Clinical Information

Ehrlichiosis and anaplasmosis are emerging zoonotic tick-borne infections caused by Ehrlichia and Anaplasma species, respectively. These obligate intracellular, gram-negative rickettsial organisms infect leukocytes and cause a potentially serious febrile illness in humans.

Human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis, is caused by Anaplasma phagocytophilum, which is transmitted through the bite of an infected Ixodes species tick. The epidemiology of this infection in the US is similar to that of Lyme disease (caused by Borrelia burgdorferi and B mayonii) and babesiosis (caused primarily by Babesia microti), which all have the same tick vector. HGA is most prevalent in the upper Midwest and the Northeastern US.

Human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis, which is transmitted by the Lone Star tick, Amblyomma americanum. Most cases of HME have been reported from the Southeastern and South-Central regions of the United States. Ehrlichia ewingii, the known cause of canine granulocytic ehrlichiosis, can occasionally cause an HME-like illness in humans. Clinical features and laboratory abnormalities are similar to those of E chaffeensis infection, and antibodies to E ewingii cross-react with current serologic assays for detection of antibodies to E chaffeensis.

Most recently, Mayo Clinic Laboratories detected a new species of Ehrlichia in patients with exposure to ticks in Wisconsin and Minnesota. This new pathogen, called E muris eauclairensis, causes a similar disease to ehrlichiosis due to E chaffeensis and E ewingii and may cause more severe disease in immunocompromised hosts.

Most cases of anaplasmosis and ehrlichiosis are subclinical or mild, but infection can be severe and life-threatening in some individuals. Fever, fatigue, malaise, headache, and other "flu-like" symptoms, including myalgias, arthralgias, and nausea, occur most commonly. Central nervous system involvement can result in seizures and coma.

Diagnosis may be challenging since the patient's clinical course is often mild and nonspecific. This symptom complex is easily confused with other illnesses such as influenza or other tick-borne zoonoses. Clues to the diagnosis of anaplasmosis/ehrlichiosis in an acutely febrile patient after tick exposure include laboratory findings of leukopenia, thrombocytopenia, and elevated serum aminotransferase levels. Intra-granulocytic morulae may be observed on peripheral blood smear in approximately 70% of cases of anaplasmosis, but intra-leukocytic morulare are rarely seen in human ehrlichiosis.

Definitive diagnosis is usually accomplished through polymerase chain reaction (PCR) and serologic methods, with the preferred method varying based on the time of presentation in relation to the onset of clinical symptoms. PCR is the most sensitive and specific method of detection in the first week of illness, whereas serology is the preferred method after this period. 

The Mayo Clinic PCR assay is capable of detecting and differentiating A phagocytophilum, E chaffeensis, E ewingii, and E muris eauclairensis.

It is important to note that concurrent infection with A phagocytophilum, Borrelia burgdorferi, and Babesia microti is not uncommon, as these organisms share the same Ixodes tick vector. Additional testing for these pathogens, including Lyme disease serology, may be indicated.