title: Evolution and genetic diversity of Theileria.
abstract: Theileria parasites infect a wide range of domestic and wild ruminants worldwide, causing diseases with varying degrees of severity. A broad classification, based on the parasite's ability to transform the leukocytes of host animals, divides Theileria into two groups, consisting of transforming and non-transforming species. The evolution of transforming Theileria has been accompanied by drastic changes in its genetic makeup, such as acquisition or expansion of gene families, which are thought to play critical roles in the transformation of host cells. Genetic variation among Theileria parasites is sometimes linked with host specificity and virulence in the parasites. Immunity against Theileria parasites primarily involves cell-mediated immune responses in the host. Immunodominance and major histocompatibility complex class I phenotype-specificity result in a host immunity that is tightly focused and strain-specific. Immune escape in Theileria is facilitated by genetic diversity in its antigenic determinants, which potentially results in a loss of T cell receptor recognition in its host. In the recent past, several reviews have focused on genetic diversity in the transforming species, Theileriaparva and Theileriaannulata. In contrast, genetic diversity in Theileriaorientalis, a benign non-transforming parasite, which occasionally causes disease outbreaks in cattle, has not been extensively examined. In this review, therefore, we provide an outline of the evolution of Theileria, which includes T. orientalis, and discuss the possible mechanisms generating genetic diversity among parasite populations. Additionally, we discuss the potential implications of a genetically diverse parasite population in the context of Theileria vaccine development.
authors.affiliation: National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido, Japan; Veterinary Research Institute, Peradeniya, Sri Lanka.
authors.forename: Thillaiampalam
authors.initials: T
authors.lastname: Sivakumar
authors.affiliation: National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido, Japan.
authors.forename: Kyoko
authors.initials: K
authors.lastname: Hayashida
authors.affiliation: Division of Collaboration and Education, Research Center for Zoonosis Control, Hokkaido University, Sapporo, Hokkaido, Japan.
authors.forename: Chihiro
authors.initials: C
authors.lastname: Sugimoto
authors.affiliation: National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido, Japan. Electronic address: yokoyama@obihiro.ac.jp.
authors.forename: Naoaki
authors.initials: N
authors.lastname: Yokoyama
mesh_terms: D000818:Animals; D005075:Biological Evolution; D014644:Genetic Variation; D054884:Host-Pathogen Interactions; D006801:Humans; D016794:Theileria / Q000235:genetics* / Q000472:pathogenicity / Q000502:physiology; D013801:Theileriasis / Q000469:parasitology*; D014774:Virulence
keywords: Evolution; Genetic diversity; Immunity; Theileria