Passive immunization against Histomonas meleagridis does not protect turkeys from an experimental infection
Résumé
Histomoniasis or blackhead is a disease of gallinaceous birds, caused by the protozoon Histomonas meleagridis. As recent regulatory action has removed almost all drugs against this disease from the European market, the development of new prophylactics has become crucial. Identification of the protective immune mechanism would facilitate the choice and development of a vaccination strategy to prevent histomoniasis. In this study, turkeys were either actively or passively immunized and challenged to assess the role of antibody-mediated immunity in the protection against this disease. Active immunization was performed either by experimental infection and treatment or by intramuscular injection with lysed H. meleagridis. Passive immunization was achieved by intraperitoneal administration of the pooled, concentrated and neutralizing antisera from immunized donor animals to naïve turkeys. A significantly higher IgG response was observed after infection and treatment, compared to the response after intramuscular injection, which in turn was higher than those of the placebo and control birds. While active immunization by intramuscular injection of dead H. meleagridis antigens appears not to be protective, immunization of turkeys by infection and treatment did induce protection against histomoniasis. However, no significant level of protection could be observed in the passively immunized birds. These results suggest that serum antibodies to H. meleagridis might not be a key component in the protection against this parasite. It is however possible that antibody concentrations were not sufficient at the mucosal site. Therefore, further investigation on mucosal immune responses is necessary.
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