Seroprevalence of Anaplasma phagocytophilum in the equine population of Southwest Virginia

Background: Equine granulocytic anaplasmosis (EGA), caused by the organism Anaplasma phagocytophilum, is a tick-borne disease of clinical importance in Southwest Virginia. The disease is recognized worldwide and causes pyrexia, anorexia, limb edema, and lethargy. Diagnosis in endemic areas is often based on clinical signs, but confirmation of infection can be made via detection of morulae on a peripheral blood smear or polymerase chain reaction analysis (PCR) at the time of disease or by serologic detection of antibodies 2-4 weeks post infection. There is growing interest in stall-side methods for diagnosis of various equine diseases which has led to an increased use of the SNAP 4DX Plus Test® for vector-borne diseases.

Objectives: Determine seroprevalence of antibodies to A. phagocytophilum in the equine population of Southwest Virginia and changes in seroprevalence compared to samples taken 6 years earlier. Determine the percentage of horses with clinical signs consistent with EGA that were positive for A. phagocytophilum infection and assess common presenting clinical signs, hematologic variables, and confirmatory diagnostic test results.

Animals: Seroprevalence was evaluated in horses presented for routine annual Coggins testing in 2013 and 2019-2020. Clinical features of disease and diagnostic test results were evaluated in horses presenting with clinical signs compatible with A. phagocytophilum infection from September 2019-August 2020.

Methods: Seroprevalence was determined using the IDEXX SNAP 4DX Plus Test® on serum collected from horses presenting for annual Coggins testing in 2013 and 2019-2020. Samples collected in 2013 had been stored at -7580 degrees F since collection. Age, sex, county of residence, and month of sampling were statistically analyzed in the seroprevalence population. Horses presenting with clinical disease consistent with EGA from September 2019-August 2020 had the following diagnostic tests performed: complete blood count (CBC), blood smear for morulae detection, polymerase chain reaction (PCR) analysis, immunofluorescence antibody testing (IFAT), and the IDEXX SNAP 4DX Plus Test®.

Results: Seroprevalence of A. phagocytophilum in the equine population of Southwest Virginia increased from 8.5% in 2013 to 11.2% in 2019-2020, although this increase was not statistically significant. In the 2019-2020 population, month of sampling was significantly associated with presence of antibodies to A. phagocytophilum. Positive samples were more common from November-February than other times of the year. When the two sample time periods were combined, sex was significantly associated with presence of antibodies to A. phagocytophilum with geldings more likely to be seropositive. Within the clinical case population, 35% of horses with clinical signs compatible with equine granulocytic anaplasmosis had confirmed infection. The most common hematologic abnormality in affected horses was thrombocytopenia. PCR analysis was the most sensitive diagnostic test to diagnose infection followed by identification of morulae on blood smears.

Conclusions: Seroprevalence of A. phagocytophilum is similar to other endemic areas in the United States and appears to be increasing over time. In active clinical cases, diagnosis is best made via PCR or detection of morulae on a blood smear. The SNAP 4DX Plus Test® was not appropriate for diagnosis of active EGA in acute cases.

Seroprevalence of Anaplasma phagocytophilum in the equine population of Southwest Virginia