ehrlichiosis

Ehrlichiosis refers to human illnesses caused by obligate intracellular bacteria in the family Anaplasmataceae, most commonly Ehrlichia chaffeensis, which causes human monocytic ehrlichiosis (HME), and Anaplasma phagocytophilum, which causes human granulocytic anaplasmosis (HGA). Less commonly, Ehrlichia ewingii and other related organisms can cause disease in humans. The organisms are transmitted by ticks.

Vects and geography vary by species. E. chaffeensis is primarily associated with the lone star tick (Amblyomma americanum) in parts of the United States, while A. phagocytophilum is transmitted by Ixodes species (such as I. scapularis) in North America and elsewhere. Infections often occur in spring and summer when tick activity is high. Most cases occur in adults, but children can also be affected.

Clinical features commonly include fever, severe headache, malaise, myalgia, and sometimes nausea, vomiting, or abdominal pain. A rash occurs more frequently with HME and is less common with HGA. Laboratory findings often show leukopenia and thrombocytopenia with elevated liver enzymes. Severe illness can involve central nervous system symptoms, pneumonia, hepatitis, or multi-organ failure, especially with delayed treatment or in older adults.

Diagnosis relies on clinical suspicion supported by laboratory tests. Polymerase chain reaction (PCR) can detect the organism early in illness, while serology with indirect immunofluorescent antibody (IFA) testing confirms infection later. Morulae may be seen in white blood cells on blood smear but are not consistently present.

Treatment is with doxycycline for both adults and children, typically 7–14 days or longer after fever resolution. Alternatives are limited and used when doxycycline is contraindicated. Early treatment improves outcomes; most cases are mild to moderate, though severe disease can occur.

Prevention focuses on tick bite avoidance, including repellents, protective clothing, prompt tick checks, and prompt removal of attached ticks. Co-infection with other tick-borne pathogens can occur.