In biomedicine, the word cancer is multifaceted. A single term is used to describe a wide range of diseases that share many characteristics but can be very different in ethology or origin. The answer to the question «Can cancer be contagious?» is neither yes nor no, because while many cancers cannot be transmitted between individuals, others are associated with infectious pathogens. For example, some viruses can cause cancer in humans, such as the papilloma virus, which causes skin or uterine cancer, or some retroviruses, which cause leukaemia.
On the other hand, infectious tumours have been described in some animals and they are transmitted by direct contact without the intervention of pathogenic microorganisms. These transmissible cancers are clonal lines of infectious somatic cells that spread by physical transfer of live cancer cells and infect individuals of their «kind». Few such cancers are known to occur in nature, but they have been detected in Tasmanian devils, marsupial carnivores endemic to the Australian island of the same name. One has also been found in dogs, and others in natural populations of a type of hamster and in about five species of bivalve molluscs.
In the case of Tasmanian devils, two different types of transmissible cancer have been described with respect to the tissue characteristics of the respective tumours. Their respective karyotypes also show highly characteristic aberrant chromosomal rearrangements.
These two cancers are devil facial tumour 1 (DFT1) and devil facial tumour 2 (DFT2). Uncontrolled cell growth causes malignant facial and oral tumours that spread between individuals through small bites. Transmission by contact between female devils and their pups has also been described. The spread of the facial tumour eventually affects the animal’s ability to feed and leads to premature death. No clinical trials of chemotherapy have been successful in treating the disease. The tumour cells grow undetected by the marsupial’s immune system.
DFT1 cells originate from a female specimen – they have two XX sex chromosomes – but can be found in infected individuals of both sexes. In addition, they have lost one of the large chromosomes and have undergone several rearrangements resulting in four «new» chromosomes. DFT1 tumour cells have continued to evolve in this pattern and differentiated lineages are now recognised. DFT2 cells, on the other hand, originate from a male specimen with XY chromosomes and have been found almost exclusively infecting male devils or with loss of the Y chromosome in infected females.
The spread of DFT1 tumours was first observed in north-eastern Tasmania in 1996 and has spread widely across the island. In contrast, DFT2 tumours were discovered on a peninsula in south-eastern Tasmania around 2014 and appear to be confined to this area for the time being. The rapid decline of the Tasmanian devil population associated with DFT1 transmissible tumours, down 80% since 1996, is pushing the species towards extinction. A DFT1 vaccine based on the technology used in one of the covid-19 vaccines is currently being tested.
Remember the Tasmanian Devil from the Warner cartoons? It was created in 1954 and I would say it bears little resemblance to the real marsupial. Shortly after it started appearing in cartoons, a Warners executive ordered it to disappear because he found the character distasteful. However, Taz was eventually «revived» due to popular support. Will the vaccine save the species?