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 hyb rids (Oreochromis niloticus x O. aureus), healthy carp (Cyprinus carpi o), diseased mullet (Mugil cephalus) and striped hybrid bass (Morone s axatilis x M. chrysops), pond water and poultry manure. Streptococci w ere divided into two groups according to biochemical characteristics: (a) alpha-hemolytic, mannitol+, usually isolated from rainbow trout (O ncorhynchus 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 dis solved oxygen levels and high nitrite concentrations on the susceptibi lity of the fish to infection. Both stress factors proved to increase the mortality. No additive effect was seen when these factors were app lied simultaneously. By using the same experimental infection model, i t was also possible to produce streptococcal infection in carp. The di seased Tilapia had a significantly lower hematocrit (p<0.05) and highe r 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.