Mycobacteriosis in Chesapeake Bay Striped Bass

The striped bass, Morone saxatilis, is one of the most commercially and recreationally important finfishes in Chesapeake Bay. Since the late 1990’s, Bay striped bass have exhibited endemic mycobacteriosis, a chronic bacterial disease thought to be fatal. Disease signs include skin ulcers and nodular lesions on internal organs, predominantly the spleen. Some age groups exhibit splenic disease prevalence >75% and skin ulcer prevalence approaching 50%.  A suite of mycobacteria has been cultured from bass but two new species, Mycobacterium shottsii and M. pseudoshottsii, are most commonly associated with disease.  Recent stock assessments have shown an increase in non-fishing mortality (M) alarming fisheries managers and suggesting the disease as a possible underlying cause.  We implemented studies to better understand mycobacterial ecology, the role of stress in disease expression and disease impacts on the stock. This has required development of rapid and specific molecular diagnostics, as identification of mycobacteria by traditional cultural methods is costly and time-consuming. We developed qPCR-based tools to detect and quantify M. shottsii and M. pseudoshottsii in fish tissues and environmental matrices including water and sediment. Results indicate that M. pseudoshottsii is widely distributed within water and sediments of the Bay, whereas M. shottsii is not.  M. pseudoshottsii is also found at high prevalence and densities in prey species of striped bass (menhaden, anchovies), suggesting possible vector involvement.  qPCR is also being applied to laboratory challenges examining mycobacterial transmission routes and impacts of temperature on disease expression. To elucidate the potential for disease-associated mortality in striped bass, we initiated two field efforts.  The first uses the Chesapeake Bay Multispecies Monitoring and Assessment Program (ChesMMAP), a fisheries-independent trawl survey conducted by VIMS (Latour). Application of new force-of-infection epidemiological models to apparent disease prevalence data obtained by this survey provided first evidence of disease-associated mortality in striped bass.  A second field survey in the Rappahannock River applies tag-recapture methods to investigate disease progression and disease-associated impacts on bass. Results indicate that disease progresses over time with little or no healing. Findings also strongly indicate occurrence of disease–associated mortality in striped bass. These studies will provide a better understanding of basic disease dynamics, mycobacterial sources/transmission routes, environmental/anthropogenic modulators of disease expression and impacts on stock health. Additionally, these studies should offer better stock management strategies that account for impacts of infectious disease.