Immunization of mice against Trypanosoma cruzi. The effect of size of dose, and route of injection, of immunizing and challenge inocula.

Groups of CD-1 mice were immunized with a vaccine prepared from freeze-thawed cultured epimastigotes of Trypanosoma cruzi, strain Y, with or without saponin as adjuvant. The vaccine was injected by the subcutaneous (s.c.) or intraperitoneal (i.p.) route. Immunized and control mice were challenged with homologous blood trypomastigotes by the s.c. or i.p. route and the former route was found to produce a far more severe and uniform infection in both control and immunized mice. Subcutaneous challenge of vaccinated mice demonstrated an improvement in the protective effect of the vaccine when saponin was included, but following i.p. challenge equally good protection was achieved with or without saponin. Thirty-nine of 40 immunized mice challenged i.p. survived the infection. A strong correlation was shown between the size of both immunizing and challenge inocula and the protective effect of the vaccine. Immunized mice which survived challenge were resistant to reinfection, but all were shown to carry persistent sub-patent parasitaemia.