Comparison of Seroprevalence and Identification of Risk Factors for Theileria equi in Horses From Vector-Free and Infested Areas in Southern Brazil.

The apicomplexan hemoprotozoan Theileria equi is a tick-borne pathogen that causes disease in equids, and together with Babesia caballi, causes equine piroplasmosis (EP). Many ticks are associated with EP around the world, and in South America three species may be related: Dermacentor nitens, Amblyomma sculptum, and Rhipicephalus microplus, as they are commonly found in horses. Among the species cited above, only R. microplus is found in Rio Grande do Sul state, Southern Brazil. In addition, this state has the only area legally free of R. microplus in Brazil. This study aimed to compare the seroprevalence for T. equi on farms located in a tick-free area (23 farms, 215 horses) and a tick-infested (25 farms, 141 horses) adjacent areas, as well as to identify potential risk factors for exposure to T. equi. Blood was collected from 356 horses from both areas, and later an indirect enzyme-linked immunosorbent assay was performed to detect anti-T. equi antibodies. Besides the blood collection, questionnaires and interviews were conducted in an attempt to identify potential risk factors. The R. microplus-free zone had 6% seropositive horses against 70% in the tick-infested zone. Previous observation of ticks on horses of the herd increased four times the odds of a horse to be seropositive for T. equi, and by three times if ticks were observed on cattle which share paddocks with horses on the farm. The results showed a large difference in T. equi seroprevalence between tick-infested and free areas, and also emphasized the relevance of R. microplus in the epidemiology of T. equi. The study also reveals the potential of the establishment of a T. equi-free zone for horse breeding in Southern Brazil.