Population dynamics of Theileria sergenti in persistently infected cattle and vector ticks analysed by a polymerase chain reaction.

Theileria sergenti Shintoku stock consists of 2 parasite populations bearing 2 allelic forms of p33/32, an immunodominant piroplasm surface protein. Parasite population changes during parasite passages among cattle and tick vectors, and during persistent infection in individual calves were analysed by using allele-specific polymerase chain reaction (PCR). The parasite DNAs were prepared from piroplasms from calves which had been infected with Shintoku stock by inoculation of sporozoite stabilates or parasitized erythrocytes, and from sporozoite stabilates which had been prepared from Shintoku stock-infected ticks. Changes in a dominant parasite population were demonstrated during transmission from calves to vector ticks and from infected ticks to calves. Parasite population changes were also apparent during persistent infection in cattle over several months, and this change is thought to occur under host immune pressure. The results of this study indicate that expression of diverse forms of p33/32 may play a role in parasite persistence within mammalian hosts and its transmission from tick vector.