Infection, immunity and the development of local skin reactions in rabbits infected with cyclically-transmitted stocks of Trypanosoma congolense.

Abstract

The development of local skin reactions following bites by tsetse flies (Glossina morsitans) infected with Trypanosoma congolense was observed in normal rabbits, in rabbits with concurrent infections with T. congolense and in rabbits challenged after immunization by infection and treatment. Local reactions developed in normal rabbits within seven days of fly bite and were resolved by approximately 19 days after infection. In rabbits with concurrent infections no typical local reactions developed when they were bitten by infected tsetse flies. By allowing a single infected fly to bite a rabbit sequentially it was established that suppression of reaction development to homologous secondary challenge occurred within three to eight days of primary infection. Suppression of reaction development also occurred in infected rabbits challenged with an antigenically heterologous trypanosome stock within 14 to 18 days of primary infection, suggesting that reaction suppression is not associated with the specific immune response. It was also found that no neutralizing antibodies had developed against the parasites used for secondary infection by 28 days post-challenge. Rabbits were immunized by allowing an infected tsetse fly to feed on a rabbit and seven days later, when local reactions had appeared, treating it with homidium chloride. All rabbits challenged within 75 days of immunization by a tsetse fly infected with a homologous trypanosome stock resisted re-infection and some animals were immune up to 300 days after treatment. In most of the immunized animals specific neutralizing antibody activity against homologous parasites was low, suggesting that local expression of immunity in the skin might be more important in resistance to cyclically transmitted infections than serum antibodies. No local reactions developed on immune rabbits challenged with a homologous trypanosome stock but immunized rabbits challenged with heterologous trypanosome stocks developed typical local reactions.