Lyme Disease, 2006

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

In 2006, 913 confirmed Lyme disease cases (17.7 cases per 100,000) were reported (Figure 1). This is similar to the 918 cases (17.9 per 100,000) in 2005 and the record number of 1,023 cases (20.0 per 100,000) in 2004. 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 866). In 2006, an additional 24 cases were classified as probable Lyme disease. Five hundred twenty-eight (58%) confirmed case-patients in 2006 were male. The median age of case-patients was 40 years (range, <1 to 98 years). Physician-diagnosed erythema migrans was present in 719 (79%) cases. Two hundred fifteen (24%) cases had at least one late manifestation of Lyme disease (including 176 with a history of objective joint swelling and 28 with cranial neuritis) and confirmation by a positive immunoglobulin G antibody test. Onsets of illness peaked in July (44% of cases), corresponding to the peak activity of nymphal Ixodes scapularis in mid-May through mid-July.

Lyme disease co-infections with HA and babesiosis can occur from the same tick bite. During 2006, six (1%) Lyme disease case-patients also had objective evidence of HA, and two (<1%) had objective evidence of babesiosis. Because of under-detection, these numbers may underestimate the true frequency of co-infections.

Most Lyme disease case-patients in 2006 either resided in or traveled to endemic counties in east-central Minnesota or western Wisconsin. As in 2006, Crow Wing County continued to have the highest number of Lyme disease case exposures (103 [19%] of 444 cases who reported a single county of exposure). Three hundred eighty-six (42%) cases occurred among residents of the metropolitan area. However, a minority of these residents (47 [19%] of 248 case-patients with known exposure) were likely exposed to infected I. scapularis in the metropolitan area, primarily Anoka and Washington Counties. Risk for Lyme disease continues to be high in certain counties at the northern and western edges (Becker, Beltrami, Clearwater, Hubbard, and Itasca Counties) and southeastern edge (Houston County) of Minnesota’s endemic area. About half of Lyme disease case-patients in 2006 (285 [53%] of 541 cases with a known activity) were exposed to deer ticks while on vacation, visiting cabins, hunting, or during outdoor recreation.

For a discussion of the recent increase in tick-borne disease in Minnesota and the distribution of ticks that transmit Lyme disease and other tick-borne diseases, see “Expansion of the Range of Vector-borne Disease in Minnesota” in the March/April 2006 issue (vol. 34, no. 2) of the DCN.

• For up to date information see>> Lyme disease
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2006