Anaplasma spp.

Anaplasma phagocytophilum is a small, gram-negative, obligate intracellular coccobacillus belonging to the family Anaplasmataceae (related to Ehrlichia). It is transmitted by Ixodes scapularis and Ixodes pacificus ticks—the same vectors responsible for Lyme disease and Babesia microti, leading to potential co-infections. The organism preferentially infects granulocytes (neutrophils), where it forms characteristic intracytoplasmic inclusions called morulae (mulberry-shaped clusters of bacteria), visible on Giemsa or Wright staining of peripheral blood smears. Clinically, human granulocytic anaplasmosis (HGA) presents with fever, headache, myalgias, malaise, and sometimes gastrointestinal symptoms 1-2 weeks after tick exposure. Laboratory findings often include leukopenia, thrombocytopenia, and elevated liver transaminases. Severe cases can lead to organ failure, opportunistic infections, or hemorrhage, particularly in immunocompromised or elderly patients. Diagnosis is confirmed via PCR (most sensitive/specific), peripheral blood smear morulae detection, or serologic testing (indirect immunofluorescence assay showing rising IgG titers). Doxycycline is the treatment of choice, even in children, due to its efficacy against intracellular organisms and low risk of dental staining with short courses. Untreated infections can be fatal, especially in immunocompromised individuals. Anaplasma is often discussed in comparison to Ehrlichia chaffeensis (causes human monocytic ehrlichiosis, infecting monocytes rather than neutrophils, transmitted by Lone Star tick—Amblyomma americanum) — a classic board differentiation point. Epidemiologically, HGA is most common in the northeastern and upper midwestern United States, correlating with Lyme disease distribution.