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Theileria annulata and Theileria parva both possess a major surface antigen on the sporozoite stage of the life-cycle, called SPAG-1 and p67, respectively. In each case, these antigens are vaccine candidates and have been shown to induce a degree of homologous protection in earlier work. These antigens share sequence homology and are serologically cross-reactive. Here, we confirm that these antigens confer protection against homologous species challenge. More importantly, they mutually confer a degree of cross-species protection raising the prospect of a common vaccine in the future.

This study examines several aspects of a polymorphic, immunodominant molecule (PIM) found in the protozoan parasite, Theileria parva. The antigen is present in all T.p. parva stocks examined, and in the related subspecies, T.p.bovis and T.p.lawrencei. It is the predominant antigen recognized by antisera from immune cattle on Western blot analysis of schizont-infected lymphocytes, and is the only antigen which has been shown to react with anti-schizont monoclonal antibodies (MoAbs) on Western blots or in immunoprecipitations. The antigen shows polymorphism in both size and expression of antibody epitopes among the different stocks of T. parva. The antigen is present in sporozoites as well as schizonts.

Sera from cattle infected with three Trypanosoma congolense clones (ILNat 2.1, ILNat 3.1 and ILRAD 588) derived from different stocks were analysed for the presence of specific antibodies against the surface glycoproteins (VSGs) of the infecting trypanosomes using the solid and liquid-phase radioimmunoassays and the neutralization of infectivity test. High levels of IgM, IgG1 and IgG2 antibodies against the VSGs of the infecting variable antigen types (VATs) as well as other VATs that arose during the course of infection were detected. In addition, 11 out of 12 infected animals showed recurrent peaks of antibody activity against the infecting trypanosomes. The recurrent peaks of antibody activity against the VSGs of the infecting organisms would suggest either a reappearance of trypanosomes of the infecting types or emergence of organisms that bear similar surface determinants. In contrast to the studies on murine trypanosomiasis, there was little or no antibody activity against 2,4,6-trinitrophenylated bovine serum albumin (TNP-BSA) in sera from these cattle.

Cattle were vaccinated with Brucella abortus (S19) vaccine during acute (25 days) and chronic (25 weeks) Trypanosoma congolense and chronic Trypanosoma vivax (25 weeks) infections in order to determine the effect of such infections on the antibody response to the vaccine. It was found that the specific antibody responses of IgG1 and IgG2 sub-classes were profoundly depressed (80%) in both the acute and chronic infections with T. congolense. Whereas IgM antibody response was also profoundly depressed (90%) in cattle with the acute infection, it was only 50% depressed in those with chronic infection. There was no depression of IgG1, IgG2, or IgM in cattle infected with T. vivax. These animals, however, had no detectable parasitaemia at the time of vaccination and thereafter. These results suggest that during acute infection with T. congolense depressive mechanisms could be acting on the afferent arm of the immune response, namely, antigen recognition and/or processing.