A rare form of transmissible cancer has been identified in brown catfish living in a lake shared by the United States and Canada. Tumor cells appear to pass from one animal to another, an exceptional phenomenon in animals and never previously described in a freshwater ecosystem.
The alert dates back to 2012, when biologists observed numerous dark lesions on fish in Lake Memphremagog. This body of water stretches between Vermont and Quebec. The tumors resembled melanomas, cancers that develop from pigment-producing cells.
Catfish with a large black spot captured by scientists.
Photo: Vermont Fish and Wildlife Department
Initial hypotheses suggested pollution or infection. However, genetic analyses showed that tumors taken from several fish shared a very close identity. They also differed from the genetic makeup of the animals that carried them.
This result corresponds to a particular mechanism. Cancer cells from a first fish could survive in the environment, then become established in another individual. They would then behave as an independent cellular population, rather than as a spontaneously occurring tumor.
The researchers compared these cells to those of different fish. This analysis indicates that the studied melanomas likely descend from the same tumor lineage. However, the study does not yet describe the precise mode of transmission between animals.
Reproduction could promote close contact between fish. Water and sediments could also play a role, but this possibility remains to be demonstrated. The authors also observed an association with arsenic in certain areas, without establishing a causal link.
Transmissible cancers remain very rare in the animal world. Cases are known in Tasmanian devils, some dogs, and several mollusks. This new observation therefore broadens the field of study of these diseases and could help understand their evolution.