Four Theileria annulata cell lines were characterised at low passage levels using two polymorphic markers and then used to infect calves. Their virulence seemed to be related to the number of genotypes present within the cell line. In all, 3 of the 4 cell lines were cultured up to passage 100 or 200 and inoculated into calves. Their characterisation using the same markers indicated that the attenuation was related to a reduction in the parasite polymorphism down to a single genotype. The immunogenicity of the three attenuated cell lines was assessed in calves using two types of challenge. Optimal protection was observed against homologous challenges. The level of immunity to heterologous challenges appeared to decrease with attenuation and seemed to depend on the cell line used.

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This study describes polymorphism in Theileria annulata, an intracellular protozoan parasite of bovine leucocytes and red blood cells. Fifty-three different stocks of T. annulata, isolated from 17 sites (districts) in Tunisia, have been characterized by anti-parasite monoclonal antibody (MAb) reactivity, glucose phosphate isomerase (GPI) isoenzyme electrophoresis, and Southern blotting with two genomic DNA probes. These appears to be considerable diversity amongst T. annulata stocks from Tunisia, no two isolates being identical, even those from animals on the same farm. Two distinct antigenic populations were detected by MAb 7E7. They were defined by negative and positive cells in the indirect fluorescent antibody test. The percentage of positive cells in different isolates ranged between 0 and 100%. The population variation seen by GPI analysis and DNA probes was greater; 7 different GPI phenotypes were identified amongst the stocks studied, while DNA probes T. annulata Tunis (TaT) 17 and 21 detected up to 5 different variants. The majority of isolates were shown to contain more than one parasite population, the number of variants per isolate ranging from 1 to 4. No correlation between particular parasite phenotypes or genotypes and their geographical site of isolation was observed. Selection of parasite populations in vivo and in vitro is also discussed.

Six stocks of Theileria annulata isolated from the Sudan and nine stocks of T. parva, isolated in Kenya and Malawi were grown in bovine lymphoblastoid cell lines. Lysates prepared from the infected cultures were examined electrophoretically on thin layer starch gels for evidence of glucose phosphate isomerase polymorphism. The six stocks of T. annulata showed major variations in their parasite enzyme patterns but no variation was detected in nine stocks of T. parva.