Infection of the host cells ofTheileria annulata withT. parva

Conclusions

The host cells ofT. annulata can be infected withT. parva. With the superinfection of untreatedT. annulata cultures it is not clear if only the few cells not carrying a schizont were susceptible to infection withT. parva or if cells with aT. annulata schizont also became infected withT. parva. In the latter case,T. parva parasites, while going through non-dividing stages (sporozoite, trophozoite, early schizont), may have been diluted out by the rapidly growing and dividing cells transformed byT. annulata.

When the propagation of the cells infected byT. annulata was stopped by the schizonticidal treatment, a high infection rate withT. parva was achieved. However, eventuallyT. annulata-infected cells reappeared in the cultures and by 6 weeks appeared to be overgrowing theT. parva-infected cells. Nevertheless, for a period of 2–3 weeks the cultures consisted of cells, more than 99% of which were infected withT. parva. When inoculated into cattle at a dose of 6×10⁵, these cells failed to initiate infection or induce immunity. In view of the fact that as few as 10⁴ of these cells infected withT. annulata can induce infection and a serological response in cattle, these results indicate that the difference in the capacity of the twoTheileria species to transfer infection is a property of the parasite rather than the host cell type which becomes infected.