Aetiology, epizootiology and pathology of ?rusty-yellow? skin discolouration of tilapia species Oreochromis niloticas and Tilapia zilii

Abstract

The resence of a rusty-yellow discolouration of the skin of the two tilapia species being cultured at the ifrican Regional Aquaculture Centre (ARAC) was studied using clinical and epizootiological methods. The condition affected the tilapia species only. Ante-mortem and post-mortem examination revealed that the discolouration was 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, most1 in adult fish. Microbiological and histopathological studies, as well as experimental infection triai demonstrated that a rosette-shaped, filamentous gram-positive bacterium, was the aetiologic agent. This organism was characterised as an Actinomyces species based on standard microbiological techniques. Correlation between physico-chemical parameters of the pond water and the prevalence of the skin discolouration, suggested a ossible relationship to low dissolved oxygen. The prevalence of the skin discolouration was observecfto increase over the &month culture eriod, being somewhat reater in a non-integrated than in an integrated culture system at harvest. Eioorts to control the coniition with available chernothera eutics (mostly disinfectants and antibiotics) were ineffective although the organism was observed to ge sensitive to terramycin.