Seasonal distribution of borreliae in Ixodes ricinus ticks.

Ixodes ricinus ticks were collected by flagging vegetation of a mixed oak forest in South Moravia (Czech Republic) at regular two-month intervals from March 1991 to March 1992 and examined for borreliae by darkfield microscopy. Mean annual proportions of infected ticks were 17.2% (15.4% to 21.2% monthly) in females (F), 18.6% (11.8% to 25.9%) in males (M), and 16.3% (12.4% to 20.9%) in nymphs (N); the differences among monthly values were insignificant. However, monthly proportions of intensively infected ticks containing more than 100 borreliae fluctuated widely, from 0.0% to 7.7% (annual mean 3.3%) in F, from 0.0% to 5.6% (mean 2.4%) in M, and from 0.0% to 5.7% (mean 1.9%) in N; the differences among months were significant, and the peak prevalence values were found in July (F, M) or November (N). Mean numbers of borreliae per infected tick reached their peak in September for both adult and nymphal ticks while they were generally low in spring. The highest risk of infection with tick-borne borreliae for vertebrates (including man) occurred in the study area during the month of July; in that month, one either female or nymphal I. ricinus containing more than 100 borreliae was encountered, on the average, every 92 minutes of flagging.