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 2000, 79 confirmed or probable cases of HGE were reported (1.6 per 100,000 population). This number represents a 119% increase in cases from the prior high of 36 cases reported in 1999. 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.
Forty-eight (61%) cases reported to MDH in 2000 were male. The median age of cases was 52 years (range, 2 to 83 years). The distribution of illness onsets was bimodal, with a peak in May and June (50% of cases) and a smaller peak in October and November (21% of cases). The latter peak likely was due to bites from infected adult I. scapularis ticks. Recent evidence suggests that I. scapularis may be able to transmit the agent of HGE to people within 24 hours after attaching to the person. Thus, while daily removal of attached ticks may prevent Lyme disease (where 1 to 2 days of tick attachment are necessary to transmit Borrelia burgdorferi), the practice may not prevent all cases of HGE. Additionally, co-infections of Lyme disease and HGE from the same tick bite are possible. During 2000 eight (10%) of 79 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).