Comparison of passive and active immunization of fish against streptococcosis (enterococcosis)

Passive immunization of rainbow trout, Oncorhynchus mykiss (Walbaum), was carried out to determine the persistence of anti-Streptococcus sp. antibodies (ASA) raised in sheep, rabbits or rainbow trout. The protection afforded by passive immunization was compared with the protection obtained from active immunization by immersion in or intraperitoneal (i.p.) injection with formalin-killed cells. Assessments were undertaken concurrently for up to 3 months post-immunization (PI) to evaluate the practical potential of passive immunization. Passively administered sheep and rabbit antibodies were detected in fish sera by enzyme-linked immunosorbent assay for more than 60 days after i.p. injection. Fish responded immunologically to these antibodies and the highest humoral responses to sheep and rabbit ASA occurred at 2 months PI. The relative per cent survival (RPS) of rainbow trout challenged with virulent Streptococcus sp. after an i.p. injection (0-1 ml 100 g-1 fish body weight) of sheep, rabbit or fish ASA was: 88·8, 50 and 0% after 1 month; 33·3, 6·8 and 6·8% after 2 months; and 13·3, O and 6·6% after 3 months PI, respectively. Fish immunized actively had an RPS of 88·8 and 11·1% after 1 month, 38·1 and 4·7% after 2 months, and 36 and 0% after 3 months PI for the i.p. injection and immersion routes, respectively.