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

Meagre (<i>Argyrosomus regius</i>) 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 <i>Mycobacterium</i> 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 <i>Mycobacterium</i> spp. specific quantitative PCR (qPCR), in situ hybridization (ISH) assay, and microbiological analyses showed negative results for the detection of <i>Mycobacterium</i> 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 <i>M. chelonae</i> 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 <i>M. chelonae</i>-specific probe in ISH failed to identify this bacterial species in granulomas. Collectively, results do not support the role of <i>M. chelonae</i> in the development of granulomas and suggest rejecting the hypothesis of a potential link between NTM and SG.