Iberian Veterinary Pathology Meeting 2026

Abstract

Fish mycobacteriosis is a major cause of morbidity and mortality in aquaculture, leading to significant economic losses worldwide and raising concerns within the One Health framework. The disease is caused by non-tuberculous mycobacteria (NTM), including Mycobacterium pseudoshottsii, an emerging pathogen in European aquaculture that is phylogenetically related to zoonotic species such as M. marinum and M. ulcerans, highlighting potential risks for human health, particularly among aquaculture workers and fish handlers. Control remains challenging due to the absence of effective treatments or vaccines.

This study aimed to characterize granuloma development and the associated innate immune response in European sea bass (Dicentrarchus labrax), a key species for European food production. Eight naturally infected fish were examined through histopathology and immunohistochemistry. Granulomas were classified into four stages based on necrosis, inflammatory infiltrate, and fibrosis. Early (Type I) granulomas consisted of loosely organized macrophages and epithelioid cells. Intermediate (Type II) lesions showed central necrosis surrounded by epithelioid cells. Advanced (Type III) granulomas displayed necrosis, a foamy cell interface, and a thin fibrotic capsule, whereas late (Type IV) granulomas were characterized by extensive necrosis and a thick fibrotic capsule. Bacillary load increased with granuloma progression, localizing mainly in necrotic areas and macrophages. The innate immune response, assessed through inducible nitric oxide synthase (iNOS) expression, was restricted to macrophages and lymphocytes and absent in epithelioid cells, suggesting a role in modulating granuloma organization and disease progression.

These findings provide novel insights into host–pathogen interactions and granuloma dynamics in M. pseudoshottsii infection. This knowledge contributes to improved surveillance, diagnosis, and control strategies in aquaculture. Ultimately, this research supports sustainable fish production, reduces potential zoonotic risk, and reinforces the interconnected health of animals, humans, and aquatic ecosystems within the One Health framework.