Canadian and US Fish Found With Rare Contagious Skin Cancer

Scientists have discovered a rare form of skin cancer in brown bullhead catfish that can be passed from one fish to another in a lake that spans the northeastern United States and Canada, according to a study published July 22 in the journal Nature.Biologists headed by a team at the University of Vermont found catfish covered in mysterious black skin lesions in Lake Memphremagog had passed the tumors to other fish in the freshwater lake.“This makes it to our knowledge the first documented transmissible cancer in fish or in a freshwater ecosystem,” the biologists wrote.The lesions were found to be malignant melanomas and were observed in an unusually high population of trout at the lake by anglers and biologists starting in 2012.Testing showed the cancer cells behaved more like parasites than conventional tumors, moving from fish to fish, according to the study.“At first, we thought this disease might be a virus, but that wasn’t panning out,” said Julie Dragon, a scientist at the University of Vermont and co-leader of the study.The researchers suspected a contaminant or a pathogen linked to pollution. Instead, they discovered it to be melanoma, a skin cancer.“This was surprising,” Dragon said. “And we wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight.”The lake provides drinking water for more than 175,000 people, but the cancer poses no known risk to humans, the researchers said.The researchers took a “deep dive” into the genetics of the cancer cells and discovered the cells were much more closely related to each other than to their host fish, Dragon said.The research team is still working to try to understand how the cancer cells spread between the animals.“It seems to only happen in larger fish that are of spawning age,” said Mark Henderson, a fish biologist and study co-leader. “Maybe some part of spawning behavior leads to the spreading of the cancer between animals.”The scientists theorize the cancer could spread when the bullhead cluster tightly in shallow water, allowing the physical contact to transmit tumor cells. Naturally occurring arsenic may also play a role, according to the study.Photomicrograph showing a melanoma on a fin of a brown bullhead fish. In the upper right-hand part is normal skin (epidermis). The epidermis has large clear cells and a layer of normal melanocytes along the basement membrane of the epidermis which is where they normally are located. Most of the rest of the tissue is composed of neoplastic melanocytes that invade up into the epidermis as well as into the dermal and subdermal areas. U.S. Geological SurveyThe discovery of contagious cancers could help scientists understand how cancer evolves and how it may be passed on to others.Transmissible cancers are a biological phenomenon that aren’t supposed to exist, but have now been confirmed in more than a dozen species, according to the study.This is the fourth type of spreadable cancer identified in animals and the first in any fish or freshwater species.In the past, researchers have discovered Tasmanian devils could transmit facial tumor disease, dogs could catch venereal tumors from other dogs, and shellfish could pass along leukemia-like diseases.“As we continue to identify example of transmissible cancers, it becomes increasingly clear that they may be more common and ecologically important than previously thought.”Other fish could also be found to have the ability to transmit cancer, the authors said.“This finding that transmissible cancers, previously detected only in dogs, devils and bivalve species, also exist in brown bullheads raises the question of whether other fish species may have transmissible cancers as well,” the biologists said.