Transmissible cancer: A strange biological phenomenon

Everyone knows how common viruses work. A drink is shared, a doorknob is touched, a sneeze is spread. Come flu season, friends and families routinely begin washing their hands and keeping their distance to prevent viral genomes from entering their cells, injecting their genetic material, and hijacking their bodies. Now imagine a similar phenomenon, but with cancer. 

In rare cases, some tumors have developed the ability to spread from one organism to another, comparable to an infectious disease or virus. The most well-known example of a contagious tumor has been observed in Tasmanian devils as facial tumors, with 80% of the population succumbing after simply mating or biting. 

Upon learning about these tumors, it’s natural to wonder: why can a Tasmanian devil catch cancer like a common cold, but humans can not?

To answer this question, it’s useful to look toward the “perfect storm” theory crafted by Australian integrative ecologist, Beata Ujvari. This theory breaks down the criteria for cancer transmission into three essential parts: a cancer cell’s ability to survive in transit, a tumor’s degree of cell shedding, and the adaptability to a foreign host. Essentially, in order for cancer to become “contagious,” a tumor must shed a large number of cells, and then these cells must be able to survive outside of its host. Finally, if these free-roaming cancer cells are able to survive, they must also evade the immune defenses of the other organisms in order to become transmissible. For this reason, it is extremely rare to observe this process in nature. 

The Tasmanian devil has become susceptible to transmissible cancer due to its low genetic diversity and inbreeding in combination with environmental stressors. In a recent 2024 study led by the Deakin University Centre for Integrative Ecology, researchers observed that season and geographic location influenced the expression of immune genes responsible for the infection of Tasmanian devil facial cancer. While more research still needs to be conducted, this study helps to point out how vital transmissible cancer research can be for the community’s general understanding of cancer. Knowing that environmental stressors such as season and geography play an interesting role in the fatal cancer of Tasmanian devils, could researchers continue to explore the connection between the environment and immune gene expression in humans? 

“The Tasmanian devil has become susceptible to transmissible cancer due to its low genetic diversity and inbreeding in combination with environmental stressors.”

In addition to environmental stressors, researchers are using the occurrence of transmissible cancers for different avenues of research. For example, French researcher Sophie Tissot has used transmissible cancers in hydras (small fresh-water organisms) to understand the progression of tumors. Tissot’s team utilized a specific strain of hydras that have developed spontaneous development of transmissible tumors. In this study, the team discovered that hydras that contracted the cancer had much lower rates of reproduction in response, increasing their survival fitness. It was also noted that these hydras were more susceptible to being taken out by predators and that lower food intake reduced the transmissibility of tumors. Tissot commented that the key factor influencing the rarity of transmissible cancers is ecological conditions rather than a cancer’s ability to transmit to another organism. 

Overall, while the concept of transmissible cancer is fascinating and may be a great avenue for further research, it should be noted again that cancer in humans is not contagious. Cancer is essentially due to a genetic mistake in our cells that causes uncontrollable growth and division. Unlike viruses, cancer cells in humans have not evolved to be able to replicate in other healthy organisms. This idea is extremely vital to get across because cancer patients need the love and support of their community without a false sense of fear that others could be contracting their potentially fatal illnesses. A cancer diagnosis is a burden enough without the added stigma.