Theileria: intracellular protozoan parasites of wild and domestic ruminants transmitted by ixodid ticks

Abstract

Theileriaare economically important, intra-cellular protozoa, transmitted by ixodid ticks, which infect wild and domestic ruminants. In the mammalian host, parasites infect leukocytes and erythrocytes. In the arthropod vector they develop in gut epithelial cells and salivary glands. All four intra-cellular stages ofTheileriasurvive free in the cytoplasm. The schizont stages of certainTheileriaspecies induce a unique, cancer-like, phenotype in infected host leukocytes.Theileriaundergoes an obligate sexual cycle, involving fusion of gametes in the tick gut, to produce a transiently diploid zygote. The existence of sexual recombination inT.parvahas been confirmed in the laboratory, and is presumed to contribute to the extensive polymorphism observed in field isolates. Key parameters inT. parvapopulation dynamics are the relative importance of asymptomatic carrier cattle and animals undergoing severe disease, in transmission of the parasite to ticks, and the extent of transmission by nymphs as compared to adult ticks. Tick populations differ in vector competence for specificT. parvastocks. Recombinant forms ofT.parvaandT. annulatasporozoite surface antigens induce protection against parasite challenge in cattle. In future, vaccines might be improved by inclusion of tick peptides in multivalent vaccines.