               Fumigants
Baldi et al. 2001                 Vineyard workers         FNG
Bazylewicz-Walczak et al. 1999    Greenhouse workers       OP
Bellin and Chow 1974              Factory workers          OP, CAR
Calvert et al. 1998               Fumigators               Fumigants
Ciesielski et al. 1994            Farmworkers              Multiple
Cole et al. 1997                  Farmers, some            OP, CAR, FNG
                                    applicators
Cole et al. 1998                  Farmers, some            OP, CAR, FNG
                                    applicators
Daniell et al. 1992               Farmworker applicators   OP
Davignon et al. 1965              Apple farmers            Multiple
Engel of al. 1998                 Farmworkers              OP
Farahat et al. 2003               Farmworker applicators   OP, CAR, PYR
Fiedler et al. 1997               Fruit tree farmers       OP
Gomes et al. 1998                 Farmworkers              Multiple
Kamel et al. 2003                 Farmworkers              Multiple
Korsak and Sato 1977              Occupational exposure    OP
Levin et al. 1976                 Pesticide applicators    OP
London et al. 1997                Fruit farm applicators   OP
London et al. 1998                Fruit farm applicators   OP
McConnell et al. 1994             Farmworkers              OP
Miranda et al. 2002               Hospital patients        OP
Misra et al. 1985                 Commercial applicators   OP
Nishiwaki et al. 2001             Rescue workers           OP
Ohayo-Mitoko et al. 2000          Farmworker applicators   OP, CAR
Pilkington et al. 2001            Sheep dippers            OP
Rodnitzky et al. 1975             Pesticide applicators    OP
Rosenstock et al. 1991            Farmworkers              OP
Ruijten et al. 1994               Flower bulb farmers      FNG
Sack et al. 1993                  Commercial applicators   Multiple
Savage et al. 1988                Registry                 OP
Smit et al. 2003                  Farmers                  Multiple
Stallones and Beseler 2002        Farmers, spouses         Multiple
Steenland et al. 1994             Pesticide registry       OP
Steenland et al. 2000             Commercial applicators   OP
Stephens et al. 1995              Sheep dippers            OP
Stokes et al. 1995                Apple orchard            OP
                                    applicators
van Wendel de Joode et al. 2001   Pesticide applicators    DDT
Wesseling et al. 2002             Farmworkers              OP, CAR

Reference                         Exposure measure (c)           No.

Ames et al. 1995                  Mild poisoning                  45
Anger et al. 1986                 High pesticide use              74
Baldi et al. 2001                 Apply pesticide                528
                                  Work in vineyards              173
Bazylewicz-Walczak et al. 1999    Work with plants                26
Bellin and Chow 1974              AChE inhibition                 83
Calvert et al. 1998               High pesticide use             123
Ciesielski et al. 1994            Self-report; AChE inhibition   202
Cole et al. 1997                  Apply pesticide                144
Cole et al. 1998                  Apply pesticide                144
Daniell et al. 1992               Apply pesticide                 49
Davignon et al. 1965              Apply pesticide                441
Engel of al. 1998                 Current formwork                67
Farahat et al. 2003               Apply pesticide                 52
Fiedler et al. 1997               Cumulative exposure             57
Gomes et al. 1998                 Past and current formwork      226
                                  Current formwork                92
Kamel et al. 2003                 Years of work                  288
Korsak and Sato 1977              High cumulative exposure        16
Levin et al. 1976                 Current pesticide use           24
London et al. 1997                Cumulative exposure            163
London et al. 1998                Cumulative exposure            164
McConnell et al. 1994             Poisoning                       36
Miranda et al. 2002               Poisoning                       52
Misra et al. 1985                 Apply pesticide                 22
Nishiwaki et al. 2001             Poisoning                       56
Ohayo-Mitoko et al. 2000          AChE inhibition                256
Pilkington et al. 2001            Cumulative exposure            612
Rodnitzky et al. 1975             Current pesticide use           23
Rosenstock et al. 1991            Poisoning                       36
Ruijten et al. 1994               Apply pesticide                131
Sack et al. 1993                  Apply pesticide                 37
Savage et al. 1988                Poisoning                      100
Smit et al. 2003                  Apply pesticide                216
Stallones and Beseler 2002        Poisoning                       69
Steenland et al. 1994             Poisoning                      128
Steenland et al. 2000             Apply pesticide                191
Stephens et al. 1995              Apply pesticide                146
Stokes et al. 1995                Apply pesticide                 68
van Wendel de Joode et al. 2001   Years apply pesticide           27
Wesseling et al. 2002             Poisoning                       81

Reference                         Comparison group                  No.

Ames et al. 1995                  Friends                            90
Anger et al. 1986                 Fumigators, low exposure           29
Baldi et al. 2001                 Farmworkers, not exposed          216
Bazylewicz-Walczak et al. 1999    Greenhouse workers, not exposed    25
Bellin and Chow 1974              Faculty, students, staff           56
Calvert et al. 1998               Friends, neighbors                120
Ciesielski et al. 1994            Local population                   42
Cole et al. 1997                  Local population                   72
Cole et al. 1998                  Local population                   72
Daniell et al. 1992               Slaughterhouse workers             40
Davignon et al. 1965              Local population                  162
Engel of al. 1998                 Local population                   68
Farahat et al. 2003               Clerks, administrators             50
Fiedler et al. 1997               Berry farmers, storeowners         42
Gomes et al. 1998                 Domestic workers                  226
Kamel et al. 2003                 Local population                   51
Korsak and Sato 1977              Low cumulative exposure            16
Levin et al. 1976                 Farmers                            24
London et al. 1997                Farmworkers, not applicators       84
London et al. 1998                Farmworkers, not applicators       83
McConnell et al. 1994             Friends, siblings                  36
Miranda et al. 2002               Cattle ranchers, fishermen         39
Misra et al. 1985                 Hospital workers                   20
Nishiwaki et al. 2001             Rescue workers, not exposed        52
Ohayo-Mitoko et al. 2000          Farmworkers                       152
Pilkington et al. 2001            Farmers, ceramic workers          160
Rodnitzky et al. 1975             Farmers                            24
Rosenstock et al. 1991            Friends, siblings                  36
Ruijten et al. 1994               Population                         67
Sack et al. 1993                  Students, staff                    35
Savage et al. 1988                Multiple sources                  100
Smit et al. 2003                  Fishermen                          44
Stallones and Beseler 2002        Other farm residents              692
Steenland et al. 1994             Friends                            90
Steenland et al. 2000             Friends, blue-collar workers      189
Stephens et al. 1995              Quarry workers                    143
Stokes et al. 1995                Population                         68
van Wendel de Joode et al. 2001   Guards, drivers                    27
Wesseling et al. 2002             Farmworkers                       130

Abbreviations: CAR, carbamates; FNG, fungicides; PYR, pyrethroids.

(a) Only studies of chronic exposure in adults [greater than or
equal to] 18 years of age with comparison groups are included. Two
studies (Baldi et al. 2001 and Games et al. 1998) evaluated two exposed
groups. In four cases, two references report studies of different
neurologic outcomes in the same population: Cole et al. (1997, 1998);
Levin et al. (1976) and Rodnitzky et al. (1975); London et al. (1997,
1998); and McConnell et al. (1994) and Rosenstock et al. (1991).
Studies are listed alphabetically. (b) Identifies the chemical
emphasized by the study; participants may have been exposed to others.

(c) Exposure measure that was used for evaluation of relationship of
chronic exposure to neurotoxicity; if more than one measure was used
for analysis, then the one providing the most specific information on
individual exposure is listed.

Table 2. Studies of chronic pesticide exposure and neurotoxicity:
neurologic outcomes.

                                  Symptoms,   Cognitive
Reference                          affect     function

Ames et al. 1995                      0           0
Anger et al. 1986                     1           1
Baldi et al. 2001                                 1
Bazylewicz-Walczak et al. 1999        1           1
Bellin and Chow 1974                  1
Calvert et al. 1998                   0           1
Ciesielski et al. 1994                1
Cole et al. 1997                                  1
Cole et al. 1998
Daniell et al. 1992                               0
Davignon et al. 1965
Engel et al. 1998
Farahat et al. 2003                               1
Fiedler et al. 1997                   0           0
Gomes et al. 1998                     1           1
Kamel et al. 2003                                 1
Korsak and Sato 1977                  0           1
Levin et al. 1976                     1
London et al 1997
London et al. 1998                    1
McConnell et al. 1994
Misra et al. 1985                     1
Nishiwaki et al. 2001                             1
Ohayo-Mitoko et al. 2000              1
Pilkington et al. 2001                1
Rodnitzky et al. 1975                             0
Rosenstock et al. 1991                1           1
Ruijten et al. 1994
Sack et al. 1993
Savage et al. 1988                    1           1
Smit et al. 2003                      1
Stallones and Beseler 2002            1
Steenland et al. 1994                 1           1
Steenland et al. 2000                 1           0
Stephens et al. 1995                              1
Stokes et al. 1995                    1
van Wendel de Joode et al. 2001       1           1
Wesseling et al. 2002                 1           1

                                  Psychomotor   Vibration
Reference                          function     sensitivity   Balance

Ames et al. 1995                       0             0           0
Anger et al. 1986                      1             0
Baldi et al. 2001                      1
Bazylewicz-Walczak et al. 1999
Bellin and Chow 1974
Calvert et al. 1998                    1             0
Ciesielski et al. 1994
Cole et al. 1997                       0
Cole et al. 1998                                     1
Daniell et al. 1992                    1
Davignon et al. 1965                                             1
Engel et al. 1998
Farahat et al. 2003
Fiedler et al. 1997                    1
Gomes et al. 1998                      1
Kamel et al. 2003                      1             0           1
Korsak and Sato 1977
Levin et al. 1976
London et al 1997                      1             0
London et al. 1998                                   0
McConnell et al. 1994                                1
Misra et al. 1985
Nishiwaki et al. 2001                  0             0           0
Ohayo-Mitoko et al. 2000
Pilkington et al. 2001                               0
Rodnitzky et al. 1975
Rosenstock et al. 1991                 1
Ruijten et al. 1994
Sack et al. 1993                                                 1
Savage et al. 1988                     1
Smit et al. 2003
Stallones and Beseler 2002
Steenland et al. 1994                  1             1           0
Steenland et al. 2000                  1             0           1
Stephens et al. 1995
Stokes et al. 1995                                   1
van Wendel de Joode et al. 2001        1             0
Wesseling et al. 2002                  1

                                              Nerve
Reference                         Tremor   function (a)

Ames et al. 1995                                0
Anger et al. 1986                               0
Baldi et al. 2001
Bazylewicz-Walczak et al. 1999
Bellin and Chow 1974
Calvert et al. 1998                             1
Ciesielski et al. 1994
Cole et al. 1997
Cole et al. 1998
Daniell et al. 1992
Davignon et al. 1965                1
Engel et al. 1998                               0
Farahat et al. 2003
Fiedler et al. 1997
Gomes et al. 1998
Kamel et al. 2003
Korsak and Sato 1977                            1
Levin et al. 1976
London et al 1997
London et al. 1998                  0
McConnell et al. 1994
Misra et al. 1985
Nishiwaki et al. 2001
Ohayo-Mitoko et al. 2000
Pilkington et al. 2001
Rodnitzky et al. 1975
Rosenstock et al. 1991
Ruijten et al. 1994                             1
Sack et al. 1993
Savage et al. 1988                              0
Smit et al. 2003
Stallones and Beseler 2002
Steenland et al. 1994                           0
Steenland et al. 2000               0           0
Stephens et al. 1995
Stokes et al. 1995
van Wendel de Joode et al. 2001
Wesseling et al. 2002

1 indicates the study found some relationship of pesticide exposure to
the general category of outcome, although not necessarily for all
tests; 0 indicates no relationship was observed for any test.

(a) Peripheral nerve conduction, EEG.