Passive immunization against Histomonas meleagridis does not protect turkeys from an experimental infection

Histomonosis or blackhead is a disease of gallinaceous birds, caused by the protozoan 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 histomonosis. In this study, turkeys were either actively or passively immunized and were then challenged to assess the role of antibody-mediated immunity in the protection form this disease. Active immunization was performed either by experimental infection and treatment or by intramuscular injection with lysed H. meleagridis. Passive immunization was attempted by intraperitoneal administration of pooled, concentrated, neutralizing antisera from immunized donor animals to naive turkeys. A significantly higher IgG response was observed after infection and treatment than after intramuscular injection, which in turn was higher than the responses of placebo and control birds. While active immunization of turkeys by intramuscular injection of dead H. meleagridis antigens appeared not to be protective against histomonosis, immunization by infection and treatment did induce protection. However, no significant level of protection could be observed in the passively immunized birds. These results suggest that serum antibodies to H. meleagridis may not be a key component in the protection against this parasite. It is, however, possible that the concentration of antibodies at the mucosal site is insufficient. Therefore, further investigation on mucosal immune responses is necessary.