Aetiology, epizotiology and pathology of ‘rusty yellow’ skin discolouration of tilapia species Oreochromis niloticus and Tilapia zilli

Abstract

The presence of a rusty-yellow discolouration of the skin of the two tilapia species being cultured at the African 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, mostly in adult fish. Microbiological and histopathological studies, as well as experimental infection trials 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 possible relationship to low dissolved oxygen. The prevalence of the skin discolouration was observed to increase over the 4-month culture period, being somewhat greater in a non-integrated than in an integrated culture system at harvest. Efforts to control the condition with available chemotherapeutics (mostly disinfectants and antibiotics) were ineffective although the organ- ism was observed to be sensitive to terramycin.