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Causes of Cancer in Animals

Reviewed ByLaurie Hess, DVM, DABVP, The MSD Veterinary Manual
Reviewed/Revised Modified Jul 2025
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Like people, pets tend to develop cancer more often as they get older. Researchers are still trying to understand why age is linked to cancer. One idea is that aging weakens the immune system, making it easier for cells to become precancerous (more likely to become cancerous). In addition, the longer a pet lives, the more exposure it has to environmental factors that can lead to cancer.

Cancer is complex and can be caused by a mix of genetic, environmental, and nutritional factors.

Are Some Breeds More Likely to Develop Cancer?

Statistics show that some breeds of dogs are more likely to develop cancer than others. For example, Golden Retrievers, Boxers, Bernese Mountain Dogs, and Rottweilers tend to have higher cancer rates, suggesting that genetics play a role.

What Other Factors Might Cause Cancer in an Animal?

Some cancers have known causes. For example, prolonged exposure to sunlight and secondhand smoke are notable risk factors. Certain viruses, like the feline leukemia virus, can also lead to cancer in pets. Some cancers can even be sexually transmitted.

Vaccinations are crucial for pet health, but in rare cases, cancer can develop at the site of a vaccine later on. Because of this, vaccines are now administered at specific locations in the body (such as the lower leg), especially in cats, so that if cancer were to develop, it would be easier to remove and treat.

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