Therapeutic and prophylactic effects of a protective monoclonal antibody (MAb-001) against the pathogenic haemoflagellate Cryptobia salmositic

We recently produced a species-specific monoclonal antibody (MAb-001) against a surface antigen of Cryptobia salmositica Katz 1951. When a single dose (4 % body of weight in mi) of MAb-001 was injected into fish infected with C. salmositica, subsequent parasitaemias were significantly (p < 0.05) reduced at 48 h after treatment. Parasitaemias were still significantly lower than in control fish (injected with cold-blooded vertebrate Ringer's solution, CBVR) 1 wk after injection of MAb-001. There was no significant difference in parasitaemias between injection of a single dose or a double dose of MAb-001. Injection of MAb-001 into fish before infection and at 1 wk after infection did not prevent infection. Parasitaemias in fish which received MAb on Days 0, 1, 2, 3, 4, 5, 6 (daily injected fish) were significantly higher than in fish which received CBVR. The production of complement-fixing antibody against C. salmositica in MAb daily injected fish was significantly lower than in those injected with CBVR. Antibodies against MAb-001 were detected in fish injected daily with MAb-001. These results suggest that a single dose of MAb-001 is therapeutic and perhaps can be used to prevent mortality of infected fish. Injection of MAb-001 every day in the first week after infection induced significant antigenic competition between parasites and MAb-001. Consequently, these fish produced less complement-fixing antibody against C. salmositica and this resulted in higher parasitaemias than in CBVR-injected fish.