Lyme Disease, 2019

Lyme disease is caused by Borrelia burgdorferi, a spirochete transmitted to humans by bites from Ixodes scapularis, the blacklegged tick. Recently, a new species, B. mayonii, has also been identified as a cause of human disease, and 10 cases have been reported in Minnesota residents since 2013, 1 in 2019. In Minnesota, the same tick vector also transmits the agents of babesiosis, human anaplasmosis, one form of human ehrlichiosis, and a strain of Powassan virus.

In 2019, 915 confirmed Lyme disease cases (16 cases per 100,000 population) were reported. In addition, 612 probable cases (physician-diagnosed cases that did not meet clinical evidence criteria for a confirmed case but that had laboratory evidence of infection) were reported. Overall, the number of reported cases of Lyme disease has been increasing despite yearly fluctuations, as evidenced by the median number of cases from 2010 through 2018 (median, 1,203; range, 896 to 1,431) compared to the median from 2000 to 2009 (median, 915; range, 463 to 1,239) (Figure 1).

Of the confirmed Lyme disease cases reported, 551 (60%) cases were male, and the median case age was 48 years (range, 2 to 90). Physician-diagnosed erythema migrans (EM) was present in 650 (71%) cases. Three hundred fifteen (34%) cases had one or more late manifestations of Lyme disease (including 231 with a history of objective joint swelling, 69 with cranial neuritis including Bell’s Palsy, 4 with lymphocytic meningitis, 15 with acute onset of 2nd or 3rd degree atrioventricular conduction defects, and 7 with radiculoneuropathy) and confirmation by Western immunoblot (positive IgM ≤30 days post-onset or positive IgG). Of the 840 cases with known onset dates, onset of symptoms peaked from June through August, with 65% of EM cases experiencing symptom onset in June or July. This timing corresponds with peak activity of nymphal I. scapularis ticks in mid-May through mid-July. The majority of cases either resided in or traveled to endemic counties in north-central, east-central, or southeast Minnesota, or Wisconsin.

• For up to date information see: Lyme disease
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2019