The significance of Aeromonas and Pseudomonas bacteria in association with disease outbreaks in feral and aquaculture fish production is of paramount important. Seven isolates of Aeromonas hydrophila (4), A. veronii (2) and Pseudomonas aeruginosa (1) recovered from normal and ulcerative affected Tilapia species in Mtera Dam were examined for virulence. Invitro experiment was conducted in 10 disinfected 20L glass aquaria filled with chlorine free water. 200 healthy Oreochromis niloticus (50–100gm) were used in which 20 fish were stocked in each aquarium. Two aquaria stocked with 10 fish each were used as control. The fish were acclimatized for two weeks prior to the infection experiment. Each fish except the control were intramuscularly injected with 0.1 ml of the experimental bacteria (concentration, 2.4×108 CFU/ml) using 21/gauge sterile needle. The infected fish were observed for 14 days. The injected bacteria were then recovered from the experimental fish and subjected to morphological, biochemical and antibiotic susceptibility tests. Results showed that; 112 out of 180 infected fish developed clinical abnormalities such as skin darkness, scales detachment, blindness and large irregular hemorrhages on the body surface, fin necrosis, exophthalmia and eye cataract/trachoma within four days and mortality rate of 95%. The recovered strains were motile, gram- negative, and were resistant to Ampicillin, Streptomycin, Amoxyllin and Novobiocin. This study concluded that Aeromonas and Pseudomonas species are responsible for the ulcerative disease outbreaks in Mtera Dam. However, the study is not conclusive as to whether the same bacteria are responsible for development of eye cataract and blindness to the infected fish.
