A phagocytophilum, a gram-negative intracellular bacterium, infects neutrophils and, occasionally, eosinophils in a variety of mammals, including dogs and cats.1,8 Although the organism is primarily vector-transmitted by the bite of an Ixodes spp tick, transmission via blood transfusion has been described in humans,9 so dogs and cats that donate blood should be tested for infection prior to donation.10 Because of the shared Ixodes spp tick vector, concurrent infection with Borrelia burgdorferi has been reported in dogs and cats and may contribute to more severe clinical manifestation than either infection alone.11-15
Geographic distribution of anaplasmosis follows that of the tick vector, and the seroprevalence of A phagocytophilum in dogs and cats varies by region.2,4,12,15-18 Anaplasmosis has been reported with highest incidence in the midwest, northeast, and western United States.2,16 Infection has also been reported in Canada, Europe, Asia, and South America, although more studies are needed to understand vector transmission in areas not endemic for Ixodes spp.3,4,6,17,19-24
Risk for exposure and/or infection is increased in animals that live in an Ixodes spp-endemic area, receive acaricidal medications inconsistently, and have increased outdoor access, particularly to natural tick habitats. Transmission to mammals occurs within 24 to 48 hours of tick attachment.25,26 Disease prevalence corresponds with increased Ixodes spp tick activity and is therefore highest in the late spring and autumn. Seropositive prevalence is increased in older dogs and cats, as these animals have had more years of exposure.1,7,12,19 Prevalence of exposure to A phagocytophilum and incidence of clinical disease are not equal. Most dogs that test positive for A phagocytophilum infection have no history of clinical illness, and cats that test positive for A phagocytophilum serum antibodies may not have signs of clinical disease.12,27 This discordance complicates testing and confirmation of disease.