Human Ehrlichia Muris-Like Infections Among Minnesota Residents: Summary of a Novel Tick-Borne Pathogen, Minnesota, 2009-2012

BACKGROUND:  Human ehrlichiosis, caused by Ehrlichia chaffeensis or E. ewingii, is a tick-borne illness not considered endemic to Minnesota. In 2009, Mayo Clinic detected Ehrlichia DNA in blood samples of Minnesota and Wisconsin patients with a unique nucleotide sequence 98% similar to E. muris, an Asian species not previously identified in North America. Since then, the Minnesota Department of Health (MDH) has been working with Mayo to identify additional infections with the E. muris-like agent (EML). Mayo Clinic is currently the sole commercial laboratory providing the EML PCR assay. Field studies have identified EML in Ixodes scapularis, a tick endemic to Minnesota and the primary vector for Borrelia burgdorferi and other pathogens, although no vector has been definitively established for the EML agent. This study summarizes the initial 4 years of Minnesota’s EML surveillance.

METHODS:  Reports of EML infection from 2009-2012 were identified, and case data were reviewed. Confirmed infections were defined as those PCR-positive for the EML agent. Cases meeting the clinical case definition for ehrlichiosis/anaplasmosis and that were PCR-positive for EML were classified as confirmed cases.

RESULTS:  Since 2009, 22 confirmed EML infections (17 male, 5 female) resulting in 17 confirmed EML cases have been identified in Minnesota residents. The most common presenting symptoms were fever (17), malaise/fatigue (15), myalgia (14), and arthralgia (9). Thrombocytopenia was identified in 13 patients, and 10 patients were leukopenic at the time of illness. Two patients had a history of organ transplantation, including one hospitalized for 3 days; another patient with immunosuppression was hospitalized for 10 days. Fifteen patients had no underlying medical conditions. All patients fully recovered after doxycycline treatment, and no adverse outcomes were identified. Of patients with exposure history, 18 reported having known tick exposure; 11 had history of tick bite. Most tick exposure likely occurred in central Minnesota.

CONCLUSIONS:  Confirmed EML infections from 2009-2012 resulted in mild, acute illness in most patients. Additional seropositive ehrlichiosis reports received by MDH, until now presumed to be probable E. chaffeensis infections despite minimal detection of its tick vector (Amblyomma americanum) may represent additional, undetected EML infections. Location of exposure in EML patients mirrors those where known I. scapularis-borne infections occur, providing further evidence that I. scapularis may be the primary vector for EML. Continued investigation of reported EML infections, broader use of PCR testing, and further development of an EML-specific serologic assay are needed to expand understanding of human EML infections.