Factors Influencing the Transmission and Incidence of Tropical Theileriosis (Theileria Annulata Infection of Cattle) in Morocco

A longitudinal epidemiological investigation of tropical theileriosis was carried out in an endemic region of Morocco during the 1991 disease season. This involved approximately 220 cross-bred cattle on 15 farms. Data were collected on the frequency of existing infections, the size of the vector tick population and the incidences of new infections and clinical disease, and these were then analysed using statistical models. The prevalence of subclinically infected carriers of Theileria annulata, the number of adult Hyalomma detritum, the vector, and the probability of becoming newly infected with T. annulata increased significantly with the age of cattle, although the age effect on new infections may be a result of increased tick numbers on older animals. The probability of clinical disease in newly infected cattle was not significantly influenced by age or by the number of adult ticks, but was significantly positively associated with the cattle population on the farm. The number of H. detritum nymphs counted on cattle in the autumn was related significantly to the previous adult tick count on the same animal.