Mycobacterium pseudoshottsii is a slowly growing photochromogenic mycobacterium closely related with M. marinum, well known as fish and human pathogen. M. pseudoshottsii was reported for the first time in 2005, during an acute case of mycobacteriosis in wild striped bass Morone saxatilis in Chesapeake Bay (Maryland and Virginia, USA). After this first case, the species was isolated in USA in several other fish and locations (New York Bight, Rhode and Corsica rivers). Other cases of mycobacteriosis caused by M. pseudoshottsii have been reported in farmed marine fish (Seriola quinqueradiata, S. dumerili, S. lalandi, Epinephelus septemfasciatus) in western Japan. The first case in Europe involved a European sea bass Dicentrarchus labrax farm in southern Italy.
In this work, the first case of mycobacteriosis caused by M. pseudoshottsii, in farmed red drum Sciaenops ocellatus is reported.
Twenty-one red drum from a farm in South-East Italy were previously analysed in September 2018: only the culture was performed, because fish were frozen. In June 2019 a second sampling in the same farm was performed: 15 fish were collected for culture and histological evaluation. The animals were necropsied and splenic and renal miliary nodules were detected. Portions of gills, liver, spleen and kidney were sampled for histology and portions of spleen and kidney, were collected for the isolation of nontuberculous mycobacteria in solid media (Löwenstein-Jensen and Stonebrinks medium). The tissues for histopathology were processed by standard paraffin wax techniques, stained with haematoxylin-eosin (HE) and Ziehl-Neelsen (ZN) acid-fast staining.
Slowly growing acid-fast bacilli were isolated in 7 fishes (1 in 2018, 6 in 2019) and subjected to DNA amplification by PCR and Sanger sequencing of 16S rRNA, hsp65 and rpoB genes. Biomolecular analysis leading to the identification of isolates as M. pseudoshottsii after comparing the sequences obtained with those present in public database (GenBank).
Microscopically, lesions were detected in all organs analysed. In spleen and kidney, the lesions were so severe that the normal organ architecture appeared completely destroyed. The lesions were characterized by multiple disseminated necrotizing granulomas. The lesions displayed large amounts of acid-fast bacilli.
To our knowledge, this is the first report of mycobacteriosis in red drum caused by M. pseudoshottsii.
The severity of the episodes described, suggests the need for constant health monitoring to clarify the role that this mycobacterium can play for Mediterranean farms. Further investigation will be needed to better understand the risk for other fish species and for humans, in relation to the proven pathogenicity of species like M. pseudoshottsii (e.g. M. marinum). Another critical point to clarify will be the way of entry of this mycobacterium in the Mediterranean Sea by use of biomolecular techniques.