Diagnosis of human granulocytic ehrlichiosis: state of the art.

Human granulocytic ehrlichiosis is an emerging zoonosis caused by Anaplasma phagocytophilum and transmitted through the bite of infected Ixodes scapularis. It is prevalent in the Midwest and Northeast United States and also in Europe, and it presents as a nonspecific febrile illness a few days after a tick bite usually between late spring and fall. Most cases present in adult patients with a mild form of the disease, although it can be severe with multiorgan failure, particularly in the elderly and in the immunocompromised. Routine laboratory abnormalities include leukopenia with a left shift, lymphopenia, and thrombocytopenia. These abnormalities are more frequently present during the first week of illness and then tend to normalize; therefore their absence should not exclude the diagnosis. Specific tests to confirm the diagnosis during the acute phase include microscopic detection of morulae in granulocytes, culture of A. phagocytophilum, and polymerase chain reaction. Of these methods, culture appears to have the greatest sensitivity during the acute phase prior to antimicrobial treatment. Serology has an important role in the confirmation of the diagnosis when used in paired specimens and when high cutoff titers by indirect fluorescence antibody assay (> or = 640) are used to diagnose a recent infection.