Human ehrlichiosis and anaplasmosis refer to infections due to bacteria belonging to the family Anaplasmataceae. Species responsible for the majority of human infections are Anaplasma phagocytophilum, which causes human granulocytotropic (or granulocytic) anaplasmosis (HGA); Ehrlichia chaffeensis, which causes human monocytic ehrlichiosis (HME); Ehrlichia ewingii; and Ehrlichia muris eauclairensis. Ehrlichia canis is also a very rare cause of human disease. This summary focuses on HGA.

Tick vectors and specific mammalian hosts (eg, deer, dogs, mice, and cattle) form the natural zoonotic cycle for the bacteria of human anaplasmosis. Infection follows bites from infected ticks. The vectors for A phagocytophilum include various tick species belonging to the genus Ixodes in the United States (primarily from the blacklegged tick [Ixodes scapularis] and the western blacklegged tick [Ixodes pacificus]) and Europe (Ixodes ricinus complex [mainly Ixodes ricinus and Ixodes persulcatus]).

The incidence of human infection corresponds with the distribution of the arthropod vectors. In the United States, HGA is most commonly reported in New England and the north-central states as well as localized areas along the western coast. Similar to other tickborne illnesses, the majority of disease occurs in the warmer months of spring and summer and is more frequently noted to occur in men and those older than 40 years. HGA has been reported from several European countries.

Symptoms of HGA typically occur 5-21 days after a bite from an infected tick. Smptoms and signs are nonspecific and include fever (virtually ubiquitous), headache, malaise, myalgias, and lymphopenia. The spectrum of disease ranges from asymptomatic to severe illness and, rarely, death. Immunocompromised individuals are at a higher risk for more severe illness, with an increased risk of mortality. HGA has a lower mortality than HME: less than 1% versus 3%.

Other symptoms may include nausea, vomiting, diarrhea, cough, arthralgias, stiff neck, and confusion. In more severe illness, symptoms may also include severe respiratory distress, sepsis-like illness, rhabdomyolysis, hemorrhage, and opportunistic infections (including herpes simplex virus and fungal infections). Central nervous system involvement is less common in HGA than HME, although various peripheral nervous system disorders including facial palsy, brachial plexopathy, and demyelinating polyneuropathy have been reported. Similarly, rash is rarer in HGA than in HME (less than 2.5% of cases).

Transmission of A phagocytophilum has occurred through blood transfusion and perinatally.