Survey on the Presence of Equine Tick-Borne Rickettsial Infections in Southcentral United States

Abstract

Although ticks are known vectors of pathogens across a range of hosts, there is limited research on emerging tick-borne diseases of horses in the United States. Tick surveys from other regions suggest Rickettsia spp. and Ehrlichia spp. may be clinically relevant in horses. To better understand the transmission risk of these tick-borne rickettsial disease agents to horses, ticks were collected from horses in Oklahoma. Ticks for the current study (306 Amblyomma americanum, 20 Dermacentor albipictus, 19 D. variabilis, and 7 A. maculatum) were evaluated for Rickettsia spp. and Ehrlichia spp. using polymerase chain reaction and sequence analysis. Serum samples from infested and noninfested horses were evaluated for antibodies to R. rickettsii using indirect fluorescence antibody testing and Ehrlichia spp. and Anaplasma spp. using a commercial enzyme-linked immunoassay. Of the horses with tick infestations, 71.4% hosted at least one tick with a rickettsial agent detected. Rickettsia spp. were identified in 25.9% (91/352) of the ticks tested with R. amblyommatis (80.2%; 73/91) most often detected. Ehrlichia spp. were identified in 2.8% (10/352) of the ticks tested with E. ewingii most often identified. Serologic screening revealed no horses with antibodies to R. rickettsii or Anaplasma spp., but 29.6% of the examined horses had circulating antibodies to Ehrlichia spp. Together, these results demonstrate the presence of Rickettsia spp. and Ehrlichia spp. in equine ticks and evidence of past or current infection with Ehrlichia spp. in Oklahoma horses which strongly suggests there is a need to explore the relationship between these agents and equine health.

Keywords: Amblyomma; Dermacentor; Ehrlichia; Horse; Rickettsia; Tick.