Piscine mycobacteriosis - Involvement of bacterial species and reflection in pathology

Piscine mycobacteriosis is a lethal disease with zoonotic potential, found worldwide in both fresh and marine fish. More than 20 strains of Mycobacterium spp. are known to persist in fish so far, but the pathogenicity is currently unknown for most of them. However, M. marinum is reported as one of the most pathogenic agents for fish and is involved in zoonotic cases. We examined 47 different cases from two zoological gardens, where fish tuberculosis was identified or previously suspected during the last ten years. We collected PCR and sequencing data, which were then compared to previously collected clinical data and pathology. The clinical signs caused by Mycobacterium spp. were similar in all the cases, except for cases infected by M. marinum, which lacked the presence of skin lesions. Lesions seen in histology caused by M. marinum tended to be more acute and severe compared lesions caused by other Mycobacterium spp. The majority of M. marinum cases have been reported within marine fish. In contrast to previous studies we detected this species to be the predominant bacteria present within freshwater fish. Interestingly, we detected M. holsaticum in one of the seawater systems used in this project, being the first report of this Mycobacterium species shown to be present in a fish.