Histopathology of sea bass (Dicentrarchus labrax) experimentally infected with mycobacterium marinum and treated with streptomycin and garlic (Allium sativum) extract

The histopathology and serological effects of an experimental infection of Mycobacterium marinum in sea bass Dicentrarchus labrax were studied. Following an intraperitoneal injection of 8.8 x l0⁴ M. marinum cells (t₀), histopathological examination did not reveal presence of acid-fast bacteria within the first 4 weeks in the spleen, normally the most severely affected organ in spontaneous infections, nor in any other visceral organ. Between the 4th and 6th weeks, a rapid development of granulomata was observed in the spleen which, remarkably, preceded the actual detection of mycobacteria in sections of these lesions at the end of week 6. At week 9, the splenic lesions appeared indistinguishable from those typical of a natural infection. At this time, two antibacterial substances to which M. marinum displayed in vitro sensitivity, streptomycin and allicin (as garlic extract), separately and in combination, were injected intraperitoneally once every second week for 12 weeks. In some individuals which underwent such antibiotic treatment, evidence of regression of the splenic lesions was noted at weeks 18-22. At a later stage (not before weeks 24-26), signs of lesion regression were noted also in some of the fish from the positive (infected/untreated) control. The granulomata appeared reduced in number and size and the splenic tissue reorganized. Residual lesions were replaced with new epithelioid cells and macrophages; relatively few mycobacteria or none at all were present. Antibody activity against M. marinum was monitored throughout the experiment. All the sera at t₀ revealed specific antibody titers, suggesting previous exposure to the pathogen. All the experimentally infected fish displayed a similar pattern of fluctuations in antibody titers, indicating a relationship between immune activity and the disease course. A sudden increase of antibody titers recorded at week 24, i.e., soon after the antibiotic treatments were discontinued, suggests an immunosuppressive effect of streptomycin and coincided with the beginning of a healing process, as observed histologically. M. marinum was re-isolated from the surviving fish at the end of the experiment (week 32), indicating that eradication of the pathogen was not achieved with any of the treatments.