Abstract

Historically, a variety of disease outbreaks have been associated with fish morbidity and mass mortalities in the Chesapeake Bay, the largest and most productive estuary in North America. In the late 1990s, an epizootic of mycobacterial infections was reported in Chesapeake Bay striped bass (Morone saxatilis), and subsequent reports indicated that over three-fourths of subpopulations of striped bass in the Bay were infected, primarily older fish.² The present study investigated regional and age class differences in mycobacterial infections among younger striped bass in the Chesapeake Bay, and identified putative risk factors for infection. Approximately 2,000 0- to 4-year-old striped bass, a limited number of spawning stock, and bycatch species were evaluated for microbiology, histopathology and parasitology. Mycobacterial isolates were grouped according to gas chromatography fatty-acid methyl-ester profiles and multi-locus sequencing. Twenty-nine groups of mycobacteria were discerned including M. scrofulaceum, M. septicum, M. interjectum, M. triplex/M. montefiorense, M. szulgai, M. moriokaense, M. duvalii, M. avium, M. terrae, M. pseudoshottsii/M. marinum and M. shottsii, and several putative new species. The majority of mycobacteria groups observed had host overlap. The prevalence of mycobacterial infection increased with age, up to 59%. In certain age classes, location of capture was associated with significantly higher prevalence in fish from the Pocomoke River compared with fish from the Upper Bay, the Choptank River and the Potomac River. The presence of parasitic copepods, isopods, acanthocephalans, nematodes and trichodinid ciliates was associated with an increased prevalence odds ratio for mycobacterial infection. The presence of bacteria other than mycobacteria was associated with a decreased prevalence odds ratio for young-of-the-year fish. Gender was not a risk factor for mycobacterial infection in the fish evaluated in this study, however gonads from some mature fish were infected. In addition, mycobacterial infections were observed in Atlantic menhaden (Brevoortia tyrannus), an important prey species, as well as blueback herring (Alosa aestivalis) winter flounder (Pleuronectes americanus), striped killifish (Fundulus majalis), mummichog (Fundulus heteroclitus), largemouth bass (Micropterus salmoides), weakfish (Cynoscion regalis), spot (Leiostomus xanthurus) and white perch (Morone americana).¹ The high prevalence of infection in Atlantic menhaden raises new questions about the epidemiology of mycobacteriosis in the Chesapeake Bay and the threat it poses to this critical keystone fish species. Atlantic menhaden is the largest commercial fishery in the Bay, and is commercially harvested for their protein that is processed for use in animal feeds, as well as for their oil. Menhaden oil, which is rich in omega-3 fatty acids, is widely used in human and veterinary dietary supplements, and in cosmetics. This study reveals that multiple species of mycobacteria infect multiple species of Chesapeake Bay fishes that represent different ecological niches. Hence, mycobacterial colonization in Chesapeake Bay fishes appears to be more complex than the one pathogen-one host scenario. Further, vertical and food-borne transmission within the Bay cannot be ruled out. Data from this and ongoing studies refocus attention on the complex, anthropogenically accelerated changes that may be altering the distribution of emerging diseases worldwide. The Chesapeake Bay is an important ecosystem with a large urban component, so understanding the epidemiology of this multispecies epizootic may improve not only the health of local regional biota but also the prediction of other emerging infections.

Acknowledgements

We thank Larry Pieper, Kevin Rosemary, and Cindy Driscoll from the Maryland Department of Natural Resources for their invaluable support and assistance in specimen collection and processing. We also thank Sarah Poynton from The Johns Hopkins University School of Medicine for her expertise in parasite identification, and Renate Reimschuessel from the Center for Veterinary Medicine, US Food and Drug Administration for her valuable insights. Support for this study was provided by the National Oceanographic and Atmospheric Administration, Chesapeake Bay Office, by the Centers for Disease Control and Prevention through the Maryland Department of Health and Mental Hygiene, and by the Maryland Agricultural Experiment Station.

References

1.  Kane AS, Stine CB, Hungerford L, Matsche M, Driscoll CP, Baya AM 2007. Mycobacteria as an environmental portent in Chesapeake Bay fishes. Emerging and Infectious Diseases. 13(2): February 2007. Available from http://www.cdc.gov/EID/content/13/2/329.htm

2.  Rhodes MW, H Kator, I Kaattari, D Gauthier, W Vogelbein and CA Ottinger. 2004. Isolation and characterization of mycobacteria from striped bass Morone saxatilis from the Chesapeake Bay. Diseases of Aquatic Organisms. 61: 41-51.