Strains of Anaplasma phagocytophilum from horses in Ohio are related to isolates from humans in the northeastern USA

Abstract

ABSTRACT The tick-borne obligatory intracellular bacterium Anaplasma phagocytophilum infects granulocytes of humans as well as domesticated and wild animals, causing a febrile disease collectively called granulocytic anaplasmosis. The host species specificity and zoonotic potential of A. phagocytophilum strains remain unclear. In the USA, human granulocytic anaplasmosis (HGA) is primarily reported in Wisconsin and Minnesota, the northeastern coastal states, and northern California. In horses, A. phagocytophilum causes equine granulocytic anaplasmosis (EGA). Although EGA has been reported in the same US regions as HGA, horses with a definitive diagnosis of EGA have not been reported in Ohio, where the incidence of HGA is low. In this study, samples from archived blood smears or whole blood from three horses from three different counties in Ohio that presented in 2018, 2020, and 2021 with clinical signs and laboratory findings consistent with EGA were evaluated by PCR and sequencing. Sequences of ankA (encoding ankyrin A) of the three A. phagocytophilum strains were identical and related to strains isolated from humans in the northeastern USA yet distinct from those in California or Europe based on phylogenetic analysis. Each Ohio strain contains unique A. phagocytophilum p44 genes encoding the immunodominant major outer membrane proteins, underscoring the antigenic diversity of A. phagocytophilum strains that cause EGA in Ohio. From 2018 to 2023, 14 additional cases of EGA were identified by serodiagnosis in Ohio and nearby locations in Pennsylvania, pointing to a heretofore unrecognized prevalence of EGA and the risk of transmission of anaplasmosis to humans and animals in this region. IMPORTANCE The tick-borne obligatory intracellular bacterium Anaplasma phagocytophilum infects humans as well as domesticated and wild animals, causing a febrile disease collectively called granulocytic anaplasmosis. The epidemiology and the host species specificity and zoonotic potential of A. phagocytophilum strains remain unclear. In this study, ankA (encoding ankyrin A) and p44 gene sequences of A. phagocytophilum were determined in clinical specimens from horses in Ohio and compared with those found in A. phagocytophilum strains from various hosts and geographic regions. With increasing numbers of seropositive horses, the study points out the unrecognized prevalence and uncharacterized strains of A. phagocytophilum infection in horses and the importance of A. phagocytophilum molecular testing for the prevention of equine and human granulocytic anaplasmosis.