Human granulocytic ehrlichiosis (HGE) is an emerging tick-borne bacterial disease in Minnesota. Although the agent of HGE has not been named, it is thought to be similar or identical to two veterinary pathogens (i.e., Ehrlichia equi and Ehrlichia phagocytophila). HGE is transmitted to humans by Ixodes scapularis (deer tick or black-legged tick), the same tick that transmits Lyme disease. During 2001, 93 confirmed or probable cases of HGE were reported (1.8 per 100,000 population). This number represents an 18% increase in cases from the prior high of 79 cases reported in 2000 and a 158% increase in cases since 1999 (36 cases). The national surveillance case definition for a confirmed case of HGE includes a compatible clinical illness with a four-fold increase in HGE antibody titer by IFA, a positive polymerase chain reaction, or intracytoplasmic morulae and an IFA antibody titer to HGE >1:64. Probable HGE cases have a compatible clinical illness and either an IFA serologic titer to HGE >1:64 or detection of intracytoplasmic morulae.
Fifty-seven (61%) case-patients reported to MDH were male. The median age of case-patients was 57 years (range, 3 to 95 years). The peak of illness onset was in June (47% of cases). Co-infections of Lyme disease and HGE from the same tick bite are possible, and during 2001, three (3%) HGE cases also met the case definition for early-stage Lyme disease (with a physician-diagnosed erythema migrans). People are at most risk of HGE in the same east-central Minnesota counties where the risk of Lyme disease is greatest (see section on Lyme disease, page 28).