Pathological Changes Associated with Aeromonas hydrophila Dose-Dependent Infection and Heat Stress in Red Hybrid Tilapia

Aeromonas hydrophila constitutes a normal microflora of fish. Heat stress in red hybrid tilapia may induce higher mortality if the fish are infected with Aeromonas. In this study, 192 red tilapia fish were divided into eight groups, A--H, each consisting of 24 red hybrid tilapia. Groups A, B, and C were inoculated with 0.1 mL of [10.sup.12] CFU/mL, [10.sup.10] CFU/mL, and [10.sup.8] CFU/mL of A. hydrophila respectively, while group D was inoculated with 0.1 mL of phosphate-buffered saline (as control for the heat-stressed groups); and heat stress was introduced from day 8 through the day 14 post-infection (pi). Groups E, F, and G were infected with similar doses as in A, B, and C respectively, with no heat stress, while Group H served as negative control for non-heat stress. Mortality was higher in the heat-stressed groups, at 44.47%, as against 16.7% for the non-heat- stressed groups. The clinical signs included hemorrhages on the abdomen, mouth, and dorsal fin, and abdominal distension most predominantly in the heat-stressed groups. Gross lesions included enlargement of the kidney and the spleen and yellowish ascitic fluid, the histopathological lesions included pyknosis and desquamation of the renal tubular epithelium, congestion, necrosis, and fatty changes in the liver, while the spleen showed congestion of the splenic blood vessels. The lesions were seen predominantly in the heat-stressed groups, most severely in group A. The effect of the different doses was more significant in group A compared to the other groups. This study revealed that heat stress predisposes red hybrid tilapia infected with A. hydrophilia to increased disease severity and higher mortality, with the degree of initial infection playing a significant role. Keywords: Congestion, heat stress, hemorrhagic mouth, mortality