Figure 1: A carcinogenic cloud over city.
What are the known etiologies of cancer in animals? Unfortunately, veterinary research has not come as far as research in humans in this area. Certainly, most dogs and cats with cancer are middle-aged to older animals, but the exact effect of an animal's age on the incidence of cancer is not well understood. In general, cancer is thought of as a disease of advancing age. One must hypothesize that, as cells continually divide through the progression of life, there is an increased chance of genetic mutation due to cell division "accidents" and to the actions of carcinogens (cancer-causing agents) on DNA and RNA (fig.1).Also, depression of the normal immune response in the older animal may play a part in the increased incidence of cancer seen with advancing age.
The more we can learn about what causes cancer in both humans and animals, the more likely we will be to find ways to prevent it. Epidemiologists study patterns of disease in the population to look for factors that affect the risk of developing these diseases, including cancer.
Figure 2: Tobacco products are highly associated with the development of cancer in humans.
In the laboratory, researchers try to determine what actually happens when normal cells become cancerous, which should help to better define actual causes for the malignant transformation. Our current understanding of the causes of cancer is incomplete, but it is clear that cancer is not caused by an acute injury, such as a bump or bruise. It is becoming clear that cancer develops gradually as a result of a complex mix of factors related to environment, nutrition, and heredity. Scientists have identified many risk factors that increase the chance of getting cancer (fig.2) with.In man, about 80% of all cancers are related to the use of tobacco products, to what we eat and drink, or, to a lesser extent, to exposure to radiation or carcinogens in the environment. Exposure to substances such as metals, dust, chemicals, or pesticides may increase the risk of cancer.
Figure 3: UV radiation from sunlight can cause skin cancer in white animals.
Asbestos, nickel, cadmium, uranium, radon, vinyl chloride, benzidene, and benzene are well-known examples of carcinogens that may act alone or along with another carcinogen, such as cigarette smoke. It is clear that some people are more sensitive than others to factors that can cause cancer, and this is probably the case in animals as well.
Actual causes have been identified for only a few tumors of animals.The transmissible venereal tumor in the dog is spread by implantation of cells during breeding, and the canine oral papilloma ("warts") of young dogs is caused by a virus. In cats, cattle, mice, and poultry, lymphoma (cancer of the lymph nodes) has been proven to be caused by a virus. In man, several forms of lymphoma have been shown to be of viral origin as well, but no virus has as yet been recovered from lymphoma in the dog. Squamous cell carcinoma in white cats, white dogs, in Hereford cattle (which have white faces), and on the faces of horses with white facial markings, especially around their eyes and on their noses, has been shown to be caused by ultraviolet (UV) radiation from the sun's rays (fig.3).
Figure 4: Dalmatian with squamous cell carcinoma on the abdomen due to "sunbathing" on her back.
Dogs that "sunbathe" on their backs, exposing their relatively hairless groin region to sunlight for hours, are very prone to develop squamous cell carcinoma over the skin over their abdomen (fig.4-5). The sun's UV rays are strongest during the summer from about 11 a.m. to about 4 p.m.The risk from exposure to the sun is greatest at this time, too, when the sun is high overhead and shadows are short. As a rule, it is best to keep animals out of the sun during these hours if possible. Recently, certain vaccines have been found to cause sarcomas in a small number of cats at the site of vaccination; research is ongoing to determine which cats are most likely to be affected and to make vaccines that do not lead to malignant transformation of cells (fig.6).
Some other known risk factors can be avoided; for example, neutering female dogs before their first heat cycle nearly eliminates the risk of mammary cancer. Other risk factors, such as those that are inherited, are unavoidable.A small number of cancers (including malignant melanoma and cancers of the breast, ovary, and colon) tend to occur more often in some human families than in the rest of the population.
Figure 5: Dalmatian with squamous cell carcinoma.
As has been discussed already, certain breeds or bloodlines of dogs or cats may also have a much higher incidence of certain kinds of cancer than is normal for the species. It is not always clear whether a pattern of cancer in a human or animal family is due to heredity, factors in the environment, or just to chance. Still, if close relatives of your pet have been affected by cancer, or if your veterinarian informs you that the breed of your particular dog or cat is at high risk for developing some specific type of cancer, it is important to follow your veterinarian's advice to avoid any predisposing causes for that cancer type, if they are known.
Figure 6: A feline vaccine-induced sarcoma.
Have regular checkups performed by your veterinarian so that, if the cancer develops, it is likely to be found early. Undoubtedly, many types of tumors have an etiological cause or causes that are as yet to be discovered. This is an area in which research in both human and animal cancer continues to be needed. Until we have better elucidated the various causes of cancers, our attempts at "shotgun" treatment will be symptomatic at best and will seldom lead to a cure.
