Lyme Disease, 2008

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

In 2008, 1,050 confirmed Lyme disease cases (20.1 cases per 100,000 population) were reported (Figure 1).

This is a 15% decrease from the record number of 1,239 cases reported in 2007 but is slightly higher than the median number of 915 cases (range, 913 to 1,239 cases) reported from 2004 through 2007. The frequency of Lyme disease since 2004 has been considerably higher than the median number of cases reported annually from 1996 through 2003 (median, 374 cases; range, 252 to 867). Six hundred seventy (64%) confirmed case-patients in 2008 were male. The median age of case-patients was 40 years (range, <1 to 95 years). Physician-diagnosed erythema migrans was present in 770 (74%) cases. Two hundred eighty-seven (27%) cases had one or more late manifestations of Lyme disease (including 214 with a history of objective joint swelling, 55 with cranial neuritis, 3 with lymphocytic meningitis, 12 with radiculoneuropathy, and 7 with acute onset of 2nd or 3rd degree atrioventricular conduction defects) and confirmation by a positive Western immunoblot. Onsets of illness were elevated from June through August and peaked in July (41% of cases), corresponding to the peak activity of nymphal I. scapularis ticks in mid-May through mid-July.

Lyme disease co-infections with anaplasmosis and babesiosis can occur from the same tick bite. During 2008, nine (1%) Lyme disease case-patients also were confirmed or probable cases of anaplasmosis, and two (<1%) were confirmed cases of babesiosis. Because of under-detection, these numbers likely underestimate the true frequency of co-infections.

Most case-patients in 2008 either resided in or traveled to endemic counties in north-central, east-central, or southeast Minnesota or in western Wisconsin. Of the cases exposed to I. scapularis ticks in Minnesota, Crow Wing County continued to have the highest number of Lyme disease case exposures (62 [14%] of 433 cases who reported a single county of exposure). Four hundred thirty-nine (42%) cases occurred among residents of the metropolitan area, of whom only a minority (25%) were likely exposed to I. scapularis ticks in the metropolitan area, primarily Anoka and Washington Counties. Nearly two-thirds of Lyme disease case-patients in 2008 (460 [63%] of 726 cases with a known activity) were most likely exposed to I. scapularis ticks while on vacation, visiting cabins, hunting, or during outdoor recreation.

Image of Figure 1.

 

Updated Friday, 19-Nov-2010 15:16:31 CST