Brown bullhead catfish melanoma represents a novel transmissible cancer

Brown bullhead catfish in Lake Memphremagog, straddling the border between Vermont, USA, and Quebec, Canada, have been found to suffer from a rare form of melanoma that appears to be transmissible. Researchers identified the condition after noticing a high prevalence of raised black skin lesions on the fish, which were later confirmed as malignant melanomas through histopathological analysis. Between 2014 and 2017, 23 to 37 percent of sampled brown bullhead exhibited such lesions, significantly higher than rates observed elsewhere. The phenomenon was first noted in 2012 by both anglers and biologists who observed a growing number of affected fish in the lake. Scientists have since investigated possible causes, including environmental factors linked to the 2011 flooding caused by Tropical Storm Irene, which may have introduced contaminants into the lake's ecosystem. Given that over 175,000 people rely on Lake Memphremagog for their drinking water, concerns about potential contamination are heightened. Genomic studies conducted on the tumorous tissues revealed distinct genetic profiles compared to the surrounding healthy tissues of the same fish. These findings suggest that the cancer might not originate solely within each individual fish but instead could be spreading among them. This hypothesis aligns with characteristics of transmissible cancers, which are rare in nature but have been documented in certain animal species. Transmissible cancers differ from typical cancers in that they can move from one organism to another. They begin as conventional cancers in a single individual before the cancerous cells migrate to infect others. Such cancers have been observed in dogs, Tasmanian devils, and some bivalve species. In dogs, the transmissible venereal tumor is relatively common and often survivable, whereas in Tasmanian devils, the facial tumors are typically fatal and have emerged independently on two occasions. Researchers examining the brown bullhead melanomas found similarities with these known transmissible cancers. The genetic makeup of the tumors suggested that they might be derived from a single ancestral cell line that has since spread throughout the fish population. If confirmed, this would represent the first instance of a transmissible cancer in fish, marking a significant addition to the list of known transmissible diseases in wildlife. The discovery raises questions about how the cancer spreads. Unlike the well-documented cases in mammals and mollusks, the mechanism by which the melanoma transfers from one fish to another remains unclear. Possible routes include direct contact during mating or feeding behaviors, or through intermediate vectors such as parasites or waterborne particles. Further research will be needed to determine the exact mode of transmission. Biologists are monitoring the situation closely, as understanding the dynamics of this transmissible cancer could provide insights into cancer biology and potentially inform strategies for managing similar conditions in other species. The brown bullhead serves as an important indicator of water quality, making the health of this population particularly relevant to human communities dependent on the lake's resources. Scientists plan to conduct further genomic analyses and track the spread of the melanoma across different regions of the lake. Comparative studies with unaffected populations in nearby lakes may help establish whether the cancer is confined to Lake Memphremagog or if it has begun to spread beyond its current range. The ongoing investigation aims to uncover the origins of the cancer, its transmission pathways, and its implications for both aquatic ecosystems and human health.