Morbidity, mortality, and risk factors associated with seropositivity in a longitudinal calf study, Narok, Kenya

East Coast fever (ECF), caused by <i>Theileria parva</i> causes devastating loss to livestock keepers in sub-Saharan Africa. A longitudinal study was carried out in Narok County to estimate the incidence and risk factors associated with <i>Theileria parva</i> infection among calves. Calves were recruited within 5 weeks of birth and visited every six weeks to collect farm and calf level factors. A total of 1849 serum samples were collected and tested using an indirect enzyme-linked immunosorbent assay (ELISA). A multilevel multivariable logistic regression was used to determine associations between the seropositivity and different predictor variables. A total of 318 calves were recruited where 62 seroconverted during the follow up period. The overall risk of exposure was 26 %, with ECF-specific morbidity risk of 14.5 %. The cumulative incidence rate was 37 cases per 100 calf-years with higher incidence rate in agro-pastoral system. The crude mortality risk was 5.1 %, but four calves manifested classical signs of ECF resulting to clinical case-fatality risk of 44.4 %. In the final multivariable model, four variables were found to be significantly associated with <i>T. parva</i> seropositivity; calf age (OR 6.9; P < 0.01), calf sex (OR 1.4; P = 0.02), acaricide application (OR 0.5; P < 0.01) and spraying specific body parts (OR 2.5; P = 0.002). The results present the <i>T. parva</i> exposure patterns based on serological response in calves from birth to one year. These results will support evidence-based and effective practices for the management and control of ECF to mitigate the impact on productivity in the livestock sector.