Abstract

Channel catfish that survive infection with the parasitic ciliate, Ichthyophthirius multifiliis (Ich), are resistant to subsequent challenge, and have serum antibodies that immobilize the parasite in vitro. Our laboratory has characterized the proteins (referred to as immobilization antigens, or i-antigens) that stimulate the production of immobilizing antibodies with the idea that these proteins also elicit protective immunity. Vaccine trials were carried out to directly test the ability of i-antigens to elicit protective immunity. Using an immobilizing mouse monoclonal antibody (G3-61) as ligand, i-antigens from Ich isolate G5 were purified to homogeneity by immunoaffinity chromatography. Fifty channel catfish, each weighing 10-15 g, were injected intraperitoneally (ip) two times at a 14 day interval with 10 µg of purified i-antigen in complete or incomplete Freund's adjuvant. Negative control fish were injected similarly with either BSA, or an irrelevant 14kD Ich protein. Positive control fish were injected ip with live theronts, 8000 and 10000 cells per fish at a 5 week interval, without adjuvant. (This dose of live organisms has been shown previously to elicit protective immunity.) All fish were challenged 12 weeks after the first injection. 72% of the fish injected with purified i-antigen, and 59.2% of fish injected with live organisms survived challenge. All negative control fish died within 16 days of exposure to infective parasites. Furthermore, there was a significant difference (p < 0.001, Kruskal-Wallis test ANOVA) in the median days to death among the control fish and the small percentage of fish that died following vaccination with i-antigen or live theronts. The serum antibody response in immunized fish was determined by in vitro immobilization. Fish injected with i-antigen or live Ich theronts had a mean immobilizing antibody titer of >600 at week 9 after the first injection, while those injected with BSA or 14kDa had no response. Cutaneous mucus antibodies against i-antigen tested by ELISA were only detectable at week 11 after the first injection. These results indicate that vaccination with purified i-antigen elicits protective immunity and point the way to the development of an effective subunit vaccine against Ichthyophthirius multifiliis.

Acknowledgements

This work was supported by USDA National Research Initiative Competitive Grants Program. The authors wish to thank Jane Noe, Joanne Maki for their technical assistance.

References

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