Published online by Cambridge University Press: 21 August 2009
INTRODUCTION
The genus Theileria comprises tick-transmitted sporozoan protozoa that are the causative agents of a variety of disease syndromes in domestic and wild ruminants, and are collectively responsible for economic losses amounting to hundreds of millions of dollars annually in sub-Saharan Africa and Asia. Theileria are unique among protozoa, in that certain species are capable of immortalizing either mammalian lymphocytes, or cells of the monocyte/macrophage lineage that they infect. Theileria has been included within a subphylum designated the Apicomplexa, based on the common possession of an apical complex containing secretory organelles involved in invasion, or establishment, in the cells of their mammalian and invertebrate hosts. However the evolutionary and functional equivalence of the apical complex between different genera and hence the taxonomic validity of the Apicomplexa remains unclear. Analysis of 18S ribosomal RNA gene sequences demonstrates that the genus Theileria is phylogenetically most closely related to Babesia, a genus of tick-borne protozoan infective to the red cells of mammals including domestic livestock, and more distantly to the genus Plasmodium which causes malaria in humans and other species of vertebrates (Allsopp et al., 1994). There are similarities, but also significant differences, in features of the life cycle, genome organization and mammalian host immune responses to infection between Theileria and Plasmodium.
Economically important Theileria species that infect cattle and small ruminants are transmitted by ixodid ticks of the genera Rhipicephalus, Amblyomma, Hyalomma and Haemaphysalis. Theileria species infective to domestic ruminants are summarized in Table 14.1.
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