New Virus Identified in Melanistic Skin Lesions on Smallmouth Bass from the Chesapeake Bay Watershed

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Smallmouth bass (Micropterus dolomieu) are important game fish and a top predator species in the Susquehanna and Potomac Rivers. Smallmouth bass disease and population declines in portions of the Susquehanna and Potomac River watersheds have raised concern among the fishing public and resource managers about the long-term sustainability of fish populations. In addition, visible skin lesions have been observed, including black areas of skin discoloration called melanistic hyperpigmentation or “blotchy bass syndrome.” Although the melanistic lesions have been recognized for many years, the causative factors are currently unknown.

Smallmouth bass

Smallmouth bass skin with melanistic lesions.

(Credit: Vicki Blazer, USGS National Fish Health Research Laboratory. Public domain.)

The U.S. Geological Survey (USGS) collaborated with the Pennsylvania Fish & Boat Commission, Pennsylvania Sea Grant College program, and State management agencies, to answer questions about the melanistic hyperpigmentation observed in smallmouth bass. Between 2012 and 2018, scientists used a combination of routine fish population surveys, fish health monitoring surveys, tissue collections, and genetic and microscopic analyses to better understand the prevalence, mechanisms of development, and potential risk factors associated with melanistic lesions in smallmouth bass.

The prevalence of melanistic lesions varied among sites from 3 to 13 percent of the fish collected depending on the year and site. As part of routine population surveys, the prevalence of melanistic lesions in the main stem of the Susquehanna River was greater than in a tributary, the Juniata River. Additional fish were collected as part of ongoing fish health monitoring surveys, which indicated the prevalence of melanistic lesions in smallmouth bass in the Susquehanna River and tributaries was greater than in the Potomac River and tributaries.

Sections of normal skin and melanistic lesions from smallmouth bass were preserved for microscopic and molecular analyses. USGS scientists examined the lesions microscopically and determined that the lesions were noncancerous and that the melanocytes normally present in the dermis had proliferated and (or) migrated to the epidermis.

Skin lesion on smallmouth bass

A hyperpigmented melanistic skin lesion in a Smallmouth bass from the Chesapeake Bay Watershed.

(Credit: Vicki Blazer, USGS National Fish Health Research Laboratory. Public domain.)

The Pennsylvania Sea Grant College Program funded the sequencing of 16 normal skin samples and 16 melanistic lesion samples to develop a partial transcriptome—the ribonucleic acid (RNA) instructions inside a cell. The abundance of 55 genes associated with contaminant exposure, melanogenesis, immune response, and a unique viral helicase-like gene were measured. Eleven of 55 genes analyzed were differentially expressed between melanistic and normal skin tissues. Three of those were unique gene sequences, two of which were unidentified and one was a viral gene. The viral gene was detected in 12 of the 16 melanistic areas and in no normal skin samples. The viral gene sequence indicated a similarity to a newly recognized viral family, Adomaviridae, that has been associated with disease in other fish species.

Further research is underway to fully characterize the virus and understand the mechanisms involved in the formation of melanistic lesions and to determine the role of environmental factors, such as temperature and chemical contaminants, in initiation and progression of the lesion.

This research was funded by the Pennsylvania Fish & Boat Commission, the Pennsylvania Sea Grant College Program, and the USGS Environmental Health Program (Contaminants Biology and Toxic Substances Hydrology) of the U.S. Geological Survey Ecosystems Mission Area, and the Environments and Fisheries Program.