Fazel Pourahmad and Mostafa Nemati
Mycobacteriosis is a progressive disease of a wide range of wild and captive marine and freshwater fish species.Mycobacterium marinum, M. fortuitum and M. chelonae are the most frequently reported species to be involved in the disease, although several new species of Mycobacterium have recently been reported to be involved. In the present study, rainbow trout, Oncorhynchus mykiss, and Atlantic salmon, Salmo salar, were inoculated intraperitoneally with 107 cells of M. salmoniphilum (NCIMB 13533) and maintained at 15°C. Infected fish were sampled for histopathology, bacteriology, and polymerase chain reaction (PCR)at weeks 4, 6, 8, 10 and 12 postinfection. While a number of rainbow trout died or were clinically ill by week 3 post-infection, there were no clinical abnormalites in the Atlantic salmon except for anorexia. Pronounced diffuse granulomatous inflammation was seen in the peritoneal cavity of the salmon, comprising large numbers of mainly epithelioid macrophages, and an occasional multinucleated giant cell. By contrast, the response in the trout was minimal. Multifocal granulomas
were not observed in the internal organs of either species of fish as previously reported to occur in naturally infected fish. Some acid-fast bacteria were detected in the early stages of infection in rainbow trout and in all cases of Atlantic salmon by Ziehl-Neelsen staining. Unlike Atlantic salmon, from which Mycobacterium was isolated from the anterior kidney at all sample times, no bacteria were detected after 8 weeks in rainbow trout. Using speciesspecific probes, PCR was used to confirm the presence of the bacterium, especially in samples which were positive by culture. The presence of bacteria in internal organs of Atlantic salmon over the duration of the trial indicated a sub-clinical infection and reflects the chronic nature of mycobacteriosis in this species.
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