Evidence for a common protective antigenic determinant on sporozoites of several Theileria parva strains.

The antigenic relationship amongst sporozoites of several Theileria parva strans isolated from different parts of East Africa was investigated. It was found that antibodies to T. parva ( Muguga ) sporozoites neutralized the infectivity of homologous sporozoites as well as those of T. parva (Uganda), T. parva lawrencei , T. parva ( Kiambu 5), T. parva ( Kabete 1) and T. parva ( Marikebuni ). The results correlated with those obtained from an in vivo challenge, in that cattle immunized against T. parva ( Muguga ) were either solidly resistant or developed only a transient parasitosis when challenged with T. parva lawrencei or T. parva (Uganda). The immunity exhibited by these animals paralleled the level of anti-sporozoite antibodies, such that animas with high titres were completely protected whereas those with low levels developed transient parasitosis. Furthermore, monoclonal antibodies made against sporozoites of T. parva ( Muguga ) were also capable of neutralizing the infectivity of sporozoites of other strains. These results indicate that the outcome of cross-challenge may depend on the level and avidity of the anti-sporozoite antibodies at the time of challenge. In addition, these findings show that there is a common protective antigenic determinant on sporozoites of all the T. parva strains investigated in this study.