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Despite organization into a single species, Anaplasma phagocytophilum, of bacteria from diverse hosts, there is evidence that different hosts vary in susceptibility to local strains of this bacterial pathogen. In particular, there is evidence that clinical hosts (humans, horses, and dogs) in the western part of the United States may be infected with strains of bacteria that differ from those in the reported wildlife reservoir, the dusky-footed woodrat. There is a lack of spatial overlap between clinical cases with woodrat exposure, genetic studies show highly distinctive strains in woodrats compared with those from clinical hosts, and experimental studies failed to transfer infection from woodrats into horses. These data suggest that multiple distinct A. phagocytophilum strains could circulate in western U.S. ecosystems. Host barriers to different bacterial strains would suggest the need for careful discrimination of A. phagocytophilum strains and further research to understand the host–pathogen interactions that result in differential outcomes in infection. [ABSTRACT FROM AUTHOR]

Anaplasma phagocytophilum is a zoonotic obligate intracellular bacterium known to be transmitted by ticks belonging to the Ixodes persulcatus complex. This bacterium can infect several mammalian species, and is known to cause diseases with variable symptoms in many domestic animals. Specifically, it is the causative agent of tick-borne fever (TBF), a disease of important economic impact in European domestic ruminants, and human granulocytic anaplasmosis (HGA), an emerging zoonotic disease in Asia, USA and Europe. A. phagocytophilum epidemiological cycles are complex and involve different ecotypes, vectors, and mammalian host species. Moreover, the epidemiology of A. phagocytophilum infection differs greatly between Europe and the USA. These different epidemiological contexts are associated with considerable variations in bacterial strains. Until recently, few A. phagocytophilum molecular typing tools were available, generating difficulties in completely elucidating the epidemiological cycles of this bacterium. Over the last few years, many A. phagocytophilum typing techniques have been developed, permitting in-depth epidemiological exploration. Here, we review the current knowledge and future perspectives regarding A. phagocytophilum epidemiology and phylogeny, and then focus on the molecular typing tools available for studying A. phagocytophilum genetic diversity.