Update on Human Granulocytic Anaplasmosis

Granulocytic anaplasmosis (GA) is a disease of humans, domestic animals, and wildlife caused by Anaplasma phagocytophilum, formerly Ehrlichia phagocytophila, E. equi, and the unnamed agents of “human granulocytic ehrlichiosis” (HGE). This pathogen is inoculated into host skin by the bite of Ixodes spp. ticks, including I. pacificus in California and I. scapularis in the eastern U.S. After inoculation, A. phagocytophilum disseminates to the blood and is phagocytosed into host neutrophils. The clinical characteristics of GA in people vary from no symptoms to fever, headache, neurological symptoms, and occasionally death. Horses with GA may experience high fever, depression, reduced ability to eat, limb edema, jaundice, and ataxia. GA is an emerging disease in the eastern U.S. but only a handful of human cases have been reported in the western U.S., despite relatively common reports of disease in horses and dogs. Wildlife and dog-sentinel studies have clarified that infection is common in the coast range mountains and Sierra Nevada foothills, with ongoing research focusing on ecological determinants that can modify the prevalence of infection in any particular area. One known reservoir is the dusky-footed woodrat. However, important, poorly understood ecological determinants serve to modify the probability of rodent as well as human infections, including climate, vegetation, and possibly the presence of other reservoir-competent rodents and nidicolous woodrat-specialist ticks such as I. spinipalpis