Lyme disease spirochetes in ticks collected from birds in midwestern United States.

In a tick-spirochete survey conducted from all 1989 through fall 1992 in north-western Wisconsin, 4,256 birds (composed of 91 species) were examined for ticks. Infestations were recorded for 400 birds (composed of 30 species). Of 1,184 ticks taken from 335 birds (composed of 26 species), 60 (5%) Haemaphysalis leporispalustris (Packard) from 8 species of birds were infected with the Lyme disease spirochete. Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner. Similar surveys conducted in 1990 and 1991 in Minnesota and Michigan yielded 223 H. leporispalustris from 61 birds (composed of 23 species), all free of spirochetes. However, 1 B. burgdorferi-infected Ixodes scapularis (Say) was found on 1 bird species in Minnesota. Most ticks were collected in fall from ground-foraging birds such as thrushes and sparrows. These results confirm that tick-infested birds are important in disseminating Lyme disease spirochetes and may also play a role as sources for infecting ticks.