Abstract

Mycobacterial species are increasingly becoming serious pathogens of aquaculture species. We are investigating the pathogenesis of three mycobacterial species, M. marinum, M. fortuilum and M. smegmatus in the goldfish. Fish were exposed to 3 different doses of inoculum (10*7, 10*8, 10*9) and the clinical progression of the disease is being characterized. Histopathological changes at 2, 4, 6, 8 weeks show differences between the 3 doses, and the 3 species. We are further characterizing the progress of the disease by conducting colony counts of liver, spleen and kidney at the same time periods in all three species. M. marinum is the most aggressive species of the three species. Fish inoculated with the 10*9 and 10*8 doses die within one and two weeks respectively. Fish inoculated with the 10*7 dose survive up to 8 weeks. Histopathological evaluation of these fish shows a progression from an acute, necrotizing peritonitis to a chronic granulomatous peritoneal response. Occasional giant cells can be found in the granulomata. Bacteria can be found on H&E sections at most time periods. Acid fast stains, which are used confirm the presence of Mycobacteria in the sections, are frequently negative despite positive cultures and identification of the organisms by PCR probes specific for the strains.