The agent of human granulocytic ehrlichiosis (HGE) recently has been classified in the genus Anaplasma and named Anaplasma phagocytophilum. HGE is transmitted to humans by Ixodes scapularis (deer tick or blacklegged tick), the same tick that transmits Lyme disease.
During 2002, 149 confirmed or probable cases of HGE (3.0 per 100,000 population) were reported. This represents a 60% increase from the prior high of 93 cases reported in 2001 and a 314% increase from the 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 (PCR), or detection of intracytoplasmic morulae and an IFA antibody titer >1:64 to HGE. Probable HGE cases have a compatible clinical illness and either an IFA serologic titer >1:64 to HGE or detection of intracytoplasmic morulae.
Ninety-one (61%) case-patients reported in 2002 were male. The median age of case-patients was 56 years (range, 2 to 100 years). The peak in onsets of illness occurred in July (42% of cases). Co-infections with Lyme disease and HGE can occur from the same tick bite; during 2002, 20 (13%) HGE cases also met the case definition for early-stage Lyme disease with 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, including Aitkin, Crow Wing, Cass, Pine, and Morrison Counties.
For more information on HGE and other tick-borne diseases in Minnesota, including a map of high-risk areas, see the May 2003 issue of the Disease Control Newsletter (vol. 31, no. 3).