Amblyomma cajennense implicated as a vector in a large outbreak of equine piroplasmosis in southern Texas

The apicomplexan hemoparasite Babesia equi, the etiologic agent of equine piroplasmosis (EP), is endemic throughout large parts of the world. The United States has been considered by the World Organization for Animal Health (OIE) to be free of this parasite since 1978. Sporadic cases have occurred in the US, but these are attributed to diagnostic insensitivity at import and there is little evidence for endemic transmission. We report a large outbreak of EP at a ranch in southern Texas that, for the first time since the Florida outbreak of the 1970’s, appears to be the result of endemic transmission. The infection prevalence has reached 100% in some divisions of the index ranch, where more than 290 positive horses have been identified. Amblyomma cajennense (Fabricius) was collected from 78.9% of the horses that had ticks; A. maculatum Koch, Dernmacentor variabilis (Say) and Anocentor nitens (Neumann) were found on 19.7%, 16.2% and 3% of horses, respectively. When A. cajennense and D. variabilis were allowed to reattach and feed on naïve horses in the lab both species successfully transmitted B. equi. A. cajennense has not previously been shown to be a competent vector and we propose that this species is primarily responsible for endemic transmission in this outbreak. Southern Texas is the northernmost limit of the distribution of A. cajennense in the US, suggesting that endemic transmission may be limited to this region.