Earlier on this blog I talked about the first time I was in a hospital, and how I feared “catching” cancer from my grandpa. Of course this fear is ridiculous because cancer can’t be “caught” or transmitted like an infectious disease. But, what if it could? While at first glance infectious cancer sounds absolutely terrifying and absurd, there are unfortunately examples of this happening in the natural world. All of these cancers are examples of “allografts” or tissue that has been transplanted from one individual to another in the same species. In this post, I will discuss three stories of how cancer became infectious in dogs, Syrian hamsters, and Tasmanian devils.
Man’s best friend
Most people are familiar with venereal diseases, and how devastating and/or embarrassing they can be for the people who have them. Human venereal diseases are usually caused by bacteria or viruses, and many of them can actually be cured. But for dogs, unprotected sex (which is, presumably, the only type of sex they have) sometimes leads to a rare and terrifying form of transmissible cancer called Canine Transmissible Venereal Tumors (CTVT).
Can you imagine getting a tumor from a one night stand? CTVT is spread from dog to dog through sex, licking, biting, and sniffing the tumor-affected areas. Once a healthy dog’s skin comes into contact with the tumor of an infected dog, the tumor cells latch on and slowly grow into a bleeding, cauliflower-shaped mass. The tumor usually affects the penis and foreskin of male dogs, and vulva of female dogs. Rarely, it can affect the nose and mouth.
As most of us know, tumors typically start when normal cells grow uncontrollably. They are difficult to treat because our immune system simply ignores their presence. The tumor is genetically identical to us after all. CTVT, however, is not genetically related to the dogs they. Instead, they originated from Native American dogs (and possibly coyotes) that lived thousands of years ago. A part of these ancient dogs lives on in the tumors that afflict our modern-day best friends.
Luckily, chemotherapy is extremely effective in combating the tumors, and most dogs can be cured with prompt treatment. Surgery is often not used because of the infectious nature of these tumors- if any tumor cells come into contact with healthy tissue the tumor will simply grow back.
Syrian Hamsters
Contagious Reticulum Cell Sarcoma is a tumor that is spread in some hamster populations though mosquito bites, hamster-hamster biting, or, rarely, cannibalism. The cancer spontaneously arose in a colony of laboratory hamsters in the 1950’s or 1960’s at the National Institutes of Health, and much of what we know about the disease comes from research articles from this era. Thankfully, this cancer was only demonstrated experimentally, and does not exist outside of the lab (your hamsters are safe). However, hamsters do seem to spontaneously develop reticulum cell sarcomas in the wild, and research has shown that many of these spontaneous malignancies may become infectious.
Unfortunately, little information is publicly available about this unique disease, and even a search through my University database was largely unproductive. Sarcomas are cancers of the connective tissue (such as bones, cartilage, ligaments, and blood) and this disease seems to predominantly affect the blood (much like leukemia). Perhaps in the future more information will be publicly available about this disease.
Tasmanian Devils
Devil Facial Tumor Disease is an infectious tumor that arose in the 1990’s in the northeast of Tasmania. The first described case was in 1996, and it took ten years for scientists to discover that the tumor was actually a form of infectious tumor. Even though the disease arose fairly recently, it has devastated devil populations in Tasmania. In the last 20 years, there has been an overall species decline of 80% and only one wild devil colony has been found without the disease.
This tumor is spread through biting, scratching, and aggressive sexual activity. The effects of the disease are terrifying, causing incredibly large and grotesque facial tumors that destroy the underlying jaw bones. Most devils with this disease die within 6 months, usually of organ failure, secondary infection, or starvation. The disease has a 100% mortality rate. Even though the first case was described in 1996, it took ten years for scientists to realize that the cancer was infectious. Because scientists didn’t know the cause of the disease until ten years after it was first discovered, most early conservation efforts were not effective. Now, scientists fear Tasmanian Devils may become extinct.
There is, however, a glimmer of hope. Some devils are genetically immune to the tumors, and their genes seem to be increasing in Tasmanian Devil populations. Conservation efforts are also underway, with breeding programs ramping up at zoos, and healthy individuals being captured and isolated as a type of insurance policy. Some culling projects have been successful. Due to these efforts, there is some hope that Tasmanian Devils will not be lost to history forever.
Conclusion
Cancer is the second leading cause of death in the world, second only to cardiovascular disease. According to the BBC, about 16% of deaths in 2016 were caused by cancer. Luckily, humans do not have to worry about “catching” cancers or tumors of any kind. Other animals, however, are not as lucky. Dogs, Syrian hamsters, and Tasmanian Devils have to live with this reality, and they serve as a friendly reminder that the world could, in fact, be much more terrifying than it already is.
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References:
https://www.bbc.com/news/health-47371078
Kattner P, Zeiler K, Herbener VJ, et al. What Animal Cancers teach us about Human Biology. Theranostics. 2021;11(14):6682-6702. Published 2021 May 3. doi:10.7150/thno.56623
https://en.wikipedia.org/wiki/Clonally_transmissible_cancer#Contagious_reticulum_cell_sarcoma
https://en.wikipedia.org/wiki/Canine_transmissible_venereal_tumor
xenograft.blog 