Systemic Granulomatosis in the Meagre Argyrosomus regius: Fishing for a Plausible Etiology

Meagre (Argyrosomus regius) is one of the fast-growing species considered for sustainable
aquaculture development along the Mediterranean and Eastern Atlantic coasts. The emergence of
Systemic Granulomatosis (SG), a disease marked by multiple granulomas in various tissues, poses
a significant challenge in meagre aquaculture. In the current study, we investigate the association
of Mycobacterium spp. and SG in offshore aquaculture facilities in Sardinia, Italy. A total of 34 adult
seemingly healthy meagre were arbitrarily collected and analyzed, combining histological, microbiological, molecular, metagenomics, and in situ techniques to investigate the presence of pathogens.
Ziehl–Neelsen (ZN), periodic acid–schiff (PAS), and Giemsa stains were performed for the detection
of acid-fast bacteria, common parasites, and fungi within granulomas, respectively. Granulomas
were detected in 91% (31/34) of fish. The affected organs were kidney (88%), liver (47%), heart (41%),
intestine (17.6%), and brain (5%). Acid-fast staining, along with Mycobacterium spp. specific quantitative PCR (qPCR), in situ hybridization (ISH) assay, and microbiological analyses showed negative
results for the detection of Mycobacterium spp. and other bacteria implicated in granuloma formation.
However, PCR amplification and sequencing of the 65-kDa heat shock protein gene revealed the presence of M. chelonae in 13% of both formalin-fixed and frozen liver tissues. Bacterial isolation failed to
detect nontuberculous mycobacteria (NTM) and other bacteria typically associated with granulomas.
Consistently, the use of an M. chelonae-specific probe in ISH failed to identify this bacterial species
in granulomas. Collectively, results do not support the role of M. chelonae in the development of
granulomas and suggest rejecting the hypothesis of a potential link between NTM and SG.