Seroprevalence of Anaplasma species in an equine population of Southwest Virginia

Abstract

SummaryBackgroundEquine granulocytic anaplasmosis (EGA) is a tick‐borne disease of clinical importance in Southwest Virginia. The diagnosis is based on clinical signs, detection of morulae on a blood smear, PCR and/or detection of antibodies 2–4 weeks post‐infection. A growing interest in stall‐side diagnostic tests has led to an increased use of the IDEXX SNAP 4DX Plus Test® for vector‐borne diseases including EGA.ObjectivesThe objectives of the study were to determine the seroprevalence of Anaplasma spp. in Southwest Virginia and examine the clinical and diagnostic features of symptomatic horses in the same region.MethodsSeroprevalence was evaluated in horses presented for Coggins testing in 2013 and 2019–2020. Clinical and diagnostic features were evaluated in horses presenting with compatible clinical signs from September 2019 to August 2020. Seroprevalence was determined using the IDEXX SNAP 4DX Plus Test®. Complete blood count, blood and buffy coat smears for morulae detection, PCR, immunofluorescence antibody testing (IFAT) and the IDEXX SNAP 4DX Plus Test® were performed on horses with clinical signs of EGA.ResultsSeroprevalence of Anaplasma spp. was 8.5% in 2013 and 11.2% in 2019–2020 (p = 0.43). 35% (6/17) of horses with clinical signs compatible with EGA had confirmed infection via PCR analysis. Three of these had morulae detected on blood smear and one had a positive IDEXX SNAP 4DX Plus Test®. Examination slides made from the buffy coat did not improve detection of morulae. The most common haematological abnormality was thrombocytopaenia.ConclusionsSeroprevalence of Anaplasma spp. is similar to other endemic areas in the United States. In active cases, diagnosis is best made via PCR. As anticipated, the IDEXX SNAP 4DX Plus Test® was not appropriate for the diagnosis of active infections.