Relationship between Ixodes ricinus density and prevalence of infection with Borrelia-like spirochetes and density of infected ticks

During 1994-1995 we examined the density of questing Ixodes ricinus (L.) nymphs and their prevalence of infection with Borrelia-like spirochetes at 43 localities in south-central Sweden with median nymphal densities ranging from 0.1 to > 50/100 m(2). The general pattern was that the infection prevalence in nymphs increased with nymphal density at < 10 nymphs per 100 m(2), peaked (20-30%) at 10-20 nymphs per 100 m(2), and decreased at higher nymphal densities. A low infection prevalence (7-10%) in nymphs at 4 of the 5 localities with the highest nymphal densities (> 35 nymphs per 100 m(2)) at the small island of Bedaron was most likely related to the presence of large numbers of fallow deer, Dama dama (L.), which are reservoir-incompetent for Borrelia burgdorferi s.l. Johnson, Schmid, Hyde, Steigerwalt & Brenner. However, because the increase in nymphal density at Bedaron was greater than the decrease in infection prevalence, 5 of the 6 highest densities of infected nymphs occurred in the 5 localities at Bedaron. We suggest that reservoir-incompetent cervids, being more important hosts for adult I. ricinus than for the larvae, indirectly increase the density of infected nymphal ticks by feeding large numbers of adult ticks, thereby increasing the number of larvae feeding on reservoir-competent shrews, rodents, and hares.