A preliminary study of the aetiology, epizootiology and pathology of 'rusty-yellow' skin discolouration of tilapia species (Oreochromis niloticus and Tilapia zilli
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An upsurge in the prevalence of rusty-yellow discoloration of the skin of the two tilapia species, being cultured at the African Regional Aquaculture Centre (ARAC), gave a basis for an urgent clinical and epizootiological studies of the condition. The studies were therefore carried out along with the performance of some catfishes in polyculture with Oreochromis niloticus, under the integrated poultry cum fish farming system. It was observed that this condition affects the tilapia species only, and the antemortem, and postmortem examination revealed that the discolouration is restricted to the surface of the cutaneous tissue; affecting the underside of the exposed part of the scales, the fins and the skin flap covering the scale pockets, mostly in adult fish. Microbiological and histopathological studies, as well as the experimental infection of fish host incriminated a rosette shaped, filamentous, gram positive organism, as the possible aetiologic agent. The organism was characterised as an Actinomyces species based on standard microbiological techniques. Moreover, observations made on the effects of some physico-chemical parameters of the pond water on the prevalence of the skin discolouration condition, suggests a possible environmental influence. Furthermore, the prevalence of the skin discolouration was observed to increase with time, being greater in the non integrated than the integrated culture system, at harvest (four month culture period). However, efforts to control the condition with some available chemotherapeutics (mostly disinfectants and antibiotics) were abortive.