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.”
Other studies have shown that area of residence can increase the cancer risk. “The same study out of Italy showed that dogs that lived in the industrial areas had an 8.5 times higher risk of developing lymphoma than those living in urban areas,” says Moore. Similarly, a study published in 1971 in Archives of Environmental Health found that tonsillar squamous cell carcinoma was 10 times more common in animals living in cities than in those living in more rural environments. Both findings imply that urban living exposes animals to more environmental pollutants; however, they do not quantify an amount of urbanization or industrialization that significantly increases risk, or which anatomical characteristics put some individual dogs at greater risk than others. As Moore explains, other factors come into play, including long-nosed vs. short-nosed, or Scottish Terrier vs. other breeds, amount of exposure, and obesity.
The Biology of Cancer
This interplay of factors can in part be explained by the mechanism of cancer. Normally, cells have a set lifespan. They die and are replaced by new cells. During the cells’ lifespan, mutations in their DNA commonly occur, and though many are repaired, others are not, which often leads to early programmed cell death (apoptosis). Some mutations, however, result in prolonged cell survival, because mutations in the cell repair mechanisms or in the mechanisms that normally lead to programmed cell death have already taken place. In other words, these cells avoid apoptosis and divide in an uncontrolled manner. As they do so, they form an enlarging mass of cells that can invade local tissues as well as spread to other locations. Thus, cancerous cells are the result of multiple mutations occurring in the right combinations.
Because of these interactions, it can be difficult to appreciate some cancer risks. “Everyone knows about a 90-year-old guy who smoked all his life,” Moore says. “While smoking increases the risk of cancer, it may be that this person just doesn’t have the changes in his cells that can combine with the mutations caused by smoking to result in cancer.” More information is necessary. “It’s possible that the genome project will help identify genes related to getting cancer or being more at risk for getting cancer, but the risk may not present until the animal is exposed to the right environmental factors, such as herbicides, or becomes obese.”
Genes Play a Role
Clearly there are genetic factors. Says Glickman, “In the average breed, 20 to 30 percent of dogs die of cancer, but in other breeds, such as Golden Retrievers, it is probably 60 percent.” In some animals, the very traits that make them desirable as pets or especially skilled at certain tasks make them prone to certain cancers. For instance, Irish Wolfhounds and other large sighthounds bred for exceptional speed and ability to capture agile prey such as deer or rabbits are also prone to developing osteosarcoma (bone cancer). In fact, in general, dogs with legs longer than 50 cm (or a little less than 20 inches) are at risk for this cancer of the leg bones. Similarly, as noted earlier, the same trait that protects long-nosed dogs against lung cancer predisposes them to cigarette-related nasal cancers.