According to Glickman, “No one chemical type of flea and tick dip accounted for the increased risk; however, the active ingredients generally account for less than 5 percent of the total product. The remaining ingredients were labeled as ‘inert” and consisted of solvents such as benzene, toluene, xylene and petroleum distillates, many of which are themselves known carcinogens [in people and lab animals].” A second study in 2004 by Glickman also looked at the spot-on flea and tick products such as Advantage® and Frontline® but found no increase in cancer. These products are minimally absorbed into the dog’s bloodstream and consequently are not excreted via the bladder. [Ed. Note: For more about over-the-counter spot-on products, see our blog on the current (2009) EPA investigation into their toxicity.]
Research also suggests a link between 2,4-D (marketed under many names, including Ded-Weed, Lawn-Keep, Weedone, Plantgard, Miracle and Demise), a phenoxyherbicide commonly used on lawns, and cancer. The findings, however, conflict. A 1991 study published in the Journal of the National Cancer Institute reported that pet dogs in homes where 2,4-D was applied to the lawn at least four times per year had twice the risk of developing lymphoma, compared to dogs who lived where lawns were not treated. These findings were challenged by the Chemical Industry Task Force, and the ensuing reanalysis of the original data found no significant relationship between the herbicide and lymphoma in dogs. (Kaneene, John B. and Miller, RoseAnn. “Re-analysis of 2,4-D use and the occurrence of canine malignant lymphoma.” Veterinary and Human Toxicology, 41, No. 3: 164–170. June 1999).
A subsequent 1994 study in the scientific journal Cancer, Epidemiology, Biomarkers and Prevention did, however, show that dogs living around residences recently treated with 2,4-D absorbed measurable amounts of the herbicide for several days after application. Dogs exposed to lawns within seven days of treatment were 50 times more likely to have high levels of the herbicide than those exposed to lawns treated more than seven days previously. The highest concentration of 2,4-D was found in dogs walking on lawns within two days after the lawn had been treated. Despite these higher herbicide concentrations, Dr. Antony Moore, veterinary oncologist at Veterinary Oncology Consultants in New South Wales, Australia, states, “This is not sufficient to say that use of the herbicides causes a high risk for cancer. However, if you’re an owner and have a dog, why use 2,4-D when there are other equally good herbicides available?”
The link between herbicides and cancer in the general dog population may be debatable, but when one considers a specific breed, the Scottish Terrier, the link is clear. Overall, Scottish Terriers are at 18 times increased risk of transitional cell carcinoma (bladder cancer) when compared to mixed-breed dogs. When exposed to lawns treated with phenoxyherbicides four or more times a year, Scottish Terriers had a four times higher risk of developing bladder cancer than those who were not exposed to herbicide. That risk increased to seven times if they were exposed to both herbicides and insecticides.
This suggests a question: If these chemicals are carcinogenic, why are they still around? Well, says Dr. Moore, “It’s very hard to show that a factor causes cancer. We can only show that it increases the risk of getting cancer. For example, a study out of Italy showed that dogs who lived with people who used paints and solvents were at 4.6 [percent] higher risk of developing lymphoma. This finding does not tell you that because you spill solvent on the ground and a dog walks through it one time, the individual dog’s risk is higher.”