Contagious Cancer - a Devil of a Problem

Are the rare cases of cancer being spread between animals cause to for us to worry?

A cancer diagnosis is one of the most devastating diseases that affects an estimated 18.1 million people globally every year. Despite the high incidence and significant scientific progress, the complexity of the disease means that important questions remain a puzzle. While we haven’t identified the agent that triggers the disease mechanism, we do know that cancer develops when cell division machinery goes haywire and cell division grows out of control. One of the few blessings is that it is not a contagious disease for humans, but the rare cases of transmissible world in the animal world challenge this idea and have brought additional stress to the cancer problem. So cancer can jump from one host to another - myth or fact?

There are three known transmissible cancers in mammals, mystifying the scientific world. In particular, the Tasmanian devil (Sarcophilus harrisii) has garnered attention for its susceptibility to not one, but two, fatal and highly-contagious cancers. Endemic to Tasmania, Australia, these marsupial carnivores are under the threat of extinction due to the ominously named Devil facial tumour disease (DFTD). Infected devils are spotted with large and soft “lumps” of tumour tissue. As the name suggests, these primarily form around their mouths, but can invade other body parts and spread to lymph nodes, lungs and the heart. On average, affected animals die from organ failure within six months. Since first being discovered in 1996 the deadly disease has wiped out populations by an estimated 80%, with fears mounting that the whole species would be driven to extinction in a matter of decades. So while cancers that can pass from one host to another are extremely rare in nature, they can have devastating effects.

These devils have proven to be a scientific gem in more ways than one. The DFTD cancer is unique in that it originated in a single devil over 20 years ago and has developed the ability to jump between individuals. It genetically resembles the founder devil while remaining distinct from the host cells. It was thought that the cancerous cells infected and colonized open wounds in the host during fights, mating or when fighting over food - but additional findings challenged the idea. After samples collected from devils puzzled scientists, they discovered two distinct transmissible cancers that evolved independently – DFT1 and DFT2. This discovery challenged everything about the paradigm of transmissible cancer at the time and suggested that naturally occurring transmissible cancer are more widespread than previously thought. Australian state officials attempted culling as a controlling strategy up until 2010. With little success, their focus was turned to captivity breeding-and-releasing devils into the wild. Although there is no cure for the cancer, trials are being run to develop a suitable vaccine. Challenges aside, recent reports show there is light at the end of the tunnel.

According to experts in vertebrate ecology, devils are undergoing evolution at lightning speed, and building resistance to the disease. Comprehensive genetic studies of Tasmanian devils have uncovered hundreds of genetic changes in response to the disease. Biologists cautiously celebrate this as an indication of some population recovery, as well as insight into how the disease progresses over time. Over time, the cancer itself is hoped to be eliminated totally or become endemic, i.e. consistently present at low levels but limited to the island. As a consequence, reintroducing the captive devils back into wildlife could backfire and cause harm to the natives. In theory, it could supplement the number of lost animals due to the epidemic. In reality, we run the risk of diluting the wild gene pool so much we inadvertently suppress the emerging disease resistance and spark a new epidemic of DFTD. The road to recovery is long, and sometimes doing nothing is the best way to fight the devil you know.

Many questions still puzzle scientists, like how these cancers emerged and what populations are most susceptible, but this has only intensified scientific efforts to understand them. On a more promising note, this may prove to be a generous scientific resource for cancer research, directly linking genes to physiology. An opportunity to elegantly combine genetics with functionality could aid research in developing potential therapeutics. But devils aren’t the only species that have been through hell and back.

Contagious cancers are exceedingly rare, but the oldest cancer known in nature actually originated over 6,000 years ago in man’s best friend - dogs. The sexually transmitted oncogenic cell line, named Sticker’s sarcoma, is a common disease that is mostly harmless to the animals. The cancer is transferred usually during mating, when living cancer cells jump between the dogs. Thankfully, most of the canine patients respond well to chemotherapy and make a full recovery. Another potential ecological disaster is affecting clams, specifically the warty venus clam. A 2022 study found that a contagious blood cancer jumped from one clam species to another in the Atlantic Ocean and Mediterranean Sea. A technique called whole genome sequencing revealed that the leukaemia-like cancer originated in a single clam before infecting other clams in the same species, similar to the devils and dogs. But the genetic analysis found sequences from a mystery clam species, later identified as the striped venus clam. Worryingly, this suggests that the cancer is not species-specific and had actually jumped from one species to the other. Unfortunately, the genetic similarity of the cancer cells found in the warty clams in two separate location suggest that human activity is responsible for the spread from one region to another as the clams hitch a ride on passing ships. The implications for marine ecology is not fully understood but monitoring these cancers could help protect the species in the future.

The mechanism by which these cancers evade animals’ immune system is still not fully understood. What is likely to be the case is that a set of genes that encode proteins responsible for the immune system’s ability to recognize foreign molecules through our cells’ surface, known as the major histocompatibility complex, is suppressed. A suppressed environment means the cancer cells can thrive without being detected.

The looming question is whether these isolated cases of transmissible cancer are a cause for worry in the human species. Although very rare, there are some documented cases of cancer making the leap between humans. Over the last few decades, there have been two dozen instances of cancer cells being transferred from mother to child during pregnancy. However, the most well-documented instance of cancer transmission happened in 1996, when a surgeon reportedly developed a tumour-like injury after pricking his hand during an operation on a cancer patient. Other cases include four patients who developed breast cancer after receiving transplanted organs carrying the cancer. The cancer cells matched the donor, who died of a stroke and was not aware of their illness at the time. While organ transplant recipients can be vulnerable to contracting cancer from donor organs, this is rare and doesn’t warrant excessive worry. Another highly unusual case describes a patient with HIV reportedly dying from a cancer contracted from a tapeworm in his gut. What all these scientific wonders have in common is immunocompromised patients. Experts agree that the chance of a contagious cancer arising in humans is little-to-none, unless the immune system is already abnormally vulnerable. Fortunately, there is no scientific evidence of cancer being contagious in humans. As of now, one can inherit genes that predispose for developing a specific type of cancer, but not the cancer itself, or develop cancer as a result of a viral infection.

Although there is no hard evidence of cancer jumping between humans akin to the devils’, canine or marine cancer, it just goes to show that our knowledge of infectious agents and cancer is just surface-level.