Researchers have identified the first known transmissible cancer in fish, according to a paper published in the journal Nature. The cancer, a form of melanoma, affects brown bullhead catfish in Lake Memphremagog, which straddles northern Vermont and the Canadian province of Quebec.
The discovery began during annual fish censuses conducted by the Vermont Fish & Wildlife Department. In 2013, the team found catfish with dark streaks on their skin that Peter Emerson, a fish biologist with the department, described as looking like “black tar.” While the lesions were identified as melanomas, researchers found this unusual because catfish typically do not spend enough time at the surface to be exposed to the UV rays that usually cause such tumors.
Clonal Spread and Transmission
Genetic analysis revealed that the tumors’ genetics closely resembled one another rather than the host fish they came from. This suggests the cancers are clones that originated in one fish and have since spread from individual to individual. Currently, an average of 30% of catfish throughout the lake on both the U.S. and Canadian sides have the melanoma.
Researchers suspect the infection may occur during spawning. This theory is supported by the fact that the melanoma is not observed in young fish and that adult catfish aggregate in large clusters. One potential avenue for the cancer cells to invade a new host is through accidental scratching from the sharp spines located behind the fish’s pectoral fins.
Broader Implications
Transmissible cancer has previously been identified in only a few animals, including dogs, Tasmanian devils, and certain molluscs. The authors of the study suggest that these types of cancers may be more common than previously believed. They noted that the condition may have been documented as far back as when Henry David Thoreau observed a catfish in a Massachusetts river tributary with what he described as an inky-black kind of leprosy.
