This question actually comes up a lot in my practice.
When a family is first coming to terms with a cancer diagnosis, so many questions pass through their minds. Does it run in families? Do my other kids need to be checked? Is it contagious?
In humans, the answer is “No,” although now that we know cervical cancer is usually caused by Human Papilloma Virus (HPV; a sexually transmitted infection) this answer is a bit fuzzy. Although viruses like HPV that can cause cancer are contagious, cancer itself is not.
But is that true for all animals? Apparently not. Recently I came across this fascinating article about one of my favorite animals from childhood: The Tasmanian Devil.
When not chasing Bugs Bunny, Tasmanian Devils live in, well… Tasmania. They are marsupials, carrying their young in pouches like a kangaroo or opossum. They are the largest carnivorous marsupial to escape extinction.
Sadly, though, over the last decade, the population has crashed. In some areas by as much as 90%. The cause? Cancer. A cancer that is contagious!
How does that happen? The cancer, known as Devil Facial Tumor Disease, causes a tumor on the face of the Tasmanian Devil, and when an animal with such a tumor bites another Devil (which isn’t a rare event, as you might imagine), the cancer cells are transmitted to the victim and grow into a tumor. The tumor makes it hard for the animal to feed, so it starves.
Why doesn’t the animal’s immune system protect it against the cancer? In most other species, if you inject cells from one animal into another, the recipient’s immune system destroys them. That’s why organ transplants don’t work without strong immune suppressive medications. This immune defense is based on differences in a set of genes called MHC genes that are so variable that (for the most part) only identical twins share the exact same gene sequences. This holds for humans, dogs, cats, mice, monkeys, kangaroos… almost every animal.
Except, apparently, the Tasmanian Devil. Tasmanian Devil MHC genes are not very diverse, and this allows the cancer cells to evade the immune system and grow.
But the mystery does not end there. There are other animals with very little MHC diversity, like cheetahs and beavers, but they don’t have contagious cancer. Also, Devil Facial Tumor Disease is new, first spotted in 1996. This suggests that the situation is more complex than it would seem on the surface, and raises the possibility that the cancer cells have evolved in ways that make them more transmissible.
How? No one knows. It’s just one more of the many unsolved mysteries surrounding cancer and the immune system.