Читать книгу Parasitology - Alan Gunn - Страница 81

Theileria parva is principally a disease of cattle. Ticks belonging to the genus Rhipicephalus (mainly Rhipicephalus appendiculatus and Rhipicephalus zambesiensis), transmit it, and its distribution is largely determined by the presence of its vectors.

Theileria parva exhibits considerable genotypic diversity. Indeed, a single cow may harbour several distinct genotypes. This complicates vaccine design because there is a lack of cross‐protection between different strains of the parasite (Katzer et al. 2010). The virulence of theileriosis varies between regions and probably relates to the ecology of the tick vector. Where the tick life cycle stages do not usually coincide (e.g., adults plus nymphs/nymphs plus larvae/adults plus larvae), the disease tends to be less virulent (Tindih et al. 2010). This is because a highly virulent parasite would kill its cattle host before there was an opportunity for the next generation of ticks to become infected. Rhipicephalus appendiculatus and R. zambesiensis are typical three host ticks and the larval, nymphal, and adult stages exploit different hosts. After feeding (engorging), the tick drops off to moult or in the case of the adult females to lay their eggs. In subtropical and southern regions of Africa, the ticks are seasonal with one generation per year (i.e., they are unimodal) but in tropical regions where there is high rainfall, up to three generations may occur.