A preliminary study on the attenuation of Tunisian schizont-infected cell lines of Theileria annulata.

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.