Human granulocytic anaplasmosis (HGA) (previously known as Human granulocytic ehrlichiosis, or HGE ) is an infectious disease caused by Anaplasma phagocytophilum, an obligate intracellular bacterium that is typically transmitted to humans by at least three kinds of ticks, including Ixodes scapularis, Ixodes pacificus, and Dermacentor variabilis. These ticks also transmit Lyme disease and other diseases.
The bacteria infect white blood cells called neutrophils, causing changes in gene expression that prolong the life of these otherwise short-lived cells.
Ecology and epidemiology
A. phagocytophilum is transmitted to humans by Ixodes ticks. These ticks are found in the US, Europe, and Asia. In the US, I. scapularis is the tick vector in the East and Midwest states, and I. pacificus in the Pacific Northwest.
The major mammalian reservoir for A. phagocytophilum in the eastern United States is the white-footed mouse, Peromyscus leucopus. Although white-tailed deer harbor A. phagocytophilum, evidence suggests that they are not a reservoir for the strains that cause HGA.
Anaplasma phagocytophilum shares its tick vector with other human pathogens, and about 10% of patients with HGA show serologic evidence of coinfection with Lyme disease, babesiosis, or tick-borne meningoencephalitis.
Symptoms
Symptoms may include fever, severe headache, muscle aches (myalgia), chills and shaking, similar to the symptoms of influenza. Symptoms may be minor, as evidenced by surveillance studies in high-risk areas. GI symptoms occur in less than half of patients and a skin rash is seen in less than 10% of patients. It is also characterized by thrombocytopenia, leukopenia, and elevated serum transaminase levels in the majority of infected patients.
Diagnosis
Clinically, HGA is essentially indistinguishable from Human monocytic ehrlichiosis, the infection caused by Ehrlichia chaffeensis, and other tick-borne illnesses such as Lyme disease may be suspected. As Ehrlichia serologies can be negative in the acute period, PCR is very useful for diagnosis.
Treatment and Prevention
Doxycycline is the treatment of choice. If anaplasmosis is suspected, treatment should not be delayed while waiting for a definitive laboratory confirmation, as prompt doxycycline therapy has been shown to improve outcomes. Presentation during early pregnancy can complicate treatment. Doxycycline compromises dental enamel during development. Although rifampin is indicated for post-delivery pediatric and some doxycycline-allergic patients, it is teratogenic. Rifampin is contraindicated during conception and pregnancy. Currently, there is no vaccine against human granulocytic anaplasmosis.
Terminology
Although the infectious agent is known to be from the Anaplasma genus, the term "human granulocytic ehrlichiosis" (HGE) is often used, reflecting the prior classification of the organism. E. phagocytophilum and E. equi were reclassified as Anaplasma phagocytophilum.