Cancer is a complex biological process through which a normal cell acquires capabilities that cause its transformation into a tumorigenic and eventually malignant cell. If the cell is not recognized and/or eliminated by the immune system, a tumor will form.
Cancer in pets is naturally occurring and is as common in dogs and cats as it is in humans. It is a leading cause of death in dogs and cats. The similarities between pets and humans—with respect to anatomy, physiology, and tumor onset and progression—make canine tumor models a valuable tool for identifying new cancer-associated genes and for enhancing understanding of tumor molecular biology in humans.1 Because dogs and cats often share the same environments as humans, one might wonder whether there is risk for tumor development caused by pets, especially in immunocompromised humans. Although no human cancer is known to spread naturally from human to human (or animal), the existence of such diseases in dogs and Tasmanian devils raises the question of whether humans could be at risk.
Few tumors can pass from one living host to another. These tumors, called naturally transmissible tumors, emerge when a tumor cell gains the ability to pass infectious material between individuals. There are 2 known naturally occurring contagious cancers:
- Canine transmissible venereal tumors are nonfatal, affect the orogenital area of dogs, and are transmitted through direct contact between individual dogs via sexual contact or bites/licks.
- Devil facial tumor disease spreads among Tasmanian devils through fighting and biting and has close to 100% mortality.
Neither disease is infectious to humans,2 and there is no scientific evidence that humans, even if immunocompromised, are susceptible to these or any other cancers by direct transmission.
In addition, no virus transmission—including retroviruses such as human papillomavirus, which can be spread between humans—between pets and humans has been shown to cause cancer in humans.
In fact, reports have shown that pet ownership may actually decrease the incidence of cancer in humans. A population-based case-control study in the San Francisco Bay area showed that pet owners had a reduced risk for nonHodgkin’s lymphoma as compared with those who never owned a pet.3 However, this research remains to be confirmed.