Infection of mice with Neospora caninum (protozoa: apicomplexa) does not protect against challenge with Toxoplasma gondii

Abstract

Neospora caninum and Toxoplasma gondii are structurally related protozoal parasites of mammals that may cause abortion and neonatal morbidity and mortality. Groups of mice were subcutaneously inoculated with 105 live zoites of the NC-1 or NC-3 isolates of N. caninum and reinoculated with an identical number of live zoites 2 weeks later. Groups of mice which were injected subcutaneously with Hanks balanced salt solution served as controls. Three weeks after the final N. caninum inoculation, one-half of the mice were inoculated subcutaneously with 2.5 x 104 zoites of the RH isolate of the T. gondii and the other half were inoculated subcutaneously with 2.5 x 104 zoites of the GT-1 isolate of T. gondii. Serum samples taken from mice on the day of T. gondii inoculation were negative for specific antibodies to T. gondii, but mice inoculated with N. caninum had reciprocal titers of ≥ 800 to this protozoan. All of the mice died after challenge with T. gondii, and no significant differences (P > 0.05) between the survival times of mice inoculated with either isolate of N. caninum and those of control mice were seen. This study indicates that N. caninum and T. condii are distinct biologic entities and not closely related isolates.