Lyme Disease, 2010
Lyme disease is caused by Borrelia burgdorferi, a spirochete transmitted to humans by bites from Ixodes scapularis (the blacklegged tick or deer tick) in Minnesota. In Minnesota, the same tick vector also transmits the agents of human anaplasmosis, babesiosis, and a strain of Powassan virus.
In 2010, 1,293 confirmed Lyme disease cases (24.6 cases per 100,000 population) were reported (Figure 1). In addition, 667 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 1,293 confirmed cases was a 21% increase from the 1,065 confirmed cases reported in 2009 and slightly higher than the previous record number of 1,239 cases reported in 2007. The median number of 1,050 cases (range, 913 to 1,293 cases) reported from 2004 through 2010 is considerably higher than the median number of cases reported annually from 1996 through 2003 (median, 373 cases; range, 252 to 866). Seven hundred eighty (60%) confirmed cases in 2010 were male. The median age of cases was 39 years (range, <1 to 89 years). Physician-diagnosed erythema migrans (EM) was present in 987 (76%) cases. Three hundred fifty-two (27%) cases had one or more late manifestations of Lyme disease (including 226 with a history of objective joint swelling, 105 with cranial neuritis, 9 with lymphocytic meningitis, 5 with radiculoneuropathy, and 12 with acute onset of 2nd or 3rd degree atrioventricular conduction defects) 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 (39% and 35% of EM cases, respectively), corresponding to the peak activity of nymphal I. scapularis ticks in mid-May through mid-July. Most cases in 2010 either resided in or traveled to endemic counties in north-central, east-central, or southeast Minnesota, or in western Wisconsin.
B. burgdorferi co-infections with the etiologic agents of anaplasmosis and babesiosis can occur from the same tick bite. During 2010, 33 (3%) Lyme disease cases also were confirmed or probable cases of anaplasmosis, and 8 (1%) were confirmed or probable cases of babesiosis. Because of under-detection, these numbers likely underestimate the true frequency of co-infections.
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