Abstract

Mycobacteriosis is a chronic infectious disease affecting many types of fishes worldwide. Causative agents of mycobacteriosis among aquatic animals are members of the genera Mycobacterium, such as M. marinum, M. fortuitum, M. chelonei, and others, including newly described species isolated from striped bass from the Chesapeake Bay watershed. Mycobacteriosis is an emerging epizootic disease of Chesapeake Bay striped bass. An important commercial and recreational resource for the region, these affected fish are often in poor body condition and/or disfigured as a result of associated external lesions that frequently include severe skin ulceration. During 1998-99, the skin ulcers attributed to mycobacterial infection were observed in up to 50 percent of striped bass from some Virginia tributaries. This condition has also been reported in striped bass from Maryland waters as well as the Delaware Bay. Preliminary data obtained in Virginia waters of the Chesapeake Bay during the summer of 2001 suggest that up to 70 percent of striped bass taken from some areas of the Bay may be infected with bacteria that cause this disease.

Many fundamental questions remain pertaining to the etiology and pathogenesis of mycobacteriosis among Chesapeake Bay striped bass. While many researchers from different organizations have been investigating various aspects of the problem over the past few years, differences in methodologies have largely precluded direct comparisons of results. In 2002, researchers from federal and state agencies as well as academic institutions (USGS, USFWS, Maryland DNR, Virginia Institute of Marine Science and the University of Maryland) began developing standardized methods for field and laboratory techniques to be utilized in a joint study. Specifically, the multi-year study initiated in fall 2002 characterizes mycobacteriosis among striped bass at four sample sites from Chesapeake Bay tributaries in both Maryland (Potomac and Nanticoke Rivers) and Virginia (York and Rappahannock Rivers). Within a two-week period, 50 adult striped bass are harvested from pound nets at each sample site and processed on site in the field via necropsy and tissue collection. Tissues are returned to the laboratory for microbial culture and histopathological analysis. Aseptically collected samples of spleen are used for quantitative cultures with both a nonselective media and media selective for Mycobacterium sp. Samples of spleen, liver, hematopoietic kidney, gill, and skin collected in fixative are processed and examined by light microscopy. Compiled results for each fish sampled will include descriptions of presence and load of Mycobacterium sp. and other bacteria in the spleen, as well as gross and histological indications of disease. Results will subsequently be compared geographically (tributary to tributary) and chronologically (year to year). Through these standardized field and laboratory methods, our goal is to address a number of questions related to the frequency, specific etiology, clinical presentation, and geographic distribution of mycobacteriosis and its causative agents among the striped bass population in the Chesapeake Bay watershed.