The effect of environmental factors on the susceptibility of hybrid tilapia Oreochromis niloticus x Oreochromis aureus to streptococcosis

Streptococcus spp. were isolated from healthy and diseased Tilapia hybrids (Oreochromis niloticus x O. aureus), healthy carp (Cyprinus carpio), diseased mullet (Mugil cephalus) and striped hybrid bass (Morone saxatilis x M. chrysops), pond water and poultry manure. Streptococci were divided into two groups according to biochemical characteristics: (a) alpha-hemolytic, mannitol+, usually isolated from rainbow trout (Oncorhynchus mykiss) throughout the year and, more rarely, from Tilapia during the winter (water temperature 15-16 degrees C), and (b) gamma-hemolytic, mannitol- usually isolated from Tilapia (Oreochromis spp.) in warm water aquaculture (26-28 degrees C). The disease was produced experimentally in Tilapia hybrids to investigate the effect of low dissolved oxygen levels and high nitrite concentrations on the susceptibility of the fish to infection. Both stress factors proved to increase the mortality. No additive effect was seen when these factors were applied simultaneously. By using the same experimental infection model, it was also possible to produce streptococcal infection in carp. The diseased Tilapia had a significantly lower hematocrit (p<0.05) and higher buffy coat (p<0.01) than the healthy fish. Our findings suggest that Streptococcus spp. are opportunistic pathogens because they are wide spread in the aquaculture environment and because of their dependence on stress to assert pathogenity.