Histopathological evidence of mycobacteriosis in mullets from Sardinian lagoons (central-western Mediterranean)

Fish mycobacteriosis is a chronic progressive disease caused by nontuberculous (atypical) mycobacteria. It can cause low mortality in wild aquatic organisms but significant financial loss in farmed fish. The disease may be detected in fish without clinical symptoms and can be also pathogenic for humans. Its diagnosis is mainly based on histopathological evidence of granulomas and acid-fast bacteria revealed by Ziehl-Neelsen (ZN) stain. Since mycobacteriosis in the Mugilidae family has been scarcely reported worldwide (and in particular in the Mediterranean Sea), the aim of this work was to find out its occurrence in extensively reared mullets from 4 different coastal lagoons of Sardinia (Italy). This was carried out by providing a histopathological description of granulomas at different stages of evolution. Two hundred thirty-nine mullets were sampled in late summer 2014 and a complete necropsy was performed. Samples of heart, liver, spleen and kidney were formalin fixed, paraffin embedded and stained with Hematoxylin and Eosin, ZN and Masson’s trichrome stains. Small whitish nodules were macroscopically observed in spleen and 8 out of 239 specimens showed positive ZN stain granulomas for mycobacteria. Based on histological pattern, granulomas were classified in 3 categories associated to their evolutive stage: 1) early stage characterized by macrophage aggregates without necrosis; 2) intermediate stage composed of macrophages with a central core of coagulative necrosis; and 3) late stage with a central lytic necrosis delimitated by several layer of fibroblasts but without macrophages. Occasionally, this latter presented necrotic material arranged in a lamellar concentric pattern. Intermediate and late granulomas were the most frequently stages found and acid-fast bacteria were detected in each granuloma stage. Our observations demonstrate for the first time the occurrence of mycobacteriosis in extensively reared mullets from Sardinia. Furthermore, the results achieved evidence that granulomas are mainly found at different stages in the spleen of mullets, confirming the systemic nature of the disease.