Vaccination of rainbow trout against Streptococcus iniae infection: comparison of different routes of administration and different vaccines

Antibody production and clinical efficacy (relative percent survival RPS) were measured in 40±5g rainbow trout after immunization with two types of Streptococcus iniae vaccines consisting of formalin killed cells (FKC) and FKC enriched with the bacterial extracellular products (ECP) administered by intraperitoneal (i.p), immersion and oral routes at 16±1 ̊C for 18 weeks. No significant difference was found in antibody levels among the fish i.p immunized with FKC enriched with ECP plus Freunds' adjuvant (FA), FKC plus FA and FKC vaccines (P>0.05), whilst the antibody production was significantly higher in these three groups than fish immunized by immersion and oral routes of FKC and FKC enriched ECP (P<0.05). Also, fish immunized by immersion route with FKC showed significantly higher antibody titer than fish orally vaccinated with FKC with or without ECP until 12 weeks post-immunization (P<0.05). No significant difference was found in antibody titer between orally vaccinated fish and control groups (P>0.05). The RPSs ranging 82.6-100, 73.9-95 and 73.9-91.7% were obtained in the fish intraperitoneally immunized with FKC enriched ECP plus FA, FKC plus FA and FKC vaccines, respectively, compared to 0% survival for the control fish. Also, RPS in fish vaccinated by the immersion route was in the range 45.8-30.4% after 18 weeks post-vaccination. Efficacy of oral vaccination of fish with FKC plus ECP was in range of 8.7-29% and that of fish orally vaccinated with FKC resulted in 8.7-20.8% protection.