Isolation and pathogenicity of a Bacillus sp. associated with a septicaemic condition in some tropical freshwater fish species

Abstract

Summary Observations made over a three-year period at the Fish Diseases Laboratory of the African Regional Aquaculture Centre (ARAC), Nigeria, revealed the gradual emergence of a new, highly infectious septicaemic condition in some widely cultivated freshwater fish species. The broad host range included: Heterobanchus bidorsalis, Clarias gariepinus, “Heteroclarias” (a hybrid of these two species, male and female respectively), Chrysichthys nigrodigitatus, and Cyprinus carpio. Clinical signs and pathological lesions associated with the condition were tyical, irrespective of the fish species affected; natural outbreaks apeared to be associated with stress due to environmental factors. The bacterium isolated from moribund and freshly-dead fishes was identified as a Bacillus sp., based on the observed cultural, morphological and biochemical characteristics. Fish reinfection trials confirmed that the isolate was the causative agent of the condition. Antibiotic sensitivity tests showed that the organism was sensitive to tetracycline hydrochloride.