Human anaplasmosis (HA), formerly known as Human Granulocytic Ehrlichiosis (HGE), is a bacterial disease transmitted to humans by Ixodes scapularis (blacklegged tick or deer tick), the same tick that transmits Lyme disease. The etiologic agent of HA is Anaplasma phagocytophilum, a rickettsial bacterium.
The disease was first recognized during 1993 in several patients from Minnesota and western Wisconsin. Human ehrlichiosis, a similar disease, is caused by Ehrlichia chaffeensis. Human ehrlichiosis is not a common vector-borne disease.
Presentation
Onset of illness occurs 5 to 21 days after exposure to an infected tick. Common signs and symptoms include fever (often over 102°F), chills, headache, and myalgias. Nausea, vomiting, anorexia, acute weight loss, abdominal pain, cough, diarrhea, and change in mental status are reported less frequently. Highly suggestive laboratory findings include leukopenia (WBC< 4,500/mm³), thrombocytopenia (platelets <150,000/mm³), and increased aminotransferase levels. Unusual presentations may be the result of coinfections with Borrelia burgdorferi (Lyme disease agent) and/or Babesia microti (babesiosis agent), as a single feeding tick may transmit multiple disease agents.
Cases of HA acquired through blood transfusions have been documented. Include HA in the rule-out for patients who develop a febrile illness with thrombocytopenia following blood transfusion. Suspected transfusion-associated anaplasmosis should be reported to MDH and the supplying blood center.
Treatment
HA patients typically respond dramatically to doxycycline therapy (100 mg twice daily until the patient is afebrile for at least 3 days). Other tetracycline drugs also are likely to be effective. In general patients with suspect HA and unexplained fever after a tick exposure should receive empiric doxycycline therapy while diagnostic tests are pending, particularly if they experience leukopenia and/or thrombocytopenia.
