Serological and Molecular Detection of Anaplasma spp. in Blood from Healthy Horses: A Preliminary Study of Horses in East Texas

Abstract Anaplasmosis is a disease caused by the bacterium Anaplasma phagocytophilum, which is spread by infected ticks. In horses, A. phagocytophilum generally causes transient infection characterized by fever, lethargy, inappetence, ventral edema, petechiae, icterus, ataxia, recumbency, muscle stiffness, and, in severe cases, death. Following natural infection, horses retain antibodies for approximately two years, which can be detected through an immunofluorescence antibody assay. Current infections are determined through PCR assay of white blood cell DNA. For this study, whole blood was collected from apparently healthy horses located in East Texas (n=70), west Texas (n=3), New York (n=49), and New Jersey (n=11) for the determination of serum antibodies and PCR testing of bacterial DNA. Of the 133 horses, 24 tested positive for DNA presence of A. phagocytophilum, and 107 tested positive for serum antibodies. Of the 24 horses testing positive for A. phagocytophilum, 16 were positive for serum antibody presence and 8 were negative. Twenty of the msp2 positive horses were located in East Texas and 4 resided in New York. For serum antibodies, 100% of New York and New Jersey horses tested positive, while only 66% of Texas horses tested positive. This study provides evidence that a large number of horses are exposed to A. phagocytophilum and that this bacterium is present in East Texas. No Texas horse owners reported treatment for anaplasmosis, and the currently infected horses were not demonstrating signs of illness at the time of sample collection. Further research to understand the differences in disease severity amongst equine populations is warranted.