Spatial risk assessment for Lyme borreliosis in Denmark.

A study of nymphal and adult Ixodes ricinus density was performed in well-defined spruce and beech forest habitats with different levels of roe deer abundance and soil water capacity. In 35 habitats, a total of 489 larvae, 1,611 nymphs and 193 adult I. ricinus ticks were collected. Tick density was found to be influenced by roe deer abundance and soil water capacity. Based on this evaluation, a model predicting increasing number of ticks with increasing roe deer abundance and soil water capacity was suggested. A total of 1,045 nymphs and 106 adult ticks were tested for infection with Borrelia burgdorferi sensu lato Of these, 53 nymphs and 6 adults were found to be infected, leading to an general infection rate of 5% and 6% for nymphs and adults, respectively. Prevalences of Borrelia burgdorferi sensu lato in nymphal I. ricinus were found to be independent of roe deer abundance and soil water capacity. The correlation between human neuroborreliosis incidence and the estimated number of I. ricinus based on roe deer abundance and soil water capacity was examined. Differences in human neuroborreliosis incidence were found to correspond with the expected spatial differences in tick density in 12 counties in Denmark.