ARS | Publication request: Effect of immunization of channel catfish with inactivated trophonts on serum and cutaneous antibody titers and survival against Ichthyophthirius multifiliis

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2008
Publication Date: N/A

Interpretive Summary: Ichthyophthirius multifiliis (Ich) is a severe fish parasite that causes high mortality in many cultured fish. Chemical treatment of Ich infection is difficult since the parasite penetrates into fish skin and gill, the high cost involved in therapy, and the concern for food and environmental safety. Acquired immunity is an alternative way to prevent fish death against this parasite. However, various results on immune protection have been reported when using parasite trophonts as antigen. Very little is known what factors may influence fish protective immunity after immunization with trophonts. The current study evaluated influence of 1) trophont inactivation and 2) doses of trophont antigen on protective immunity of channel catfish against parasite Ich. No statistical difference was noted on anti-Ich antibodies and survival for fish immunized with differently inactivated trophonts (1% formalin, 3% formalin or freeze thawed trophonts). Amount of antigen received by fish had a great effect on fish immune protection. Fish immunized with the higher dose of trophont antigen showed higher survival than those that received low or no trophont antigen. The results in this study provide new information on factors influenced the immunity of catfish against parasite Ich when using trophont as antigen. The information will be benefiting the development of Ich vaccines.

Technical Abstract: Two trials were conducted to determine the effect of immunization of channel catfish with inactivated trophonts on serum and cutaneous antibody titers and survival against Ichthyophthirius multifiliis Fouquet (Ich). In trial I, catfish were intraperitoneally (IP) immunized with: 1) 1% formalin-inactivated trophonts, 2) 3% formalin-inactivated trophonts and 3) freeze-thawed trophonts. Positive and negative control catfish were immunized with live theronts and 5% bovine serum albumin (BSA), respectively. At day 14, 28 and 50 post immunizations, no statistical difference was noted in serum or cutaneous anti-Ich antibody titers to formalin-inactivated trophonts or freeze-thawed trophonts. The survival of catfish challenged with live trophonts ranged from 33.3% - 43.3% for the formalin-inactivated or freeze-thawed trophonts at 50 d post-immunization. The survival of the live theront and BSA-immunized catfish was 93.3 and 0%, respectively. In trial II, catfish were IP-immunized with sonicated trophonts at doses of 1) 5 trophonts g-1 fish, 2) 10 trophonts g-1 fish, 3) 20 trophonts g-1 fish, and 4) 5% BSA as the control. Fish immunized with 10 or 20 trophonts g-1 fish showed highest serum (1/210- 1/480) and cutaneous antibody titers (1/48-1/52), respectively at 22 d post-immunization and survival (63.3 -60.0%). The fish immunized with 5 trophonts g-1 fish had titers of 1/52 and 1/12 for serum and cutaneous antibody and survival of 23.3%. BSA immunized catfish had background titers and a survival of 6.7%. There was a significant correlation between doses of sonicated trophonts used to immunize and catfish survival (correlation coefficient=0.859, p<0.01). These results indicate that doses of sonicated trophonts, but not formalin-inactivated or freeze-thawed trophonts provided both serum and cutaneous antibody responses and survival to live trophont challenge.