Low prevalence of splenic mycobacteriosis in migratory striped bass Morone saxatilis from North Carolina and Chesapeake Bay, U.S.A.

Mycobacteriosis is a chronic bacterial disease causing an ongoing epizootic in striped bass Morone saxatilis in Chesapeake Bay, U.S.A. Prevalence of disease is high in pre-migratory fish, and multiple species of Mycobacterium spp. have been isolated. However, prevalence of mycobacteriosis in the coastal migratory population is unknown and is of concern to multiple coastal states, as disease-related mortality may impact the long-term health of the population. Histological examinations of spleens collected from fish caught by recreational anglers during the winter fishery in coastal North Carolina (2005-2006, n=249) and during the spring fishery in Chesapeake Bay (2006, n=120) indicated a low prevalence of mycobacteriosis (6.8% of all fish examined) in comparison to smaller, pre-migratory Chesapeake Bay fish. Genus-level PCR and subsequent sequencing of the 16-23S intergenic transcribed spacer (ITS) region revealed that all bacteria were phylogenetically related, but species is unknown. Location of survey, gender of fish, and total length of fish had no significant effect on prevalence of mycobacteriosis, parasitic granulomas, or the density of splenic granulomas (p > 0.05). These results may indicate that either granulomas resolve after Chesapeake Bay fish enter the coastal migratory population, or that there is disease-related mortality among pre-migratory Chesapeake Bay fish.

Historical presence (1975-1985) of mycobacteriosis in Chesapeake Bay striped bass Morone saxatilis.

A retrospective analysis of archived tissue blocks has revealed that mycobacteriosis was apparent in Chesapeake Bay striped bass as early as 1984. Of 37 cases available from the years 1975 to 1985, 2 fish were found positive based on histopathology and genus-specific PCR. Multi-gene sequencing places the bacteria from the 2 positive cases (1984 and 1985) within the Mycobacterium tuberculosis clade with closest resemblance to the recently described fish pathogen M. pseudoshottsii. Our data confirms that mycobacteriosis is not a new disease of Chesapeake Bay striped bass and underscores the value of archived tissues in epidemiological examinations.

Influence of nutritional state on the progression and severity of mycobacteriosis in striped bass Morone saxatilis.

Challenge studies with Mycobacterium marinum clearly demonstrate that a poor diet affects the progression and severity of mycobacteriosis in striped bass Morone saxatilis. Fish (n = 512 total, wt = 65 +/- 15 g) were inoculated intraperitoneally with 10(4) colony-forming units (CFU) g(-1) body weigth (BW) or a physiological saline solution (controls) and evaluated for 8 mo. Inoculated fish fed a low-ration diet (0.15% BW d(-1)) developed a severe, systemic infection characterized by a high bacterial load (>10(8) CFU g(-1) spleen) and poor granuloma formation, which commonly progressed to mortality by 6 wk. In contrast, inoculated fish fed an adequate ration diet (1% BW d(-1)) developed classic granulomatous inflammation of reduced severity and total body energy similar to that found in uninoculated controls (p > 0.05). After 4 wk, fish fed adequate rations maintained an equilibrium state throughout the study period, even though 10(6) CFU g(-1) spleen mycobacteria were consistently cultured. In a second study, reactivation of an acute inflammatory state was demonstrated by placing previously infected fish on reducing diets (0.073% BW d(-1)). In both studies, the energetic demand of this disease was only appreciable when associated with active, severe, inflammatory states. To our knowledge, this study is the first to demonstrate the interaction of diet and mycobacteriosis in fish.

Expanded range and new host species of Mycobacterium shottsii and M. pseudoshottsii.

Mycobacterium shottsii and M. pseudoshottsii are recently described mycobacteria commonly isolated from Chesapeake Bay striped bass Morone saxatilis. However, their distribution in striped bass outside of the Chesapeake region and their ability to infect alternative hosts have not been described. Mycobacteria identified as M. shottsii (based on fatty acid methyl ester analysis and multigene sequencing) were isolated from striped bass collected in Albemarle Sound, North Carolina, and white perch Morone americana in the Rhode River, Maryland, and detected in striped bass from the New York Bight off Long Island, New York. Mycobacterium pseudoshottsii were isolated from white perch in the Rhode and Corsica rivers, Maryland, and detected in striped bass in the New York Bight. This work demonstrates that these mycobacteria can be found outside of the Chesapeake Bay as well as in hosts other than striped bass.