Experimental Bacterial Co-infection in Nile Tilapia Shows High Pathogenicity from Lake Kariba Isolates

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Abstract Objective This study aimed to determine the pathogenicity of bacteria ( Aeromonas spp., Lactococcus garvieae, Acinetobacter spp., and Klebsiella spp.) isolated from Nile tilapia ( Oreochromis niloticus ) in small-scale aquaculture establishments on Lake Kariba through experimental infections. Materials and methods Healthy fish (50g ± 5g) were distributed among six transparent fish tanks labeled A to F. Fish in tanks A, B, C, and D were exposed to Aeromonas spp., Lactococcus garvieae, Acinetobacter spp., and Klebsiella spp., respectively, through intraperitoneal inoculation. Tank E received a co-infection of all isolates, while tank F served as the control group with an injection of normal saline. Clinical signs, mortalities and post-mortem lesions were recorded in all experimental groups, with histopathological examinations performed on liver, kidney, and spleen tissues. Results The findings indicate that Acinetobacter spp. and Klebsiella spp. exhibit low pathogenicity, evidenced by few clinical signs such as lethargy, pale skin, and fin erosion. The study further highlighted Aeromonas spp. and Lactococcus garvieae as the bacterial isolates causing significant clinical symptoms in Nile tilapia compared to Acinetobacter spp. and Klebsiella spp. following experimental infection. Conclusion Co-infection with all bacterial pathogens demonstrated very high pathogenicity, with 100% mortality reported within seven days post-infection. The study proposes the development of a polyvalent vaccine to control fish disease outbreaks in small-scale aquaculture operations on Lake Kariba, thereby sustaining aquaculture production in Zambia.

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