New Ehrlichia Species Identified In Humans
During the summer of 2009, using polymerase chain reaction (PCR) testing and DNA sequencing, Mayo Clinic identified a novel ehrlichiosis agent from humans residing in Wisconsin and Minnesota. The DNA sequence of this organism was found to be similar to that of Ehrlichia muris, a bacterial species that typically infects mice and other animals, and was thus called Ehrlichia muris-like (EML). In total, EML DNA was detected in 4 patients (3 from Wisconsin, 1 from Minnesota), and the bacteria was isolated in culture from 1 patient via collaboration with the University of Minnesota, Twin Cities. All patients presented with headache, malaise, fever, and lymphopenia, and recovered with doxycycline treatment. The epidemiologic investigation that followed involved extensive collaboration with the Centers for Disease Control and Prevention (CDC), Minnesota Department of Heath (MDH), and the Wisconsin State Laboratory of Hygiene (WSLH).
Background of Human Ehrlichiosis and Human Anaplasmosis
Human ehrlichiosis (HE) and human anaplasmosis (HA) are tick-borne infections that present with flulike symptoms and are caused by closely related, obligate, intracellular bacteria. While the 2 infections are closely related, they have distinct differences and are caused by different microorganisms. Both are most common in spring and summer, when ticks are active.
HE is caused by Ehrlichia chaffeensis and Ehrlichia ewingii, while HA is caused by Anaplasma phagocytophilum. All are transmitted through the bite of a hard-bodied tick (Ixodidae family); HE is transmitted by Amblyomma americanum (Lone Star tick) and HA is transmitted by Ixodes scapularis (blacklegged or deer tick). Unlike HE, which is uncommon in the upper Midwest, HA is the second most frequently reported tick-borne disease, after Lyme disease, in Minnesota and Wisconsin. The tick species that transmits HA also transmits Lyme disease and babesiosis, and multiple infections from a single tick bite can occur.
During investigative studies of the EML agent, Mayo Clinic collaborated with WSLH to test ticks from northwestern Wisconsin. This investigation revealed EML DNA in a single pool of Ixodes scapularis nymphs, although it is not known whether this tick serves as a vector.
HA, HE, and EML infections can only be definitively identified and differentiated by laboratory testing. PCR is considered the gold standard and is most effective during the first week of illness. Mayo Medical Laboratories offers a PCR test (#84319 Ehrlichia/Anaplasma, Molecular Detection, PCR, Blood) that can differentiate the EML organism from the agents of HA and HE.
Serologic testing also plays an important role in the diagnosis of ehrlichiosis and anaplasmosis. Ideally, an acute serum sample should be submitted for HA/HE (#81478 Ehrlichia chaffeensis(HME) Antibody, IgG, Serum or #81480 Ehrlichia Antibody Panel, Serum) during the first week of disease, followed by a convalescent serum sample 2 to 4 weeks later. Some serologic cross-reactivity exists between these organisms. A specific serologic test has not yet been developed for the EML agent and the degree of cross-reactivity of this agent with HE and HA serologic tests is unknown. Rarely, intragranulocytic or monocytic morulae may be observed on peripheral blood smear, but this is not a reliable means of diagnosing cases of human ehrlichiosis or anaplasmosis. Therefore, PCR remains the test of choice for the EML agent at this time. Due to the high rate of coinfections with other tick-borne agents, patients at risk for ehrlichiosis or anaplasmosis should also be tested for babesiosis and Lyme disease if these diseases are prevalent in the area. Mayo Medical Laboratories offers a PCR assay that will detect the presence of any of these bacteria (#83266 Tick-Borne Panel, Molecular Detection, PCR, Blood).
Currently, HA, HE, and EML are treated with doxycycline. Empiric treatment may be recommended, even if an early diagnostic test is negative. Symptoms usually subside within 24 to 72 hours of treatment and failure to rapidly respond to antibiotic treatment argues against a diagnosis of ehrlichiosis. In this case, other diagnoses should be considered.