Anaplasmosis

Often a nonspecific fever—sometimes a serious systemic illness

Symptoms usually begin roughly one to two weeks after infection. Common findings include fever, chills, severe headache, malaise, myalgia and sometimes gastrointestinal symptoms. Rash is much less characteristic than it is in ehrlichiosis or RMSF. Laboratory clues can be especially useful: leukopenia, thrombocytopenia and mild-to-moderate transaminase elevations are classic patterns.

A 2024 systematic review found that many infections are mild or subclinical, but severe complications do occur. U.S. surveillance data summarized in the review showed substantial hospitalization and a low but real case-fatality rate, with higher risk in older and immunocompromised patients. 2024 systematic review ↗

Do not wait for a positive test when the disease is clinically suspected.

CDC recommends doxycycline as first-line treatment for adults and children of all ages and emphasizes empiric treatment when anaplasmosis or another rickettsial infection is suspected. Fever typically improves within 24–48 hours after appropriate treatment. Current CDC guidance commonly uses a 10–14 day course when anaplasmosis is suspected, in part to cover possible concurrent Borrelia infection. CDC clinical care ↗

Why the blood counts and liver tests change

Anaplasmosis is unusual because the bacterium targets a cell that is itself part of the body's first-line antibacterial defense. After entering a neutrophil, A. phagocytophilum alters intracellular trafficking, delays programmed cell death and changes inflammatory signaling. The resulting disease reflects both pathogen replication and the host immune response rather than simply direct destruction of one organ.

Leukopenia and thrombocytopenia are common clues, although normal counts do not exclude infection. Mild anemia may occur. Elevated AST and ALT are frequent. C-reactive protein can rise. In more severe illness, clinicians may see kidney injury, respiratory compromise, neurologic abnormalities, coagulopathy or secondary inflammatory syndromes.

Who is more likely to become severely ill?

Older age, immune suppression, significant underlying disease and delay in appropriate antibiotic therapy are repeatedly associated with worse outcomes in observational literature. Splenic rupture has been reported rarely. Severe illness can resemble sepsis, making exposure history and the blood-count pattern especially useful.

What else can look like anaplasmosis?

Ehrlichiosis can produce an almost identical fever/leukopenia/thrombocytopenia/transaminitis pattern. RMSF may initially lack a rash. Heartland virus can mimic anaplasmosis or ehrlichiosis but does not respond to doxycycline because it is viral. Acute viral syndromes, sepsis, hematologic disease and medication reactions may also enter the differential diagnosis. In an Ixodes-endemic region, Lyme disease and babesiosis can coexist and may contribute additional findings such as erythema migrans or hemolytic anemia.

What happens after successful treatment?

CDC notes that fever usually falls within 24–48 hours of doxycycline. A lack of expected response should prompt reconsideration of the diagnosis or complications. CDC also states that doxycycline resistance and relapses after a completed recommended course have not been documented. Persistently positive antibody titers can last months or years and should not be treated as proof of active infection in a person who has clinically recovered.