Edwardsiella tarda is a gram negative bacterium belonging to the family Enterobacteriaceae, it is the causative agent of edwardsiellosis a disease which is among the major constraints in aquaculture worldwide. It causes mass mortalities of fish that results into high economic losses in both aquaculture and fisheries.
This study aimed at determining the immunoprotection of Nile tilapia against E. tarda. A total of 50 fish in five groups were used to determine the LD50, fish were exposed to E. tarda intraperitoneally by injection with dilutions 1.5x106 to 1.5x109.
In the second phase, two groups of 20 fish each were vaccinated with formalin-killed E. tarda and phosphate buffered saline for control; a booster dose was given two weeks after the first dose. Blood was collected weekly from six fish in each group for serum to determine antibody titer by agglutination in microtiter plates. Two weeks after the booster dose, all fish were challenged with 100µl of 108 CFU/ml E .tarda (LD50). Fish were monitored for 4 weeks; dead fish were recorded, examined for clinical signs and pathological changes. Bacteriology was done to confirm the presence of the pathogen in freshly dead or moribund fish. Bacterial load in the liver kidney and spleen was determined by drop plate counting from 10-fold serial dilutions of homogenized tissues. LD50 of 1.6x108 was determined in this study.
Infected Fish showed signs of skin and fin hemorrhages, ulcers, depigmentation, exophthalmia, erosion and distended abdomen externally. Grayish nodules in the spleen, kidney, congested internal organs, fluid filled intestines, black spots in the liver, mottled liver were observed internally. The lesions were more severe in the non-vaccinated groups. There was high bacterial load in the kidneys than in the spleens and livers. All the sampled dead fish were E. tarda positive which was confirmed using API 20E kits. Significantly high antibody titers were found in vaccinated fish and the Relative Percentage Survival was 32.4% indicating relative protection. No significant difference in percentage mortalities was found between groups (p>0.05), there was high bacterial load in the kidney than in the liver and spleen and the bacterial load in non-vaccinated fish was highly significant than in vaccinated fish (p<0.05). The antibody titers in the vaccinated fish were highly significant than in non-vaccinated (p<0.05). Results indicate that formalin-killed cells enhance production of specific antibodies, induce specific immunity and can confer protection to the fish. These results can be used as a baseline for vaccine development after a series of studies on different age groups of fish and doses of different formulations of vaccines under optimized conditions.