There is a deadly contagion decimating the population of an island off the coast of Australia. Upon infection, it multiplies rapidly, producing gruesome lesions that cause certain death within months. Like a zombie virus, the contagion spreads through brutal bite wounds.
But this contagion is no virus. It’s a tumor.
Cancerous tumors are not traditionally contagious diseases. Normally, cancers result from the uncontrolled growth of our own cells. Conventional cancers begin and end in one body and can’t spread from person to person. Even cancers caused by viral infections, like cervical cancer caused by HPV, involve the uncontrolled growth of our own cells. Rare contagious cancers, like those spreading across Tasmania, are unique because the cancer cells act like parasites. These cancer cells infect other bodies directly.
This deadly outbreak among Tasmanian devils is one of only eight known contagious cancers, all of them in animals and none in humans. In addition to Tasmanian devils, dogs are susceptible to canine transmissible venereal tumors, which spread through sexual contact. The handful of other known contagious cancers spread through mussel, clam, and cockle species in the Pacific Northwest.
Devil facial tumor disease (DFTD) has killed at least 80% of the Tasmanian devil population since the first case was discovered in 1996. Since then, devils have been added to the endangered species list and conservationists have been struggling to save the remaining uninfected animals.
As scientists document these rare and bizarre cases in the animal kingdom, they ask the question; can cancer become contagious in humans? The short answer is probably not. However, cancerous tumors have transmitted from one person to another in a handful of fluke accidents.
In one case, a surgeon injured his hand while removing a cancerous tumor from a patient and found cancer cells growing at the injury site months later. In another case, a young researcher pricked her finger while working with colon cancer cells and, later, found the same type of cells growing in her finger. Fortunately, in all of the documented human cases (which are rare) the tumors were not contagious. This means the cells did not come back after they were removed and the cancer did not spread to additional people.
The infected surgeon and researcher can thank their immune systems and humanity’s genetic diversity for preventing the infection from spreading. One part of the complicated human immune system is called the Major Histocompatibility Complex (MHC). It determines what is part of our body (self) and what does not belong (non-self). Pathogens like viruses and bacteria can be recognized as non-self by the MHC and destroyed by our white blood cells. Even conventional cancer cells that start in our own bodies are often recognized as non-self before they can proliferate.
However, when the immune system is suppressed, a person can become more susceptible to infections, including (although extremely rare), transmitted cancers. In a particularly bizarre case of interspecies cancer transmission, one man’s immune system was compromised due to an HIV infection and he became infected with tapeworms. Later, one of the tapeworms inside the man got cancer – and those worm cancer cells invaded the man’s body! This case was an absolute anomaly, but it shows that a weak immune system can make a person more susceptible to all sorts of infectious agents.
Human genetic diversity also aids our immune systems in the fight against the potential for contagious cancers. Genetic diversity helps the immune system differentiate self verses non-self. Generally, another person’s cells in our body (cancerous or healthy) are recognized as non-self. This rejection of non-self cells is why people must take immune suppressants to undergo an organ transplant.
But does this mean cancer cells can transmit between people who are genetically similar, like parents and their children? In very rare cases, yes they can. There are at least 14 cases on record of cancer transmission from a mother to a fetus. In these extremely rare cases, the fetus’s MHC recognized the transmitted cancer cells as self because of high genetic similarity between mother and offspring.
These cases, though, are incredibly rare and improbable. Humans generally have high genetic diversity and strong immune defenses, making the chances of a pandemic like that plaguing Tasmanian devils highly unlikely.
DFTD is particularly contagious in Tasmanian devils for several reasons. First, the cancer cells actively combat the devil’s MHC, lowering the body’s immune defense. Second, the devil population has very low genetic diversity, making it harder for their immune systems to recognize the tumors as non-self. Conservationists have observed that a dwindling devil breading population and loss of habitat – due in part to human development and climate changes on the small island – has caused low genetic diversity in devils. Lastly, the devils’ uniquely aggressive temperament – i.e. their tendency to bite each other’s faces-- makes DFTD transmission particularly easy.
Recent evidence convinced scientists that devils are particularly susceptible to the growth and spread of DFTD. Since its discovery 20 years ago, scientists had always traced the DNA in the facial tumors back to a single female devil. But in 2016, scientists discovered a second lineage of DFTD that started in a separate, male devil.
Scientists were alarmed that the rise of the second contagious cancer lineage would threaten the remaining devil population. Luckily, in early 2017, researchers designed a potential vaccine to protect non-infected devils. They also found that immunotherapy may help to regress tumors in infected devils. Between these new prevention and treatment strategies, scientists hope to save the devil species from extinction.
Humans are unlikely to become endangered by transmissible cancers like the one threatening Tasmanian devils. If a contagious cancer like DFTD were to ever arise in humans, it would have to evade our strong immune system, adapt to our comparatively high genetic diversity, and find an easy mode of transmission from person to person. Bacteria, parasites, and viruses are already vastly better at these tasks than cancer cells. For as devastating a disease as cancer can be, we can rest assured that it won’t infect others. For now, contagious cancer in humans can be left to the imagination of science fiction novelists and the makers of zombie movies.