Lyme Disease, 2002
The national surveillance definition for a confirmed case of Lyme disease includes: 1) physician-diagnosed erythema migrans (EM) where a solitary lesion is >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 of 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 2002, 867 confirmed cases of Lyme disease (17.6 per 100,000 population) were reported. This represents an 88% increase from the 461 cases reported in 2001 and a 206% increase from the 283 cases reported in 1999. During 2002, an additional 27 reports were classified as probable cases of Lyme disease. Also in 2002, 20 case-patients showed evidence of co-infection with human granulocytic ehrlichiosis, compared to only three in 2001.
Five hundred forty-six (63%) confirmed case-patients in 2002 were male. The median age of case-patients was 38 years (range, 1 to 89 years). Physician-diagnosed EM was present in 766 (88%) cases. One hundred twenty-seven (15%) cases had at least one late manifestation of Lyme disease (86 had a history of objective joint swelling and 24 reported cranial neuritis) and confirmation by a positive Western blot test. Onsets of illness peaked in July and August (71% of cases), corresponding to the peak activity of nymphal Ixodes scapularis (deer tick, or black-legged tick) in June and July.
Four hundred one (46%) cases occurred among residents of the seven-county Twin Cities metropolitan area. However, only 70 (11%) of 666 case-patients with known exposure data likely were exposed to infected I. scapularis in metropolitan counties, primarily Anoka, Washington, and extreme northern Ramsey Counties. Most case-patients either resided in or travelled to endemic counties in east central Minnesota or western Wisconsin. Of note, 187 (28%) Lyme disease cases had likely exposure in Crow Wing County. The counties with the highest incidence of Lyme disease in Minnesota were Crow Wing, Cass, Pine, Aitkin, and Morrison Counties, with incidence rates of 339, 206, 162, 105, and 98 cases per 100,000 population, respectively.
A more detailed discussion of tickborne diseases in Minnesota, including a map of high-risk areas, is available in the May 2003 issue of the Disease Control Newsletter (vol. 31, no. 3).