Lyme Disease, 2011
Lyme disease is caused by Borrelia burgdorferi, a spirochete transmitted to humans by bites from I. scapularis (the blacklegged tick or deer tick) in Minnesota. In Minnesota, the same tick vector also transmits the agents of babesiosis, human anaplasmosis, one form of human ehrlichiosis, and a strain of Powassan virus. In 2011, 1,201 confirmed Lyme disease cases (22.6 cases per 100,000 population) were reported (Figure 1). In addition, 953 probable cases (physician-diagnosed cases that did not meet clinical evidence criteria for a confirmed case but that had laboratory evidence of infection) were reported. The 1,201 confi rmed cases represent a 7% decrease from the 1,293 confirmed cases reported in 2010 but higher than the 1,065 confirmed cases reported in 2009. The median number of 1,058 cases (range, 913 to 1,293 cases) reported from 2004 through 2011 is considerably higher than the median number of cases reported annually from 1996 through 2003 (median, 373 cases; range, 252 to 866). Seven hundred fifty (62%) confi rmed cases in 2011 were male. The median age of cases was 40 years (range, <1 to 91 years). Physician-diagnosed erythema migrans (EM) was present in 863 (72%) cases. Three hundred seventy-two (31%) cases had one or more late manifestations of Lyme disease (including 262 with a history of objective joint swelling, 89 with cranial neuritis, 9 with acute onset of 2nd or 3rd degree atrioventricular conduction defects, 8 with radiculoneuropathy, 7 with lymphocytic meningitis, and 1 with encephalomyelitis) and confirmation by Western immunoblot (positive IgM <30 days post-onset or positive IgG). Onsets of illness were elevated in the summer months and peaked in June and July (26% and 44% of EM cases, respectively), corresponding to the peak activity of nymphal I. scapularis ticks in mid-May through mid-July. Most cases in 2011 either resided in or traveled to endemic counties in north-central, eastcentral, or southeast Minnesota, or in western Wisconsin.
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