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 / Master of Science / Equine granulocytic anaplasmosis (EGA) is a common tick-borne disease in the United States and worldwide. The causative bacteria, Anaplasma phagocytophilum, also infects humans, dogs, and various domestic animal species. In horses, signs of disease include fever, decreased appetite, leg swelling, and depression. Diagnostic testing that is both accurate and timely is still lacking. The point-of-care SNAP 4DX Plus Test® used to diagnose vector-borne infectious disease in dogs has been suggested for similar use in horses. The objectives of the current study were to determine seroprevalence of antibodies to A. phagocytophilum in the equine population of Southwest Virginia and to characterize the clinical signs and diagnostic test findings of horse with clinical signs of EGA. Seroprevalence was determined using the SNAP 4DX Plus Test®. Serum samples were obtained from horses presenting for annual Coggins testing in 2019-2020. Samples from 2013 were also tested to determine if seroprevalence had increased. Horses presenting with clinical signs consistent with A. phagocytophilum were examined by a veterinarian and had blood drawn for a complete blood count (CBC), blood smear evaluation, polymerase chain reaction analysis (PCR), immunofluorescent antibody testing (IFAT), and the SNAP 4DX Plus Test®. Seroprevalence in 2019-2020 was 11.2% and 8.5% in 2013. This is similar to other endemic areas in the United States and Europe. In horses sampled from 2019-2020, the month of sampling was significantly associated with presence of antibodies to A. phagocytophilum with most of the positive samples being identified in November through February. Geldings were more likely to be seropositive than mares. Thirty five percent of horses with signs consistent with EGA were confirmed to have the disease. Within this population, PCR analysis and/or detection of morulae on the blood smear were reliable indicators of disease while diagnostic techniques utilizing serology were unreliable. This is the first study to determine seroprevalence of A. phagocytophilum in Southwest Virginia. In the actively infected population, PCR and blood smear evaluation remain the most sensitive methods of diagnosis. While the SNAP 4DX Plus Test® is useful for serologic data collection, it was not appropriate for acute diagnosis of EGA.