Diagnostic investigation for the detection of mycobacteria in samples of fish feeds and tissue from sea bream and sea bass with severe granulomatous lesions

We report on a case of a sea bass and sea bream aquaculture farm in Greece, with many incidents of severe dermal and visceral granulomatous lesions detected during routine inspection and post-mortem examination. The investigation was conducted on samples of fish feeds and fish feed ingredients available commercially, and tissue samples collected from the affected population, and consisted of molecular analysis and histopathology. The former was performed using five PCR assays targeting the main mycobacterial pathogens. Histopathology examination relied on staining of tissue sections of various visceral organs with hematoxylin-eosin and ZiehlNeelsen. Presence of mycobacteria was demonstrated in the fish feeds (100% and 46.2% positive for Mycobacterium spp. and Mycobacterium avium, respectively), in the fish feed ingredients (73.3% and 33.3% positive for Mycobacterium spp. and Mycobacterium avium, respectively) and the tissue sections (76.5% and 3.9% positive for Mycobacterium marinum and Mycobacterium avium, respectively) that were examined. Sequence analysis of the PCR products was confirmatory of the specificity of the amplification process, at a 98% minimum level of identification. Histopathology revealed in all cases evidence consistent with mycobacterial infection. The result of the analysis indicates that fish are exposed to different mycobacterial species; this probably accumulates in large numbers over the years in the organic sediment collected on the seabed of the aquaculture, causing severe infections.