Abstract
Mycobacteriosis causes serious economic damages to yellowtail aquaculture in Japan. From October to November 2020, mycobacteriosis occurred in yellowtail cultured in Owase Bay, Mie Prefecture, Japan. In this study, we observed the gross pathology and histopathology of the diseased fish. We identified mycobacteria isolated from the diseased fish and investigated their drug sensitivities against 12 antimicrobial agents. Diseased yellowtail exhibited abdominal swelling and many nodules in their kidney and spleen. The head kidney and trunk kidney presented typical granulomas with a necrotic core surrounded by epithelioid cells and fibroblasts. The isolates were identified as Mycobacterium pseudoshottsii based on phylogenetic analysis using the 16S rRNA gene, hsp65 and rpoB. Low concentrations of amikacin, ciprofloxacin, kanamycin and lincomycin inhibited growth of the isolates. Two of three isolates showed sensitivity to erythromycin; the other isolate was resistant to it. Erythromycin and lincomycin that are permitted for therapeutic use in Perciformes in Japan may be useful for treating yellowtail infected with M. pseudoshottsii. However, emergence of antibiotic-resistant strains of M. pseudoshottsii due to antibiotic use requires attention in yellowtail aquaculture.
