Anecdotal evidence from fish farmers in Nigeria suggests that the hybrid of Clarias gariepinus (female) and Heterobranchus longifilis (male) are hardier to diseases, and have higher survival and growth rate when compared to their parent species. There is the need to validate this assumption and elucidate the factors that may contribute to disease resistance observed in the hybrids.
Twenty juveniles each of Clarias gariepinus (Group A), Heterobranchus longifilis (Group B) and the hybrids of Clarias gariepinus (female) and Heterobranchus longifilis (male) (Group C), were acclimatized for two weeks under the flow-through system. The fishes were fed to satiation, twice daily using commercial catfish diet. Blood samples were collected from ten fish samples in each group for haematology, and samples of the liver, spleen, kidney, gills and posterior intestine were also collected for histology. Aeromonas carviae isolated from a moribund fish in a clinical case and identified biochemically using Vitek 2- Compact (BioMerieux) was used for experimental infection of the three fish species by immersion, in 1.5x108 cfu per ml of the isolate at a rate of 5ml/L. MacFarland’s standard (0.5) was used for the bacteria count. Gross lesions and mortality were observed and recorded for 14 days. Blood samples and tissue samples were also collected from the surviving fish for haematology and histopathology respectively.
The pre-infection comparison of the Mean (±SEM) WBC, Lymphocytes and Eosinophils counts across the three groups were significantly different (p value < 0.05). The hybrids had the highest value in Mean (±SEM) WBC, Lymphocytes and Eosinophils counts followed by the Clarias gariepinus, while Heterobranchus longifilis had the least values. The pre-infection histology revealed normal tissue structures in the organs. The post-infection comparison of the Mean (±SEM) WBC, Lymphocytes and Eosinophils counts across the three groups were not different significantly (p value > 0.05). There were histopathological lesions observed in the three groups. These lesions include diffuse hepatic degeneration and coalescing, loss of gill lamellae core and mucosal arrangement, and diffuse tubular degeneration and ballooning in the renal tissue. Other organs did not have histopathological lesions. Mortality was recorded post-infection in Group A (Clarias gariepinus) only, although all the groups had mild clinical signs and gross lesions. Comparing the pre-infection and post-infection haematology, the differences in the Mean (±SEM) lymphocytes and eosinophils were significant (p value < 0.05) with the hybrids having the highest increase, followed by Heterobranchus longifilis while Clarias gariepinus had the least increase.
These results show that the hybrids have better immune response to infection as evidenced in the highest increase in lymphocyte count. This also shows that the adaptive immunity plays a major role in the line of defense against bacterial infection in African catfish species.