HME is an infection caused by Ehrlichia chaffeensis. An intracellular rickettsia-like organism infecting macrophages and monocytes, HME is found predominantly in the South Central, Southeastern, and Middle Atlantic states, where it is transmitted by the Lone Star Tick (Amblyomma americanum). Over the past decade, the geographic distribution of HME has expanded to include parts of the Northeast including New Jersey and the Long Island area of New York.
Symptoms are nonspecific and include fever, headaches, myalgias, and fatigue. The presence of a rash is variable. As with HGA, thrombocytopenia, neutropenia, and elevated liver function tests may be found. Clinical features vary widely from asymptomatic, to a mild self-limiting febrile illness, to severe or even fatal cases in immunocompromised persons.
There are several laboratory tests to assist in the diagnosis of HME. One traditional test is the direct demonstration of the organism intracellularly (in monocytes) on a thick smear of whole blood. The advantage of this test, in the hands of an experienced pathologist or technician, is that the result is available immediately. One potential disadvantage is that the organisms may not be visible below a certain level of bacteremia and is in fact negative in most cases of HME infections.
A more recently developed methodology for the direct demonstration of the organism in a clinical specimen (whole blood) is PCR. This methodology is capable of detecting minute quantities of the E. chaffeensis specific DNA, making it a much more sensitive test than visualization on thick smear.
Serologic assays are available to detect the patient antibody response to HME and include IFA testing for the IgG response, using native HME protein antigen preparations. Seropositivity can accurately identify patient exposure by demonstrating a specific immune response. A four-fold rise in titers in paired (acute and convalescent) specimens suggests recent infection.
There is a window early in the course of the infection when the patient is infected but may not yet be symptomatic, or is just beginning to become symptomatic. PCR testing at this stage is frequently positive, prior to a detectable serologic response.
The staff at the laboratory is highly trained in performing and interpreting these assays, has decades of experience in analyzing these test results in the context of various clinical situations, and is available to assist healthcare providers in interpreting tests and answering questions about the detection of tick-borne infections.