Mycobacterium pseudoshottsii in Mediterranean Fish Farms: New Trouble for European Aquaculture?

Abstract

<i>Mycobacterium pseudoshottsii</i>, a slow-growing mycobacterium closely related to <i>M. marinum</i>, has been isolated only in wild fish in the United States and in Japanese fish farms to date. Here, we report cases of mortality in three farmed fish species (<i>Dicentrarchus labrax</i>, <i>Sparus aurata</i>, and <i>Sciaenops ocellatus</i>) caused by <i>M. pseudoshottsii</i> in Italy. Samples underwent necropsy, histology, and culture with pathogen identification based on PCR and sequencing of housekeeping genes (16S rRNA, <i>hsp65</i>, <i>rpoB</i>). Multifocal to coalescing granulomatous and necrotizing inflammation with acid-fast bacilli were observed in the parenchymatous organs, from which <i>M. pseudoshottsii</i> was isolated and identified. Phylogenetic analysis confirmed the results of gene sequencing and allowed subdivision of the isolates into three distinct groups. <i>M. pseudoshottsii</i> poses a potential threat for Mediterranean aquaculture. Its origin in the area under study needs to be clarified, as well as the threat to the farmed fish species.