New Rare Contagious Cancer Discovered in Bullhead Catfish

In Lake Memphremagog, a glacial body of water spanning Vermont to Quebec, concerning numbers of brown bullhead catfish have been found with raised black lesions over the past 13 years. Researchers investigated, and discovered that these lesions were actually skin tumors. Around a third of all brown bullhead catfish tested between 2014 to 2017 were found to have skin cancer.

While skin cancer does normally occur in brown bullhead catfish, the incidence rates seen in other water bodies are usually not this high. Further genomic investigation found that it was likely that a single variant of the cancer has been spreading from one fish to another, rather than developing independently in each fish. Currently, this is the first transmissible cancer discovered in fish.

Lake Memphremagog is the source of drinking water for over 175,000 people in Canada and the US. Brown bullhead catfish are also used in ecological studies to monitor water quality. The results of the studies suggest that a cancer-causing contaminant or contagion may be present in the water.

One possible theory for the higher cancer incidence rates observed in brown bullhead catfish is that flooding from 2011 tropical storm Irene has since polluted the lake, reducing water quality and leading to health issues in the fish.

Cancer cases in brown bullhead catfish are much more frequent after sexual maturity, suggesting that it’s possible that the cancer spreads during the mating period. The fish is also known to be a solitary species, and mating is the only time that they gather in close clusters, potentially providing regular opportunities for cancer spread.

Chemical pollutants or hormonal imbalances may weaken the immune systems of bullhead catfish, increasing the risk of cancer transmission. One study has found that affected fish had higher levels of arsenic, a Class 1 carcinogen, on their skin. Residents near the lake have also raised concerns over a landfill near the south of the lake, although investigations found that cancer incidence rates were not any higher near the dump, than away from it.

Currently, not all researchers agree that the cancer is transmissible. The next step for research will be to confirm in laboratory tanks if the cancer really is transmissible from an affected fish to a healthy fish. Researchers are also keen to investigate the origins of the cancer, and to confirm the causes of the cancer moving forward.

It will also be important to observe how this cancer will affect brown bullhead catfish populations. For example, in Tasmanian devils, a rare but aggressive cancer has spread, and led to a 90% population decline and threatens extinction for the species. Luckily, the brown bullhead population in Lake Memphremagog has remained stable so far.