Lyme Disease, 2000
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 2000, 465 cases meeting the case definition for a confirmed case of Lyme disease were reported (9.5 per 100,000 population). This number represents a 64% increase in cases from the prior high of 283 cases reported in 1999 (Figure 5). During 2000, an additional 13 reports were classified as probable cases of Lyme disease.
Two hundred eighty-one (60%) confirmed cases were male. The median age of cases was 35 years (range, 1 to 87 years). Physician-diagnosed EM was present in 399 (86%) cases. Ninety-one (20%) cases had at least one late manifestation of Lyme disease (56 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 July (44% 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, 249 (54%) Lyme disease cases in 2000 occurred among residents of the seven-county metropolitan area. However, only 62 (13%) cases likely were exposed to infected I. scapularis in metropolitan counties, primarily Anoka, Washington, and extreme northern Ramsey counties. Most cases are reported in patients who either live in or travel to endemic counties in east-central Minnesota (Figure 6) or western Wisconsin. Several east-central Minnesota counties continue to have the highest incidence of Lyme disease in Minnesota (e.g., Kanabec, Crow Wing, Aitkin, Pine, and Cass Counties had incidence rates of 83, 80, 77, 53, and 44 cases per 100,000 population, respectively).