A study of the lesions induced in Seriola dumerili by intradermal or intraperitoneal injection of Streptococcus dysgalactiae.

Fish of the species Seriola dumerili were infected experimentally with the pathogen Streptococcus dysgalactiae. Intradermal (ID) injection of S. dysgalactiae resulted in moderate mortality regardless of the dose of bacteria injected, whereas intraperitoneal (IP) injection caused greatest mortality in the group of fish receiving the highest dose of bacteria (10(9) colony forming units/ml). On necropsy examination of affected fish, the most striking change was microabscessation and/or granulomatous inflammation of the heart, caudal peduncle, pectoral and/or dorsal fin and olfactory region. The lesions in the atrial myocardium and arterial cone consisted of severe arterial thrombosis, granulomatous valvular endocarditis and epicarditis. S. dysgalactiae was cultured from these lesions and S. dysgalactiae antigen was demonstrated by immunohistochemistry within these tissues. The mortality in these fish is therefore considered to reflect bacterial septicaemia and systemic granulomatous inflammatory disease.