The meagre (Argyrosomus regius), a fast-growing species, is increasingly valued for sustainable aquaculture along the Mediterranean and Eastern Atlantic coasts. However, Systemic Granulomatosis (SG), a condition characterized by multifocal granuloma formation in multiple tissues, presents a major challenge for meagre aquaculture.
This study investigates the potential link between Mycobacterium spp. and SG in offshore floating cage aquaculture facilities in Sardinia (Italy; Mediterranean Sea).
In June 2022, thirty-four adult, apparently healthy meagre were randomly selected from the facility with a stocking density of 10-20 kg/m³. Environmental conditions included water salinity levels of 37-40, and a surface water temperature of 25±1.0 °C. The fish averaged 34.5±3.5 cm in total length and 389±67 g in weight. Samples were analysed using histological, microbiological, molecular, metagenomic, and in situ methods to detect potential pathogens. Ziehl-Neelsen (ZN), periodic acid-Schiff (PAS), and Giemsa stains were applied to identify acid-fast bacteria, common parasites, and fungi within granulomas.
Granulomas were observed in 31 out of 34 fish (91%), affecting organs including the kidney (88%), liver (47%), heart (41%), intestine (17.6%), and brain (5%). Acid-fast staining, alongside Mycobacterium spp.-specific quantitative PCR (qPCR), in situ hybridization (ISH), and microbiological analyses, returned negative for Mycobacterium spp., and other known bacteria involved in granuloma formation.
However, PCR amplification and sequencing of the 65-kDa heat shock protein gene identified M. chelonae in 13% of formalin-fixed and frozen liver samples. Despite this, bacterial culture did not yield nontuberculous mycobacteria (NTM) or other bacteria commonly associated with granulomas, and ISH with an M. chelonae-specific probe failed to detect this bacterium in granulomas.
Overall, the findings do not support M. chelonae as a cause of granulomas and suggest ruling out a link between nontuberculous mycobacteria (NTM) and SG. These results highlight the need for further investigation into the underlying causes of SG in meagres, emphasizing the importance of distinguishing between infectious and non-infectious factors (e.g., nutritional or autoimmune factors). Future studies should aim to identify specific pathogens or conditions contributing to granuloma formation to enhance disease management strategies in aquaculture.