In Lake Memphremagog, a body of water wedged between Vermont and Quebec, scientists are uncovering a first in fish biology: a cancer that has spread to over 30 per cent of the lake’s brown bullhead catfish population is likely contagious, according to a new study.
While transmissible cancers among wildlife aren’t unheard of, they are rare. Until now, the only documented instances have occurred in dogs, Tasmanian devils, and bivalves. In this case, the cancer infecting brown bullheads are malignant melanomas that cause raised black lesions to form across the fishes’ entire body.
It’s not clear yet if the cancer is killing the fish, says co-author of the study Julie Dragon, but researchers are collecting age and weight data to determine if there’s been a population drop in fish over a certain age, which would indicate that the cancer is fatal. “It seems like it’s not heavily impacting their ability to move or eat,” she says. “But we have seen some fish with really big growths in their mouths and in their gills, and I can’t imagine that that doesn’t eventually kill them.”
Deadly whirling disease has been detected in Lake Louise
The melanomas were first spotted by anglers in 2012, who then reported the growing number of lesioned catfish to the Vermont Fish & Wildlife Department. The department began tracking the number of infected individuals, finding 23 to 37 per cent of the brown bullhead population displayed lesions between 2014 and 2017.
According to the study, melanomas among brown bullheads aren’t uncommon, but the rate at which the cancer was documented in Lake Memphremagog was considerably higher than other lakes across eastern North America. Locals, many of whom depend on the lake for drinking water, feared the worst. “We had a very large flooding event in 2011 during Hurricane Irene,” says Dragon. “Folks were concerned that contaminants had gotten in the water.”
But Dragon says that a study analysing levels of different harmful chemicals in the fish failed to provide answers. “They did not find enrichment of any of these chemicals in the diseased fish or their tumors,” she says. Instead, the researchers suspected a virus or microbes. This theory was also disproved after testing.
Then, a genomic analysis study was undertaken; it showed the tumor genes were surprisingly different from genes in healthy tissues. In conventional cancers, tumor genes are largely similar to healthy host genes, but the bullhead’s tumors were more closely related to each other than to the host fish’s genes. “This is not what you would expect in most tumors that arise in any organism,” says Dragon. “And so we determined that the only explanation is that it’s a contagious cancer. It passes from fish to fish on its own.”
Cottage Q&A: Does this bass have two mouths?
Dragon says it’s still unclear what triggered the cancer, but that it is becoming more widespread. “We have found the same cancer in New Hampshire and Maine, we have collected fish from New Brunswick, and we are now getting reports from lots of people who have said they’ve seen this in Nova Scotia as well,” she says. It should be noted that it is yet to be confirmed if the specimens and sightings from New Brunswick and Nova Scotia have the same type of cancer, but Dragon says photos indicate it’s likely a match.
This also may not be the first time brown bullheads have been subject to the contagious cancer, adds Dragon. Similar lesions were documented in a 1918 paper by a researcher in Massachusetts who was then able to transmit the cancer from infected to non-infected fish through contact with tumour cells. “We can’t confirm his work, but it’s really good evidence that this [contagious cancer] has actually been around for about 100 years,” she says.
So why are we only investigating this rare biological phenomenon now? Dragon says it might be due to the unpopularity of the fish. “I’ve heard people call them trash fish,” she says. “People basically aren’t interested in these catfish, so sometimes something can be right under your nose, and if you’re not looking, you just don’t see it.” Vermont is home to some of the only people who eat brown bullheads, which is likely why it was brought to researchers’ attention in 2012, adds Dragon.
Now, Vermonters and anyone else who catch these catfish have been instructed not to eat them (or any fish that has discolorations or lesions). Dragon says there is no evidence that the cancer can be passed to other fish species or humans, but out of an abundance of caution, they’re off the menu.
Anglers who catch an infected catfish are encouraged to take a photo of it, and to put the fish in their freezer. They can send the photo to Dragon or another biologist from the study, and they will send a shipping label so the fish can be transported to their lab for study.
Related Story Is Canada the wildest place on earth?
Related Story 4 new invasive species to look out for this summer
Related Story Ew! This is why some plants stink when they bloom