Study on pathogenesis of the mycobacteriosis in european seabass (Dicentrarchus labrax)

To date the pathogenesis of atypical mycobacterioses in fish has been scarcely characterized. In this research, European sea bass (D. labrax) were experimentally infected with Mycobacterium fortuitum to study histologically the pathogenesis of the infection. Three groups of sea bass (total of 90 fish, 30 fish per group) weighing 10-12 g were used: two groups were intraperitoneally (i.p.) inoculated with different concentrations of M. fortuitum (104 and 107 cfu/10 g b.w. diluted in Phosphate Buffered Saline + 0.05% Tween 80) while a third control fish group was injected i.p. only with Phosphate Buffered Saline + 0.05% Tween 80. Three fish from each group were sacrificed at 1, 2, 3, 4, 6, 8 and 10 weeks post inoculation (p.i.). All the fish were subjected to bacteriological examination. Samples of peritoneum, kidney, liver, spleen, skin and muscle were fixed, paraffin-wax embedded and two 4-µm-thick serial sections underwent haematoxylin-eosin and Ziehl-Neelsen stains. The strain used for the experimental infection was reisolated from all the fish, starting from the 1st p.i. week. Macroscopic lesions were similar in the two infected groups, independently from the inoculum. Histological examination of the subjects at the first p.i. week revealed the presence of granulomatous peritonitis. At the second p.i. week the peritonitis was more diffused on the serosa, with rare necrotic foci. The granulomata enlarged and were surrounded by a fibrous capsule at the third p.i. week. Acid-alcohol fast bacteria were easily observed when necrosis appeared. After the third p.i. week, both initial and well organized granulomas with necrotic center coexisted in peritoneum and parenchymal tissues. The results of this research seem to indicate the high suitability of European sea bass, when inoculated with a rapidly growing Mycobacterium strain, as an in vivo model to study the pathogenesis of fish mycobacteriosis.