Lyme Disease, 2012
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 2012, 912 confirmed Lyme disease cases (17.2 cases per 100,000 population) were reported (Figure 1). In addition, 604 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 912 confirmed cases represent a 24% decrease from the 1,203 confirmed cases reported in 2011. The median number of 1,050 cases (range, 911 to 1,293 cases) reported from 2004 through 2012 is considerably higher than the median number of cases reported annually from 1996 through 2003 (median, 373 cases; range, 252 to 866). Five hundred seventy (63%) confirmed cases in 2012 were male. The median age of cases was 39 years (range, 1 to 92 years). Physician diagnosed erythema migrans (EM) was present in 564 (62%) cases. Three hundred sixty-eight (40%) cases had one or more late manifestations of Lyme disease (including 271 with a history of objective joint swelling, 79 with cranial neuritis, 8 with acute onset of 2nd or 3rd degree atrioventricular conduction defects, 10 with radiculoneuropathy, 6 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 (37% and 27% of EM cases, respectively), corresponding to the peak activity of nymphal I. scapularis ticks in mid-May through mid-July. The majority of cases in 2012 either resided in or traveled to endemic counties in north-central, east-central, or southeast Minnesota, or Wisconsin.
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