A Norwalk-like virus outbreak on the Appalachian Trail. (Features).
Each year, more than 2,500 men and women attempt to backpack the 2,160 miles of the Appalachian Trail from Springer Mountain in northern Georgia to Mount Katahdin in central Maine. Only one in 10 of these "through-hikers" complete the trip, which can take, on average, almost six months (Appalachian Trail Conference, 1994; Center for Appalachian Trail Studies, 1999; Crouse & Josephs, 1993). Long-distance hikers face a number of challenges posed by weather, terrain, and insect or animal vectors, Hikers must also contend with the metabolic demands of daily hiking as well as the demand for potable water. Over 80 percent of Appalachian Trail hikers experience injury or illness, with an average of 1.8 health problems per hiker (Crouse & Josephs, 1993). Furthermore, injuries or illnesses occur in locations remote from sources of medical care.
Among the most common health effects reported by serious hikers and other wilderness recreationists are musculoskeletal or soft-tissue injuries (which represent 50 to 70 percent of health effects) and illnesses and gastrointestinal problems (which represent 20 to 30 percent of health effects) (Montalvo, Wingard, Bracker, & Davidson, 1998; Crouse & Josephs, 1993; Gentile, Morris, Schimelpfenig, Bass, & Auerbach, 1992). Long-distance trails usually have limited sanitation facilities and limited or contaminated water supplies that increase the risk of gastroenteritis from viral, parasitic, or bacterial pathogens. Pathogens such as Norwalk-like viruses, Campylobacter jejuni, and Ciardia lamblia have been documented as causes of gastroenteritis among campers and hikers (Jenkins, Herman, Israel, Cukor, & Blacklow, 1985; Taylor, McDermott, Little, Wells, & Blaser, 1983; Crouse & Josephs, 1993; Wilson, Anderson, Holman, Gary, & Greenberg, 1982).
Norwalk-like viruses, also known as human caliciviruses, are an important cause of acute gastroenteritis, especially in settings like schools, child care centers, nursing homes, hospitals, restaurants, cruise ships, military settings and summer camps. The incubation period of this virus can be from several hours to several days, followed by an abrupt onset of nausea, vomiting, or diarrhea. Illness can last up to 48 hours with sequelae-free recovery (Hunter, 1997). Norwalk-like virus can be spread rapidly from person to person (Sharp et al., 1995; Stevenson, McCann, Duthie, Glew, & Ganguli, 1994); through food that has been handled by an infected person (Griffin et al., 1982; Nelson et al., 1992; Patterson, Hutchings, & Palmer, 1993); through exposure to recreational water (Baron et al., 1982); or from a contaminated water supply (Beller et al., 1997; Kaplan, Goodman, Schonberger, Lippy, & Gary, 1982; Lawson et al., 1991; Wilson et al., 1982). Because of the highly infectious nature of this agent, it is often difficult to determine the source of transmission, be it person to person or a single point such as a water supply or a food item.
An outbreak of gastrointestinal illness occurred among hikers on the Appalachian Trail in Virginia during May and June 1999. The purpose of the investigation reported here was to determine the extent of the outbreak, its source, and the infectious agent responsible for illness among the hikers.
On Thursday, June 10, 1999, the Alleghany/Roanoke City Health Department received calls from two Appalachian Trail hikers who had become sick with nausea, diarrhea, and vomiting on the trail between Catawba and Troutville, Virginia. These hikers reported others being sick on the trail and their suspicion that the source of this illness was a general store frequented by the hikers in Catawba. On the next day, two additional hikers in the same section of the trail reported similar illness.
Health department staff immediately interviewed nine sick hikers in Troutville and found that these hikers had begun to feel ill one to two days after leaving Catawba to resume their hike. Many were convalescing in one of several motels found in Troutville. During the next two days, Health Department staff identified 36 additional hikers ill with similar symptoms.
The preliminary investigation suggested that the illness might have started in Catawba, a small town with an all-you-can-eat restaurant and two general stores (stores A and B). Hikers went to Store A for water, prepared sandwiches, and pizza or packaged food. The proprietors offered an area adjacent to the store for overnight camping and two water hydrants for bathing and drinking. A bathroom was available for the hikers. Most hikers also went to the all-you-can-eat restaurant in town during their stay.
The local health department informed the proprietor of Store A of the possible outbreak and requested that the store post a notice of possible contamination of water supply until samples could be collected and analyzed. On the following day, water samples were collected from taps inside the store, from the backyard hydrant (Hydrant A), and from the nearby feedlot hydrant (Hydrant B) to be tested for total and fecal coliforms according to standard methods (American Public Health Association [APHA], 1998). Both backyard hydrants are in the camping area behind the store. In addition, inspectors were called in to observe food-handling practices and to monitor food temperatures during storage. Food handlers were interviewed to elicit any history of recent illness, to assess procedures used for food preparation, and to identify food sources. No food items from previous meals remained for laboratory analysis. During this visit, the owner of Store A was asked to voluntarily stop supplying water to the public.
In addition to samples collected from Store A, water was sampled from a nearby restaurant, the local post office, and four homes located near the store to determine if there was more extensive contamination of the water table in the immediate area. Concurrently, the local health department contacted the Roanoke Appalachian Trail Club to request assistance in posting advisories on the trail for hikers to filter and treat all water supplies, including well water.
On a third inspection of the store, a fluorescein dye was flushed down the toilet iii the store to see if the dye would later appear in the potable water system and thus indicate a hydrologic connection between the septic system and the well. A 10-gallon water sample was collected from the sink in store A and sent to the Center for Pediatric Research at Eastern Virginia Medical School in Norfolk, Virginia, to test for Norwalk-like viruses.
Because most hikers had become ill on the trail between Catawba and Troutville, the investigation focused on potential exposures occurring two days before and two days after hikers entered the town of Catawba. Using information from the Appalachian Trail Conference on average rates of travel over portions of the trail, the authors estimated that hikers could be intercepted in Waynesboro, Virginia, where the Appalachian Trail crosses the Blue Ridge Parkway. A team from the health department set up a station at the crossing and conducted interviews from June 16 through June 20. In order to find hikers who might have arrived in the area a day or two earlier, hotels, motels, and campgrounds in the Waynesboro area were visited. Information on hikers who were further behind on the trail, as well as on those already heading out of Waynesboro, was obtained during the interviews.
A questionnaire solicited information on date of symptom onset, duration and characteristics of symptoms, foods consumed at Store A and at the restaurant, location of campsite two days before entering Catawba and two days after, and sources and amount of water consumed during the time period of interest. Additional information was sought on whether the hikers were traveling alone or in groups and whether they had shared food and water with other hikers. Six hikers who had been ill submitted stool specimens to be tested for enteric pathogens. Acute and convalescent serum specimens were collected from eight hikers.
A person with a case was defined as any long-distance hiker in the state of Virginia reporting symptoms of diarrhea or vomiting with onset of symptoms on or after May 1. Relative risks (RR) and corresponding 95 percent confidence intervals [CIs] were calculated for consumption of specific food items from the general store or the restaurant, consumption of prepared items from the general store, consumption of other food along the trail, amount and source of water consumed, use of water filter, and travel pattern (alone or in a group). Data were analyzed with Epi Info, Version 6 (Dean et al., 1996).
Six stool specimens were tested for common enteric bacterial, parasitic, and viral pathogens, including Salmonella, Shigella, Campylohacter, E. coli 0157:H7, Crypto-sporidium parvum, Giardia lamblia, and Norwalk-like virus, by the Epidemiologic Support Group in the Virginia State Laboratory Division of Consolidated Laboratory Services (DCLS) in Richmond, Virginia. Standard microbiological procedures were used for the laboratory analyses (APHA, 1998). Serum specimens were tested for antibodies against Norwalk-like viruses at the Center for Pediatric Research (CPR) in Norfolk, Virginia. The test for Norwalk-like virus in stool specimens was repeated at CPR. All water samples were tested for fecal coliform bacteria at DCLS. The 10-gallon water sample collected from inside Store A also was tested for Norwalk-like viruses in the CPR.
Reverse transcription-polymerase chain reaction (RT-PCR) was used to detect Norwalk-like viruses in stool and water samples. A primer pair designed in the RNA polymerase region highly conserved among Norwalk-like viruses was used in RT-PCR (Jiang et al, 1999; Jiang et al., 2000). Viral RNA was extracted from the stool specimens by the Trizol method before RT-PCR was performed (Farkas et al., 2000), while viruses in the water samples were concentrated by filtering of the samples through a positively charged filter before extraction of viral RNA (Huang et al., 2000). RT-PCR products were analyzed by gel electrophoresis, and DNA bands in the gel were visualized under UV light. RTPCR products from positive specimens were then cloned and sequenced.
Serum specimens were tested for antibodies to Norwalk-like viruses with recombinant enzyme immunoassays (EIAs) specific for four strains of Norwalk-like viruses (Norwalk virus [NV]; Hawaii virus [HV]; Mexico virus [MxV]; and Grimsby virus [GrV]). A seroresponse was defined as a fourfold or greater increase of the antibody titer in a convalescent patient over the antibody titer of serum specimens taken from the same patient during acute illness.
The water supply for the taps inside Store A and for hydrants A and B, outside Store A, was a hand-dug, shallow well. An ultraviolet light had been installed to purify the store's drinking water and had been in operation for three years without replacement. Store A is a convenience store with no indoor seating, so the Virginia Department of Agriculture and Consumer Services is responsible for inspection of the water supply. Water samples from the two outside hydrants (hydrants A and B) were positive for total and fecal coliform bacteria (Table 1). These samples were taken when the ultraviolet light system was in working order. The samples taken from the sink in the food preparation area inside Store A tested positive for total coliform bacteria. Water samples from a nearby post office and four nearby homes also were positive for total coliform bacteria. Water samples from the nearby restaurant tested negative for total coliform bacteria (Table 1). In addition, the environmental inspection of Store A identifie d no major deficiencies in food-handling practices or food temperatures during storage or preparation.
The distance from the septic system to the well was estimated to meet the standard of 100 feet, and no fluorescent dye was detected in the water supply, suggesting no connection with the septic system. The area is heavily agricultural, so it is likely that for the five contaminated wells and Store A, the sources of groundwater contamination were primarily agricultural. It is also possible that a leaking or failing neighboring septic system contributed to the problem. In addition to changing the ultraviolet light in the purification system of Store A, a chlorine treatment was applied, and water samples subsequently taken from the store demonstrated no contamination. The proprietor was advised to add a chlorinator to the system, and further sampling was recommended. Although the epidemiologic investigation was conducted to determine the source of the outbreak, no further investigations were conducted to determine the source of well contamination.
The authors interviewed 70 long-distance hikers from June 16 through June 20. Their ages ranged from 19 to 66 years, with a median age of 26 years, and 46 (65.7 percent) were male. Among the hikers interviewed, 45 (64.3 percent) met the case definition for illness. The earliest onset of illness reported was May 1, with two cases reported in the latter part of May. A cluster of 29 cases occurred between May 31 and June 10, and a second peak occurred between June 14 and June 16, the date of the last reported case (Figure 1). The predominant symptoms were diarrhea (in 88.9 percent of cases), nausea (in 84.4 percent of cases), and vomiting (in 60.0 percent of cases). Almost half of the ill persons reported fever, although no temperatures were measured (Table 2). Symptoms lasted for two to 72 hours, with a median duration of 24 hours.
Of the 45 ill hikers interviewed, seven had become ill before arriving in Catawba and 38 after. Among the 38 who became ill after reaching Catawba, one became ill before leaving Catawba, 35 became ill within two days of arriving in Catawba, one became ill within six days, and one became ill within 12 days.
People who had consumed food items like pizza or sandwiches that were prepared at General Store A were almost twice as likely to become ill as people who did not consume those foods (RR = 1.8; 95 percent CI: 1.0-3.2) (Table 3). No specific food item was, however, associated with illness. Consumption of water from the store was also associated with illness (RR = 1.7; 95 percent CI: 1.0-2.7). Thirty-five out of 51 people (68.6 percent) who consumed food prepared at the store or drank the water became ill, compared with none out of nine who either completely bypassed the store or consumed only prepackaged drinks and food.
Eating at the all-you-can-eat restaurant in Catawba was not associated with illness. Approximately 50 percent of respondents who ate there became ill, compared with 62 percent of respondents who did not eat there (RR = 0.8; 95 percent CI: 0.6-1.2). Consuming food left by "trail angels" or sympathetic local residents (canned sodas, homemade brownies, homemade peanut-butter-and-jelly sandwiches) at a location about a half-day hike from Catawba was not associated with illness (RR = 1.0; 95 percent CI: 0.6-1.6).
The majority of hikers filtered or purified their drinking water (73 percent), some used iodine or chlorine to treat water (15 percent), several used a combination of methods (three percent), and a small minority did not filter their water at all (nine percent). Only one person filtered the drinking water at the store; most assumed that this source provided potable water. Hikers who filtered their water were slightly more likely to become ill than those who drank "raw" water. Traveling alone conveyed a small protective effect over traveling in groups of two or more, although the effect was not statistically significant (RR = 0.7; 95 percent CI: 0.4-1.3). Finally, camping overnight at Store A was associated with a 50 percent increased risk of illness compared with not camping at the store (RR = 1.5; 95 percent CI: 1.0-2.1).
Of the six stool samples submitted to DCLS, three were positive for Norwalk-like virus, and one was positive for Campylobacter jejuni. The RT-PCR result for Norwalk-like virus was confirmed at CPR; four out of the six samples were positive. One of the RT-PCR products was cloned and sequenced and the sequence revealed 77 percent nucleotide identity with Norwalk virus, suggesting that the outbreak was associated with genogroup I Norwalk-like viruses. All patients had seroresponses to genogroup I (NV). Seventy-five percent, 63 percent, and 63 percent had responses to members of genogroup II--HV, MxV, and GrV, respectively. These results confirmed the RT-PCR indication that a genogroup I Norwalk-like virus was associated with the outbreak. The water sample taken from Store A resulted in a single band of RT-PCR product of size similar to that of Norwalk-like virus products. The sequences of the product, however, revealed a less-than-50-percent nucleotide identity with known Norwalk-like viruses. This band may have been amplified from nucleic acids of unrelated organisms in the water supply.
Because of the unique setting, which allowed a portion of the cohort to be intercepted at a particular geographical and temporal location, the authors were able to determine the extent of the outbreak and the agent responsible. Clinical features of Norwalk-like virus infection matched the description of illness given by the hikers, and laboratory diagnosis detected Norwalk-like viruses in stool samples as well as antibodies to Norwalk-like viruses in the sera of hikers involved in the outbreak.
The authors successfully identified Norwalk-like virus in stool and sera but not in the water sample taken from the store. The process of examining water and wastewater for enteric viruses is still considered experimental. The low levels of virus contained in drinking water or recreational water require large sample volumes to increase the probability of virus detection (APHA, 1998), and the sample taken for this study may not have been large enough. Thus, the authors could not definitively implicate the water supply at Store A as the point source of the outbreak, despite positive results from fecal-coliform tests.
Nevertheless, contamination of Store A's water supply posed a potential risk to health. Evidence implicating the store came from the finding that almost 70 percent of hikers who consumed food or drank water at Store A became ill, whereas no illness occurred among those who bypassed the store or who ate only prepackaged food. No specific food items were, however, implicated. Although the hikers also gathered at the nearby restaurant, the authors found no association between having eaten at the restaurant and illness. An additional complication was that the proprietor of Store A became ill with diarrhea during, but not before, the outbreak. Because he was responsible for cleaning up the campground behind the store, he may have become infected from the hikers and, while ill, may have spread the infection by working in food preparation. Environmental surface contamination in the store and in the surrounding camping area may have been factors in the persistence of the outbreak.
An important factor for the argument against the Store A as the source of the outbreak is that several sporadic cases of illness were reported during May, prior to the peak of the outbreak (Figure 1). Norwalk-like virus infection is extremely contagious, with very high attack rates. The attack rate in this cohort was 64 percent. Given the extremely infectious nature of this organism and the opportunity presented by the gathering of hikers at Store A, it is likely that Store A might have provided the opportunity for the virus to spread very rapidly rather than have been the initial source of this outbreak.
The nature of hiking on the Appalachian Trail presented the authors with a cohort that could be intercepted and interviewed in one location. All hikers who intended to continue on the trail or who were traveling into town to take a break had to pass the interviewing station. No hiker refused to participate. In addition, the authors canvassed hotels, campgrounds, and parks in Waynesboro to find hikers who had been in town a day or two earlier. They also searched further along the trail and found hikers at the next campground. Nevertheless, they may have missed hikers who had become ill and ended their hike or who had dropped out before interviewing began.
This research suggests that health problems among hikers, specifically gastrointestinal illness, may be difficult to control. Traditional public-health methods of reducing the risk of infectious disease are difficult to apply in this setting. Water is not plentiful, so regular handwashing can be impossible. Filtering water is time consuming, so filtered water is used for drinking but not for washing. Boiling water also presents difficulties in terms of obtaining fuel and in the amount of water that can be disinfected given the small scale of equipment carried by hikers. Several hikers used iodine or chlorine for disinfection and expressed concern over the use of iodine for purification. The use of iodine tablets often produces moderately high levels of residual iodine, and such use should be limited to three months (Backer & Hallowell, 2000). Products that use charcoal scavengers to remove residual iodine are available and make this method acceptable for long-term use. In the case of person-to-person transmis sion, however, which is thought to be the source of this outbreak, filtering of water would not have provided protection against becoming ill.
Educating hikers on the nature of the Norwalk-like virus infection and potential risk factors in different environmental settings may help reduce risk and control outbreaks. Hikers should be aware that not all water sources in town are free from contamination -- especially those that rely on well water. Most hikers in this study did not filter water while in town because they assumed the water was potable. Routine inspection and monitoring of public water supply systems is critical to maintaining a safe water supply Nevertheless, wells in heavily agricultural areas can be contaminated by runoff from barns or fields.
As with any situation in which sanitation is poor and water is scarce, the risk of gastrointestinal illness increases with crowding and contact. This is suggested by the lower risk of illness incurred by those who either traveled alone or avoided camping at Store A, where opportunity for contact was highest.
[FIGURE 1 OMITTED]
TABLE 1 Water Samples Obtained from Store A and Surrounding Areas Date Location Results 6/11/99 Backyard hydrant 500 MPN total coliforms (*) 6/11/99 Backyard hydrant 900 MPN total coliforms 6/11/99 Feedlot hydrant 500 MPN fecal coliforms (**) 6/11/99 Feedlot hydrant 500 MPN fecal coliforms 6/14/99 Hand sink Positive total coliforms (+) 6/15/99 Nearby restaurant Negative total coliforms 6/15/99 Post office Positive total coliforms 6/15/99 Four nearby homes Positive total coliforms 6/15/99 Four nearby homes Positive fecal coliforms (++) (*)Read as 500 = most probable number (MPN) of total coliform organisms. (**)Read as 500 = MPN of fecal coliform organisms. (+)Read as positive or negative for total coliform organisms (nonquantitative). (++)Read as positive for fecal coliform organisms (nonquantitative). TABLE 2 Symptoms Reported in Appalachian Trail Outbreak Symptom Number of Cases Diarrhea 40 (88.9%) Nausea 38 (84.4%) Vomiting 27 (60.0%) Fever 21 (46.7%) Cramps 15 (33.3%) Headache 12 (26.7%) TABLE 3 Relative Risk and 95% Confidence Intervals for Food or Water Consumption and Hiker Travel Patterns Consumption/Travel Characteristics Relative Risk 95% CI Food prepared at Store A 1.8 (1.0-3.2) Food from nearby restaurant 0.8 (0.6-1.2) Food left by "trail angels" 1.0 (0.6-1.6) Water from Store A 1.7 (1.0-2.7) Traveling alone 0.7 (0.4-1.3) Filtering or treating water 1.9 (0.6-5.9) Camping overnight at Store A 1.5 (1.0-2.1)
Acknowledgements: The authors thank Dick Tabb, David Taylor, Sandy Moore, Jackie Zeltvay, and Karen Chaples of the Alleghany/Roanoke City Health District; Robert Tate of the Virginia Department of Health; John Dansby of the Division of Consumer Protection; Sue Yager of the Pennsylvania Department of Health; Robert Proudman and James Hutchings of the Appalachian Trail Conference; Hal Cantrill of the Roanoke Appalachian Trail Club; Brian Cagle, Public Health Consultant for the National Park Service; and, last but not least, the 1999 Appalachian Trail hikers.
American Public Health Association. (1998). Standard methods. Washington, DC: Author.
Appalachian Trail Conference. (1996). History of the Appalachian Trail Project. http://www.arconf.org/about/history/index.html (30 Dec. 2001).
Backer, H., & Hollowell, J. (2000). Use of iodine for water disinfection: Iodine toxicity and maximum recommended dose. Environmental Health Perspectives, 108(8), 679-684.
Baron, R.C., Murphy, F.D., Greenberg, H.B., Davis, C.E., Bregman, D.J., Gary, G.W., Hughes, J.M., & Schonberger, L.B. (1982). Norwalk gastrointestinal illness: An outbreak associated with swimming in a recreational lake and secondary person-to-person transmission. American Journal of Epidemiology, 115, 163-172.
Beller, M., Ellis, A., Spencer, H.L., Drebot, M.A., Jenkerson, S.A., Funk, E., Sobsey, M.D., Simmons, O.D., Monroe, S.S., Ando, T., Noel, J., Petric, M., Middaugh, J.P., & Spika, J.S. (1997). Outbreak of viral gastroenteritis due to a contaminated well. Journal of the American Medical Association, 278, 563-568.
Center for Appalachian Trial Studies. (1999). Definition of thru-hiking. Hot Springs, NC. http://www.trailplace.com/atcenter/html (30 Dec. 2001).
Crouse, B.J., & Josephs, D.J. (1993). Health care needs of Appalachian Trail hikers. Journal of Family Medicine, 36(5), 521-525.
Dean, A.G., Dean, J.A., Coulombier, D., Brendel, K.A., Smith, D.C., Burton, A.H., Dicker, R.C., Sullivan, K., Fagan, R.F., & Arner, T.G. (1996). Epi info, version 6: A word processing, database and statistics program for public health on IBM-compatible microcomputers. Atlanta, GA: Centers for Disease Control and Prevention.
Farkas, T., Jiang, X., Guerrero, M.L., Zhong, W.M., Wilton, N., Berke, T., Matson, D.O., Pickering, L.K., & Ruiz-Palacios, G. (2000). Prevalence and genetic diversity of human caliciviruses (HuCVs) in Mexican children. Journal of Medical Virology, 62, 217-223.
Gentile, D.A., Morris, J.A., Schimelpfenig, T., Bass, S.M., & Auerbach, P.S. (1992). Wilderness injuries and illnesses. Annals of Emergency Medicine, 21, 853-861.
Griffin, M.R., Surowiec, J.J., McCloskey, D.I., Capuano, B., Pierzynski, B., Quinn, M., Wojnarski, R., Parkin, W.E., Greenberg, H., & Gary, G.W (1982). Foodborne Norwalk virus. American Journal of Epidemiology, 115(2), 178-184.
Huang, P.W, Laorde, D., Land, V.R., Matson, D.O., Smith, A., Jiang, X. (2000). Concentration and detection of caliciviruses in water samples by reverse transcription-polymerase chain reaction (RT-PCR). Applied Environmental Microbiology, 66, 4383-4388.
Hunter, PR. (1997). Waterborne disease: Epidemiology and ecology. New York: John Wiley and Sons.
Jenkins, S., Horman, J.T., Israel, E., Cukor, G., & Blacklow, N,R. (1985). An outbreak of Norwalk-related gastroenteritis at a boy's camp. American Journal of Diseases of Childhood, 139, 787-789.
Jiang, X., Huang, P.W, Zhong, W.M., Farkas, T., Cubitt, W.D., & Matson, D.O. (1999). Design and evaluation of a primer pair that detects both Norwalk- and Sapporo-like calciviruses by RT-PCR. Journal of Medical Virology, 83(1-2), 145-154.
Jiang, X., Huang, P.W., Zhong, W.M., Farkas, T., Barrett, E., Guerrero, M., Ruiz-Palacios, G., Green, K.Y., Green, J., Hale, A.D., Estes, M.K., Pickering, L.K., & Matson, D.O. (2000). Diagnosis of human calciviruses by enzyme immune assays. Journal of Infectious Diseases, 181 (Suppl. 2), S349-S359.
Kaplan, J.E., Goodman, R.A., Schonberger, L.B., Lippy, E.C., & Gary, G.W. (1982). Gastroenteritis due to Norwalk virus: An outbreak associated with a municipal water system. Journal of Infectious Diseases, 146, 190-197.
Lawson, H.W, Braun, M.M., Glass, R,I.M., Stine, S.E., Monroe, S.S., Atrash, H.K., Lee, L.E., & Englender, S.J. (1991). Waterborne outbreak of Norwalk virus gastroenteritis at a southwest U.S. resort: Role of geological formations in contamination of well water. The Lancet, 337, 1200-1204.
Montalvo, R., Wingard, D.L., Bracker, M., & Davidson, T.M., (1998). Morbidity and mortality in the wilderness. Western Journal of Medicine, 168, 248-254.
Nelson, T.L., Wright, T.L., Case, M.A., Martin, D.R., Glass, R.I., & Sangal, S.P (1992). A protracted outbreak of foodborne viral gastroenteritis caused by Norwalk or Norwalk-like agent. Journal of Environmental Health, 54(5), 50-55.
Patterson, T., Hutchings, P., & Palmer, S. (1993). Outbreak of SRSV gastroenteritis at an international conference traced to food handled by a post-symptomatic caterer. Epidemiology and Infections, 111, 157-162.
Sharp, T.W, Hyams, K.C., Watts, D., Trofa, A.F., Martin, G.J., Kapikian, A.Z., Green, K.Y., Jiang, X., Estes, M.K., Waack, M., & Savarino, S.J. (1995). Epidemiology of Norwalk virus during an outbreak of acute gastroenteritis aboard a U.S. aircraft carrier. Journal of Medical Virology, 45, 61-67.
Stevenson, P., McCann, R., Duthie, R., Glew, E., & Ganguli, L. (1994). A hospital outbreak due to Norwalk virus. Journal of Hospital Infection, 26, 261-272.
Taylor, D.N., McDermott, K.T., Little, J.R., Wells, J.G., & Blaser M.J. (1983). Campylobacter enteritis from untreated water in the Rocky Mountains. Annals of Internal Medicine, 99, 38-40.
Wilson, R., Anderson, L.J., Holman, R.C., Gary, G.W, & Greenberg, H.B. (1982). Waterborne gastroenteritis due to the Norwalk agent: Clinical and epidemiologic investigation. American Journal of Public Health, 72(1), 72-74.
Corresponding Author: Lucy A. Peipins, Ph.D., Agency for Toxic Substances and Disease Registry, 1600 Clifton Rd., NE, E-31, Atlanta, GA 30333. E-mail: <email@example.com>.
|Printer friendly Cite/link Email Feedback|
|Publication:||Journal of Environmental Health|
|Article Type:||Statistical Data Included|
|Date:||May 1, 2002|
|Previous Article:||Salmonella and other Enterobacteriaceae in dairy-cow feed ingredients: antimicrobial resistance in western Oregon. (Features).|
|Next Article:||The launch of NEHA Training LLC. (Special Report).|