Prevalence of Malignant Melanoma in Brown Bullhead from Lake Memphremagog Greater than Expected—Linkages to Contaminant Exposure and Implications for Fish Population Health are Currently Unknown

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Raised black lesions observed in 30 percent of the brown bullhead collected from two sites in Lake Memphremagog from 2014 through 2017 were identified microscopically as malignant melanoma. Malignant melanoma in freshwater fishes has been reported before, but this cancer occurrence cluster is raising questions about the cause of the tumors and the implications for the long-term health of fish populations. Previous findings and histopathology characteristics measured in this study suggest that tumor development is likely associated with multiple environmental and genetic factors. Study designs are being developed to test these hypotheses.

Raised black external tumors on an adult brown bullhead fish

Raised black external tumors on adult brown bullhead (Ameiurus nebulosus) collected from Lake Memphremagog (Vermont and Canadian Border) were identified as malignant melanoma in a 2014–17 study.

(Credit: Vicki S. Blazer, U.S. Geological Survey, Leetown Science Center. Public domain.)

In 2012, brown bullhead (Ameiurus nebulosus) with large, raised, black growths were first reported from multiple areas within the Vermont portion of Lake Memphremagog. The natural lake is 1,780 square kilometers and lies within the boundaries of Vermont, United States and Quebec, Canada. The lake is important for recreational activities and is a major drinking water supply for Canadian municipalities. Bullhead and other fish can be caught in the lake from spring through fall for consumption. Melanoma, a melanocyte-derived neoplasm, has been reported in a variety of wild fishes, including brown bullhead; however, the prevalence and pathology of the growths observed in the brown bullhead in Lake Memphremagog were unknown.

Therefore, U.S. Geological Survey (USGS) scientists, in collaboration with Vermont Fish and Wildlife Department, began a study to document the prevalence of the visible lesions in surveys conducted in 2014–17 and to describe the gross and microscopic pathology findings observed in affected bullhead.

Surveys conducted from 2014 to 2017 at two sites within the lake indicated an overall prevalence of 30 percent in adult brown bullhead with a total length of 200 millimeters and above. There were only slight differences in prevalence between the sites: Hospital Cove (361 total fish collected; 29 percent affected) and South Bay (231 total fish collected; 33 percent affected). These lesions ranged from slightly raised, smooth black areas to large, nodular areas on the body surface and fins and within the oral cavity.

To determine the pathology of the lesions, subsequent microscopic analyses were completed on 20 bullhead with black lesions and 20 normal-appearing bullhead collected from Lake Memphremagog and 10 normal-appearing bullhead from Ticklenaked Pond without any lesions.

Photomicrograph showing melanoma on a fin of a brown bullhead fish

Photomicrograph showing 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.

(Credit: Vicki S. Blazer, U.S. Geological Survey, Leetown Science Center. Public domain.)

The raised black lesions observed in brown bullhead collected from two sites in Lake Memphremagog were identified microscopically as malignant melanoma with invasion into surrounding hypodermis, skeletal muscle, and bone as well as metastases to gill, ovary, and intestine. Liver neoplasms, including cholangiocarcinoma and hepatic cell carcinoma, were observed in 8 percent of the Lake Memphremagog bullhead. Neither skin nor liver neoplasms were noted in Ticklenaked Pond, a site used for comparison. Melanoma prevalence in this study was also greater than observed for bullhead collected during 2011 to 2013 at sites in Lake Erie and Lake Ontario watersheds during a previous study where the prevalence of melanoma in bullhead was rare .

There have been reports of malignant melanoma epizootics in freshwater fishes, but they are rare and the cause of this tumor outbreak is currently unknown. Genetic factors, ultraviolet radiation, chemical contaminants, and viral pathogens alone or in combination may be linked to tumor incidence. Chemical analyses of skin, water, and sediment; measurement of ultraviolet radiation in brown bullhead habitat; and molecular analyses to identify viral sequences or mutations are logical next steps to identify the associated risk factors.

Scientists on the USGS Fishing and Hunting and Immunomodulation Science Teams are continuing to develop and test hypotheses to address key questions about the interactions between pathogen and toxicant exposures and tumor development in fishes. Identification of the actual risk factors is important to prioritize mitigation and minimize risk to fish, wildlife, and humans.

This study was supported by the Vermont Fish and Wildlife Department and the USGS’s Environmental Health Programs (Contaminant Biology and Toxic Substances Hydrology); the USGS Cooperative Fish and Wildlife Research Unit and Fisheries Programs.