Lesions induced in Seriola dumerili following exposure to Streptococcus dysgalactiae by oral treatment or immersion

Abstract

Seriola dumerili were infected experimentally with Streptococcus dysgalactiae by oral dosing or immersion. There was moderate mortality after immersion in water containing defined numbers of bacteria, regardless of the dose, whereas the effect on the oral challenge groups depended on the dose of bacteria administered. The characteristic lesions were microabscesses and/or pyogranulomatous inflammation of the caudal peduncle, pectoral and/or dorsal fin, heart and olfactory region. S. dysgalactiae antigen was found within necrotic foci at these sites. There was no difference in distribution of S. dysgalactiae antigen in fish that were exposed by oral or immersion challenge. There was no difference in antigen distribution when fish that died were compared with those that survived and were killed. Immersion exposure is therefore a more effective natural route of infection than oral challenge.