Comparative severity of experimentally induced mycobacteriosis in striped bass Morone saxatilis and hybrid tilapia Oreochromis spp

Twenty striped bass Morone saxatilis and 20 hybrid tilapia Oreochromis niloticus x O. mossambicus x O. aureus each received a single intramuscular injection of 1.6 x 10(6) colony forming units per gram body weight of Mycobacterium marinum. Striped bass manifested significantly greater clinical and microscopic disease compared to tilapia. Whereas all the striped bass had died or were clinically ill by Day 8 post-infection, there was no apparent disruption of normal behaviour, physical appearance, or growth in any of the sacrificed or surviving tilapia. Histologically, granulomas in striped bass were generally larger and less discrete, with a higher proportion of heavily vacuolated macrophages, and large cores of necrotic cells. Visceral granulomas in tilapia were smaller, with a higher proportion of epithelioid macrophages, more pigment-containing cells, more peripheral lymphocytes, and virtually no central necrosis. Visceral granulomas were 18-fold more numerous in striped bass than in tilapia. Based upon histomorphometric data, mean proportions of acid-fast bacteria within pronephros granulomas were 4-fold greater in striped bass than tilapia, and striped bass granulomas averaged more than twice as large as tilapia granulomas. In the anterior kidney of striped bass, a positive correlation existed between mean mycobacterial proportions and mean necrosis scores. In tilapia, mean mycobacterial proportions correlated negatively with mean granuloma numbers, whereas there was no correlation between these parameters in striped bass. Results suggest that intrinsic functional differences in the immunologic systems of striped bass and hybrid tilapia may contribute to inter-species variation in mycobacteriosis susceptibility.