Lyme Disease, 2001

The national surveillance case definition for a confirmed case of Lyme disease includes: 1) physician-diagnosed erythema migrans (EM) (solitary lesion must be ≥5 centimeters in diameter), or 2) at least one late manifestation of Lyme disease (neurologic, cardiac, or joint) and laboratory confirmation of infection. MDH has established the following criteria for laboratory confirmation with regard to counting surveillance cases: 1) positive results of serologic testing conducted by CDC, or 2) a positive Western blot test from a clinical reference laboratory. A probable case of Lyme disease is defined as a person with at least one late manifestation of Lyme disease and laboratory evidence of infection but without a history of EM or appropriate laboratory confirmation.

During 2001, 461 cases meeting the case definition for a confirmed case of Lyme disease were reported (9.4 per 100,000 population), similar to the 465 cases reported in 2000 (Figure 4). During 2001, an additional 21 reports were classified as probable cases of Lyme disease.

Two hundred eighty-four (62%) confirmed case-patients were male. The median age of case-patients was 39 years (range, 1 to 91 years). Physician-diagnosed EM was present in 404 (88%) cases. Eighty-two (18%) cases had at least one late manifestation of Lyme disease (48 had a history of objective joint swelling, and 29 reported cranial neuritis) and confirmation by a positive Western blot test. Onsets of illness peaked in June-July (61% of cases), corresponding to the peak activity of nymphal Ixodes scapularis (deer tick, or black-legged tick) in June.

Similar to data from previous years, 209 (45%) Lyme disease cases in 2001 occurred among residents of the seven-county Twin Cities metropolitan area. However, only 41 (9%) case-patients likely were exposed to infected I. scapularis in metropolitan counties, primarily Anoka, Washington, and extreme northern Ramsey counties. Most case-patients either reside in or travel to endemic counties in east-central Minnesota (Figure 5) or western Wisconsin. Of note, 94 (25%) of 378 Lyme disease cases with known exposure data had likely exposure in Crow Wing County. Several east-central Minnesota counties continue to have the highest incidence of Lyme disease in Minnesota (e.g., Crow Wing, Aitkin, Houston, Cass, and Pine counties had incidence rates of 111, 85, 66, 59, and 49 cases per 100,000 population, respectively).

• For up to date information see: Lyme disease
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2001