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

Bacterial diseases are a major constraint to the sustainability of small-scale tilapia aquaculture in Zambia, particularly in cage-culture systems on Lake Kariba, where multiple bacterial pathogens are frequently isolated during disease outbreaks. This study evaluated the pathogenicity of Aeromonas spp., Lactococcus garvieae, Acinetobacter spp., and Klebsiella spp. isolated from diseased Nile tilapia (Oreochromis niloticus) from small-scale farms on Lake Kariba, using controlled mono-infection and co-infection experimental models. Twenty (20) healthy Nile tilapia juveniles were intraperitoneally challenged with individual bacterial isolates, a combined co-infection of all four pathogens, or sterile saline (control). Fish were monitored for clinical signs, cumulative mortality, and gross pathology over 21 days post-infection. Histopathological examination of the liver, kidney, spleen, and skin was conducted, and bacteriological re-isolation was performed to confirm pathogen dissemination. Clinical signs appeared earliest in fish infected with Aeromonas spp. and L. garvieae (1 day post-infection) and later in those infected with Klebsiella spp. and Acinetobacter spp. (2 days post-infection), with severity varying by pathogen. Co-infected fish exhibited the most severe disease, characterized by extensive external lesions, multisystem histopathological damage, rapid systemic dissemination, and 100% cumulative mortality by 7 days post-infection. Mono-infections with L. garvieae and Aeromonas spp. resulted in progressive mortality, reaching 100% by 12 and 16 days post-infection, respectively, whereas Klebsiella spp. and Acinetobacter spp. caused comparatively mild pathology and lower mortality. Bacterial co-infection resulted in markedly higher pathogenicity and faster disease progression than single-pathogen infections. These findings highlight the critical role of co-infections in tilapia disease outbreaks on Lake Kariba and underscore the need for improved biosecurity, strengthened husbandry practices, and development of polyvalent vaccines to support sustainable small-scale aquaculture in Zambia.