During a time when the global coronavirus pandemic is still a threat, the thought of cancer being as contagious as COVID-19 is unbearable. While there are no reported transmissible cancers in humans, the animal kingdom hasn’t been so lucky.

The canine transmissible venereal tumor (CTVT), a disease that affects both sexes of dogs, wolves, foxes, and coyotes, is the most widely studied transmissible cancer.

CTVT is spread when tumor cells from one dog are rubbed onto the genitals of another. Experts are still uncertain exactly how CTVT cells are spread, but the disease seems to be confined to canid species and is successfully treated with chemotherapy.

On the other hand, Tasmanian devil facial tumor disease (DFTD), first reported in 1996, is almost always fatal and has decimated approximately 90 percent of the population in some parts of the island. DFTD is characterized by a transmissible tumor spread through biting during feeding or mating.

Sadly, the devils’ immune system often doesn’t recognize cancer as it grows big enough to kill the animal by asphyxiation.

Both CTVT and DFTD are considered parasitic cancer clones because the cells are directly transferred from one animal to another. While these are the only known transmissible tumors in mammals, experts have found similar transmissible tumors in laboratory hamsters and one mysterious contagious cancer in clams.

To date, these are the only four cases of documented transmissible tumors that are confined to animals. However, continued study is crucial for all of us who call Earth home.