Clinical Pearls & Morning Reports
Published March 25, 2020
Anaplasma phagocytophilum, the cause of human granulocytic anaplasmosis, is a type of intracellular bacteria that is endemic in the New England area and is transmitted by the same ixodes tick species that transmits Lyme disease. Read the NEJM Case Records of the Massachusetts General Hospital here.
Q: What symptoms and laboratory abnormalities are associated with human granulocytic anaplasmosis?
A: Common symptoms include fevers, malaise, headaches, and myalgias; rash is an uncommon finding. Laboratory abnormalities often include leukopenia, anemia, thrombocytopenia, and elevated aminotransferase levels. In fact, white-cell and platelet abnormalities are so often present that normal values can be used to rule out this infection in most populations.
Q: Can Anaplasma phagocytophilum infection be transmitted only by the bite of an infected tick?
A: Human granulocytic anaplasmosis was first described in Wisconsin in 1990. The majority of cases occur in the upper Midwest and northeastern regions of the United States, and incidence has steadily increased in these regions since 2000. Transmission most often occurs by the bite of an infected tick, but mother-to-child transmission, transmission by blood transfusion, and transmission by slaughtering of infected animals have also been reported.
A: The diagnosis of human granulocytic anaplasmosis is often made by means of examination of a peripheral-blood smear, serologic testing, or whole-blood nucleic acid testing. Although culture is the most accurate method, its use is often limited to research settings, given the expertise and specific culture requirements needed. Examination of a peripheral-blood smear can be a useful tool for the diagnosis of human granulocytic anaplasmosis but is limited by variable sensitivity (ranging from 20 to 80%); in addition, this method is labor intensive and requires a high degree of expertise, and these factors limit its use in the clinical setting. On serologic testing, documentation of a rise in titer by a factor of 4 between the acute and convalescent phases of infection is required for diagnosis confirmation, and antibody responses can persist for months or years after infection. Of the available diagnostic methods, nucleic acid testing is the most widely used, with high sensitivity within the first week of illness.
A: Doxycycline is considered to be first-line therapy for treatment of A. phagocytophilum infection in both adults and children. This recommendation is based on in vitro data and published reports that show clinical efficacy. There are no current guidelines regarding the treatment of pregnant women with human granulocytic anaplasmosis, although small case series have shown successful treatment with the use of doxycycline. With appropriate therapy, fevers tend to resolve within 24 to 48 hours, and the majority of patients have a complete recovery after 2 months. Chronic infection has never been documented.