The study investigated disease outbreaks in farmed Oreochromis niloticus (Nile tilapia) establishing bacterial etiological agents. The study was conducted in central and southern regions in Zambia among large commercial producers . A total of 172 fish samples (fingerlings, grow out and breeders) from 6 farms in 4 districts were were aseptically collected and bacteria isolated from the kidney, liver, brain, and spleen. The isolates were identified by their morphological characteristics, conventional biochemical tests . Furthermore , virulence genes of Aeromonas spp and Lactococcus garvieae were detected through PCR . From the survey samples, a total of 97 isolates of the 3 genera of bacteria of interest were identified . Co-infection of 2 or 3 bacterial species was observed in the internal organs (brain, liver, kidney) of the diseased fish.
Among the mono-infections, Aeromonas spp was the most prevalent bacterial pathogens isolated from brood stock (25/100, 25%) and grow out fish (13/63, 20.6%).
The 2-pathogens co-infection, Lactococcus garvieae and Aeromonas spp, were the most prevalent combination of bacterial pathogen which was observed in Fingerlings (22%), grow out (15.9%) and brood stock (17%). The 3-pathogen co-infection, Lactococcus garvieae , Aeromonas spp and Flavobacterium columnare , was only recorded in fingerlings (11.1%) and brood stock (3%).
The detection of virulence genes in Aeromonas spp revealed that t he hemolysin (hly) was the commonest (50%) among single virulence genes detected in diseased grow out (26.5%) and broodstock fish (23.5%) . With Lactococcus garvieae isolates , Hemolysin 3 (hly 3) was the commonest (55.3%) single virulence gene that was detected in all diseased fish types .
To our knowledge, this is the first study to establish the occurrence of several bacteria species and their virulence genes infecting tilapia in both cage and pond culture systems in central and southern Zambia . The current study provides baseline information for future reference and fish disease management in Zambia.