The immunological relationship between <i>Typanosoma evansi</i> and <i>Trypanosoma vivax</i>: Immunization by infection, treatment and challenge findings

Abstract

Trypanosoma evansi and Trypanosoma vivax show very high cross-reactivity in serology; and previous serum neutralization studies demonstrated that anti T. evansi serum contained lytic antibodies that could lyse a large proportion of T. vivax trypomastigotes though anti T. vivax serum had no effect on T. evansi trypomastigotes. This supports the suspicion that the two parasite species are immunologically closely related and prior infection with one species could cross protect against infection with the other species. The possibility of mice infected with T. evansi then treated and challenged with T. vivax being protected against the latter parasite infection and vice versa was investigated. Mice were infected with T. evansi, then treated with Diminasan after the infection had become patent and challenged with T. vivax and vice versa. The challenged mice were monitored for patent infection through examination of their tail blood to establish whether there was any cross protection. Mice were also infected, treated and challenged with homologous trypanosome species to establish whether there was any protection after prior infection followed by treatment and homologous challenge. Mice without prior infection but treated then challenged and those without prior infection and not treated but challenged were included as unexposed and Deminasan controls, respectively. The mean survival time of the Deminasan control mice was not significantly different (p = 0.246183 > 0.05) from the mean survival time of the unexposed control mice. However, mice previously exposed to T. evansi infection then challenged with T. vivax had a significantly higher mean survival time (p = 0.022055 < 0.05) compared to the unexposed control mice. Similarly the mean survival times of mice without prior exposure to T. vivax infection but treated then challenged with T. evansi (unexposed control) and those without prior infection and not treated but challenged (Deminasan control) were not significantly different (p = 0.122966 > 0.05). The mean survival time of mice previously exposed to T. vivax infection then challenged with T. evansi was significantly higher compared to the mean survival times of the unexposed and Deminasan control mice (p = 0.01622< 0.05). Previous exposure to T. evansi infection followed by treatment and homologous challenge conferred 50% protection to mice previously infected with T. evansi then treated and challenges with T. evansi. While previous exposure to T. vivax infection followed by treatment and homologous challenge only prolonged the mean survival time of mice previously infected with T. vivax then treated and challenged with T. vivax.

The above findings, therefore, suggest that T. evansi and T. vivax are immunologically related and prior infection with one species prolongs the survival time of mice previously infected with one species when challenged with the other. Previous exposure to T. evansi infection followed by homologous challenge confers 50% protection to mice previously exposed to T. evansi infection. While previous exposure to T. vivax infection followed by homologous challenge only prolongs the mean survival time of mice previously exposed to T. vivax infection.

Keywords: Immunological relationship, T. evansi, T. vivax, cross protection