How risky is rover?: petting transfers pesticides. (Science Selections).

Although children can be exposed to pesticides through many means, including carpets, house dust, and chemically treated lawns and gardens, one important potential source is pets treated with parasite control products. According to the American Veterinary Medical Association, dogs are the most popular U.S. pets, with an estimated 53.6 million dogs owned as pets in the United States as of 1998. Those dogs could be a source of insecticide exposure to the millions of children who come into direct contact with them. Moreover, other pets and domestic animals treated for insect pests may be a large additional source of exposure.

Parasite control products often contain carbamate, pyrethroid, or organophosphate insecticides. To estimate the amount of transferable residues that children could obtain from their treated pets, J. Scott Boone and colleagues at Mississippi State University studied the organophosphate chlorpyrifos, used until recently in commercial dips used to treat dogs and cats for fleas and ticks [EHP 109:1109-1114]. The scientists found that young children could receive chlorpyrifos doses nearly equal to the U.S. Environmental Protection Agency (EPA) reference dose, or the amount a person can be exposed to daily without risk of adverse health effects over a lifetime. Although chlorpyrifos has since been withdrawn from the domestic use market, human exposure to other flea- and tick-control pesticides from pets and livestock could occur in much the same way.

Twelve dogs of similar breeds and weights were dipped with a nonprescription commercial flea dip, according to the manufacturer's guidelines, for four consecutive treatments at three-week intervals, with no shampooing in between. Another 12 dogs were shampooed in between dips. Chlorpyrifos samples were taken from the dogs' fur before and after dipping by rubbing the animals with cotton gloves.

Samples collected at 4 hours and at 7, 14, and 21 days after treatment from dogs that were not shampooed averaged 971, 157, 70, and 26 [micro]g chlorpyrifos, respectively. Samples from dogs that were shampooed averaged 459, 49, 15, and 10 [micro]g, respectively. Most of the transferable chlorpyrifos residues dissipated after 3 weeks, with the sharpest decrease after 1 week.

Humans are therefore at the greatest potential risk of exposure to chlorpyrifos shortly after treatment. Children, particularly, spend much time with pets and may be more sensitive to pesticide exposures than adults, so plausible estimates of exposure levels are needed to calculate risk. The researchers calculate that if a child plays with a dog for 5 minutes within 24 hours of the dip, petting and rubbing the dog over an 80-square-inch area, the child could be exposed to an average of 0.9-1.9 mg/kg chlorpyrifos. If the child weighs 25 kg (55 lb), the absorbed dosage could be 0.0011-0.0023 mg/kg. (The EPA's reference dose for chlorpyrifos exposure is 0.003 mg/kg/day.) By contrast, in a 79.5 kg (175 lb) adult, the absorbed dosage could be 0.0003-0.0007 mg/kg.

Activities of butyrylcholinesterase (BChE) and acetylcholinesterase (ACHE) were assayed in the dogs' blood plasma as possible biomarkers of exposure to chlorpyrifos. These cholinesterases (ChEs) break down the neurotransmitter acetylcholine. This prevents overloading of the cholinergic system, which could result in effects ranging from headache to coma. Because humans and dogs have similar percentages of BChE and AChE in their plasma, the results may have implications for human health.

BChE was inhibited by 50-75% throughout the study; AChE was inhibited by 11-18% in dogs that were not shampooed, and to a lesser extent in dogs that were. Greater plasma ChE inhibition was observed at 7 days than at 4 hours, which might reflect the bioactivation of chlorpyrifos to chlorpyrifos-oxon (the active metabolite). Surprisingly, dog plasma ChE activity did not return to control levels during the 21-day period. Indeed, ChE activity inhibition in the blood was maintained at 60-80% throughout the experiment.

Currently, there is no perfect technique for measuring human exposure to flea- and tick-control pesticides from pets and livestock and assessing the attendant risk. Given the large number of pesticides in use, however, the cotton glove dosimeter model could be a quick, reliable, and useful tool for evaluating potential pesticide exposure.