Humoral immune response of rainbow trout to two bacterins prepared from Streptococcus iniae evaluated by ELISA

This study evaluated the humoral immune response of rainbow trout (Oncorhynchus mykiss) against Streptococcus iniae, a regional fish pathogen, the aetiological cause of streptococcosis with a septicemic infection in rainbow trout cultured in Fars province as well as other provinces in Iran. For this, two different preparation of antigens including heat killed and formalin killed antigens were prepared. Ninety fish in six groups were exposed to these antigens via three routes of administration including intraperitoneal injection (107 killed bacterium/fish), immersion (107 killed bacterium/ml) and oral (107 killed bacterium/fish). For control group 15 rainbow trout injected with phosphate buffered saline were used. Fish were bled 30 days after immunization, serum sample was collected from each fish and the fish humoral response was detected by ELISA. Results showed that using both antigens by injection route induced the highest humoral response similarly. Immersed fish in both antigens showed significant humoral responses with a significant higher response by heat killed streptococcal antigens (P<0.05). Oral immunization of rainbow trout with streptococcal antigens did not reveal a significant immune response when compared with control.