Borrelia burgdorferi sensu lato and Ehrlichia spp. in Ixodes ticks from southern Norway

Abstract

We report the results of a study of the prevalence of Ehrlichia and Borrelia species in 341 questing Ixodes ricinus ticks from two locations in southern Norway. The prevalences of Borrelia burgdorferi sensu lato and Ehrlichia spp. were, respectively, 16 and 11.5% at site 1 and 17 and 6% at site 2. Prevalence and species composition of Borrelia and Ehrlichia varied with location and date of collection. The dominant Borrelia species at both sites was Borrelia afzelii, followed by Borrelia burgdorferi sensu stricto. Borrelia garinii was found in only a single tick. The dominant member of the Ehrlichia group was a recently described Ehrlichia-like organism related to the monocytic ehrlichiae. Variants of Ehrlichia phagocytophila and the agent of human granulocytic ehrlichiosis were also found. The highest prevalences for B. afzelii, B. burgdorferi sensu stricto, and the Ehrlichia-like organism were observed in May. B. afzelii was most prevalent in females, less prevalent in nymphs, and least prevalent in males, while the prevalence of Ehrlichia was highest in nymphs, lower in females, and least in males. Double infections with B. afzelii and B. burgdorferi sensu stricto and with B. afzelii and the Ehrlichia-like organism were significantly overrepresented. Tick densities were highest in May, when densities of more than 200 ticks/100 m2 were observed, and declined during the summer months to densities as low as 20 ticks/100 m2. We conclude that estimates of the prevalence of tick-borne bacteria are sensitive to the choice of date and site for collection of ticks. This is the first study of tick-borne Borrelia and Ehrlichia in Norway and the lowest reported B. garinii prevalence in Northern Europe. The prevalence of the Ehrlichia-like organism is described for the first time in questing ticks.

FIG. 1

Density and stage of ticks per 100 m² at site 1, May 1998 to July 1999.

FIG. 2

Interstadial variation in prevalence of B. burgdorferi sensu lato (Borrelia), B. afzelii (Afzelii), B. burgdorferi sensu stricto (s.stricto), Ehrlichia species (Ehrlichia), the Ehrlichia-like organism (ELO), and other Ehrlichia species. Data are for site 1, May to July 1999, and include 144 nymphs, 75 females, and 75 males divided equally between three collection dates. All occurrences of ticks containing the species indicated, including multiply positive ticks, are counted.