Pathogenicity and therapeutic studies of Aeromonas hydrophila infection in sand goby (Oxyeleotris marmoratus Bleeker)

Sand gobies (Oxyeleotris marmoratus Bleeker), 19-23 cm. in length, were injected intraperitoneally with different concentrations of Aeromonas hydrophila. The lethal dose concentrations where 50 percent of the fish died within 96 hours (96th-LD50) was 9.089 x 10 to the ninth power cells. The relatively high doses of bacteria caused ascites and petechial hemorrhage along the abdomen, peritoneum and mesenteries of infected fish. Most moribund fish were lethargic. Fish were injected intraperitoneally with a low number of A. hydrophila (1.052 x 10 to the ninth power cells) had hematological values similar to the control at 24 hours. However, infected fish with a medium number (3.092 x 10 to the ninth power cells) and high number (6.344 x 10 to the ninth power cells) of A. hydrophila had lower total erythrocytes and leucocytes counts, hematocrit, hemoglobin, plasma protein than the control at 48 hours. Plasma glucose from infectd fish was higher than the control through the experimental period. However, most of the blood parameters for the injected fish were similar to the control at 72 hours. Histopathologically, edema, hyperplasia and telangiectasis were observed in secondary gill lamellae. Internally, splenic tissues were necrotized. Other visceral organs appeared to be normal. Sand gobies injected intramuscularly with A. hydrophila developed large external ulcerative lesion on the body at the injected area. Zenker's necrosis, granulomatous inflammation and hemorrhage were observed in the ulcerative tissue area. Sand gobies continuously exposed to 1 percent sodium chloride solution after intramuscularly injection with different concentrations of A. hydrophila had the highest survival rate (10-80%) compared to those injected with 20 ppm oxytetracycline-HCl (20%) and 4 ppm potassium permanganate (0%). Fish were injected intraperitoneally with A. hydrophila followed by injection with oxytetracycline-HCl or tetracycline-HCl at a dose of 30 mg/kg of fish body weight showed 100 percent survival rate