Lyme Disease, 2009

Lyme disease is caused by Borrelia burgdorferi, a spirochete transmitted to humans by bites from Ixodes scapularis (the blacklegged tick) in Minnesota. The same tick vector also transmits the agents of human anaplasmosis and babesiosis.

In 2009, 1,065 confirmed Lyme disease cases (20.4 cases per 100,000 population) were reported (Figure 1), slightly more than the 1,050 cases reported in 2008 but 14% fewer than the record number of 1,239 cases reported in 2007. The median number of 1,037 cases (range, 913 to 1,239 cases) reported from 2004 through 2009 is considerably higher than the median number of cases reported annually from 1996 through 2003 (median, 373 cases; range, 252 to 866). Six hundred sixty-nine (63%) confirmed cases in 2009 were male. The median age of cases was 39 years (range, <1 to 87 years). Physician-diagnosed erythema migrans (EM) was present in 769 (72%) cases. Three hundred thirty-nine (32%) cases had one or more late manifestations of Lyme disease (including 228 with a history of objective joint swelling, 88 with cranial neuritis, 5 with lymphocytic meningitis, 15 with radiculoneuropathy, and 12 with acute onset of second or third degree atrioventricular conduction defects), and confirmation by a positive Western immunoblot. Onsets of illness were elevated from June through August and peaked in July (45% of EM cases), corresponding to the peak activity of nymphal I. scapularis ticks in mid-May through mid-July.

Lyme disease co-infections with the etiologic agents of anaplasmosis and babesiosis can occur from the same tick bite. During 2009, 9 (1%) Lyme disease cases also were confirmed or probable cases of anaplasmosis, and 3 (<1%) were confirmed cases of babesiosis. Because of under-detection, these numbers likely underestimate the true frequency of co-infections.

Most cases in 2009 either resided in or traveled to endemic counties in north-central, east-central, or southeast Minnesota or in western Wisconsin. Crow Wing and Cass Counties had the highest number of reported I. scapularis exposures for cases exposed in Minnesota, (91 [21%] of 424 cases who reported a single county of exposure in Minnesota). Four hundred forty-four (42%) cases occurred among residents of the metropolitan area, of whom only a minority (17%) were likely exposed to I. scapularis ticks in the metropolitan area, primarily Anoka and Washington Counties.

A revised national surveillance case definition for Lyme disease was implemented in 2008, replacing a case definition in use since 1996. The 2008 case definition clarified certain laboratory and epidemiologic criteria for case classification and added a probable case category. Comparison of 2008 and 2009 case numbers in Minnesota using both definitions demonstrated that application of the revised case definition yielded a slightly larger number of cases than the old definition (2008: 1,050 versus 1,007 cases; 2009: 1,065 versus 1,021 cases). In addition, 223 and 481 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 in 2008 and 2009, respectively.

Image of Figure 1.

 

Updated Friday, November 19, 2010 at 03:16PM