WEST LAFAYETTE, Ind. – Exposure to herbicide-treated lawns and gardens increases the risk of bladder cancer in Scottish terriers, a discovery that could lead to new knowledge about human susceptibility to the disease, according to Purdue University scientists.
A team of veterinary researchers including Lawrence T. Glickman has found an association between risk of transitional cell carcinoma of the urinary bladder in Scottish terriers and the dogs' exposure to chemicals found in lawn treatments. The study, based on a survey of dog owners whose pets had recently contracted the disease, may be useful not only for its revelation of potentially carcinogenic substances in our environment, but also because studying the breed may help physicians pinpoint genes in humans that signal susceptibility to bladder cancer.
"The risk of transitional cell carcinoma (TCC) was found to be between four and seven times more likely in exposed animals," said Glickman, a professor of epidemiology and environmental medicine in Purdue's School of Veterinary Medicine. "While we hope to determine which of the many chemicals in lawn treatments are responsible, we also hope the similarity between human and dog genomes will allow us to find the genetic predisposition toward this form of cancer found in both Scotties and certain people."
The research, which Glickman conducted with Malathi Raghavan, Deborah W. Knapp, Patty L. Bonney and Marcia H. Dawson, all of Purdue's School of Veterinary Medicine, and Indianapolis veterinarian Marcia Dawson, appears in the current (4/15) issue of the Journal of the American Veterinary Medicine Association.
According to the National Cancer Institute, about 38,000 men and 15,000 women are diagnosed with bladder cancer each year. Only about 30 percent of human bladder cancers develop from known causes. As Scottish terriers – often called Scotties – have a history of developing bladder cancer far more frequently than other breeds, Glickman and his team decided to examine the dogs' diet, lifestyle and environmental exposures for a possible link to bladder cancer.
In an earlier study, Glickman and his colleagues found Scotties are already about 20 times more likely to develop bladder cancer as other breeds.
"These dogs are more sensitive to some factors in their environment," Glickman said. "As pets tend to spend a fair amount of time in contact with plants treated with herbicides and insecticides, we decided to find out whether lawn chemicals were having any effect on cancer frequency."
Glickman's group obtained their results by surveying the owners of 83 Scottish terriers. All of the animals had bladder cancer and were of approximately the same age. Based on an 18-page questionnaire, owners documented their dogs' housing, duration of exposure to the lawn or garden and information on the particular lawn treatment used (dog owners provided either the label from the treatment bottle or, if a company sprayed the lawns directly from a truck, the name of the lawn service). The results were then compared with a control group of 83 unexposed Scottish terriers of similar age that were undergoing treatment for unrelated ailments.
"We found that the occurrence of bladder cancer was between four and seven times higher in the group exposed to herbicides," Glickman said. "The level of risk corresponded directly with exposure to these chemicals: The greater the exposure, the higher the risk."
Glickman said it is possible the active ingredient in most lawn and garden sprays – a compound known by its chemical name of 2,4-D – was to blame, although it has been thoroughly tested by the FDA for carcinogenicity. However, he said, it also is possible that one of the so-called inert ingredients in the mixture – ingredients which often make up nearly two-thirds of a treatment's volume – could be responsible for the increased risk.
"These other ingredients are thought to be inert and, therefore, are not tested or even listed on the product label," Glickman said. "But 4 billion pounds of these other untested chemicals reach our lawns and gardens every year, and we theorize they are triggering cancer in these animals, which are already at risk because of a peculiarity in their genome."
Scottish terriers' genetic predisposition toward developing bladder cancer makes them ideal as "sentinel animals" for researchers like Glickman because they require far less exposure to a carcinogen than other breeds before contracting the disease.
"You might compare them to the canaries used in coal mines a century ago," he said. "The difference is that we don't deliberately place our research animals in harm's way. We study animals that have already contracted diseases, bring them to the hospital and then try to find out what combination of genetic predisposition and environmental influence added up to make them ill."
Glickman said the similarity between dog and human genomes could lead researchers to find the gene in humans that makes them susceptible to developing bladder cancer.
"If such a gene exists in dogs, it's likely that it exists in a similar location in the human genome," Glickman said. "Finding the dog gene could save years in the search for it in humans and could also help us determine which kids need to stay away from lawn chemicals."
But Glickman emphasized that because the effect was a combination of chemical and genetic predisposition, the results do not suggest that everyone should avoid treated lawns.
"We don't want to indicate that every person is susceptible," he said. "Because this study shows that exposure to the chemicals exacerbates a genetic predisposition in Scotties towards developing TCC, it's likely that only a segment of the human population would be in similar danger.
"But we still need to find out who those individuals with the same predisposition are. Until we do, we won't know who's safe and who isn't."
As a next step, Glickman will survey children, as well as dogs, in households that have treated lawns and compare the chemicals in their urine samples with those from households where lawns have not been treated.
"It's important to find out which lawn chemicals are being taken up by both children and animals," he said. "We hope to start this spring."
Funding for this research was provided in part by the Scottish Terrier Club of America and the American Kennel Club's Canine Health Foundation.
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