A series of experiments were carried out to investigate some of the poorly understood\ud aspects of the life cycle of Theilefid parva in its rhipicephalid tick vectors. The first\ud series of experiments established that nymphae infected as larvae develop lower levels\ud of infection compared to adults infected as nymphae, while female ticks develop\ud higher infections than males. It was shown that the period of development of\ud sporoblasts into mature sporozoites took on average four days in the nyrnphal ticks\ud compared to five days in the adult ticks. Infection levels developing in different tick\ud instars or sexes appeared to be related to the number and position of type III salivary\ud gland acini. The second series of experiments established that there were considerable\ud differences in the vector competence of different stocks of Rhipicephalus\ud appendiculatus and R. zambeziensis for the transmission of Muguga and Boleni stocks\ud of Yheileria parva. Finally the study established that survival of infected R.\ud appendiculatus and the T parva they harboured was longer under quasi-natural\ud climatic conditions compared to all the laboratory conditions examined. Basically,\ud infection levels in the ticks did not affect the duration of survival of the ticks,\ud however, survival of the parasite appeared to be influenced by the intensity of\ud infection in the tick as the parasites diminished more rapidly in ticks having high\ud infections than in those having low infections. Nymphae and the parasites they\ud harboured survived for shorter periods compared to the adult ticks and their infections.\ud Data generated from these series of experiments will be used to develop quantitative\ud models of T parva dynamics in the tick vectors.\ud The relative importance of the factors influencing the levels of infection developing in\ud the tick vector were analysed statistically by the logistic and Poisson regression.\ud Factors found to play a significant role included tick instar or gender, tick stock,\ud parasite stock, the ambient climatic conditions in which infected ticks survived and the\ud day of tick repletion after infection of the bovine host. Individually, the bovine host or\ud its piroplasm parasitaernia were found to be poor predictors of infection levels\ud developing in the salivary glands of the tick vector. However, when piroplasm\ud parasitaernia was included in a model lacking the days post-repletion variable, the\ud bovine host factor became significant
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