The Western Black-Legged Tick, Ixodes Pacificus: A Vector of Borrelia Burgdorferi

Willy BurgdorferDepartment of Health and Human Services, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Epidemiology Branch, Rocky Mountain Laboratories, Hamilton, Montana 59840

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Robert S. LaneDepartment of Entomological Sciences, University of California, Berkeley, California 94720

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Alan G. BarbourDepartment of Health and Human Services, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Epidemiology Branch, Rocky Mountain Laboratories, Hamilton, Montana 59840

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Robert A. GresbrinkField Services Section, Oregon State Health Division, Department of Human Resources, Portland, Oregon 97207

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John R. AndersonDepartment of Entomological Sciences, University of California, Berkeley, California 94720

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To determine the significance of the western black-legged tick, Ixodes pacificus, as a vector of the Lyme disease spirochete, Borrelia burgdorferi, a tick/spirochete survey was conducted in northern California and southwestern Oregon from 1982 to 1984. Of 1,687 adult ticks collected off vegetation, 25 (1.48%) contained spirochetes. Of 715 ticks from Oregon, 14 (1.96%) were infected whereas 11 (1.13%) of 972 ticks from California harbored spirochetes

An isolate of 1 of the spirochetes reacted specifically when treated with monoclonal antibodies to B. burgdorferi. Polyacrylamide gel electrophoresis of a lysate of the isolate showed it to be nearly identical with 2 isolates of B. burgdorferi

Of the 25 infected I. pacificus, 17 had spirochetes in their midgut only; the remaining 8 ticks showed a generalized infection of all the tissues, with midgut, central ganglion and ovary or testes showing heavy spirochetal infections. Decreased immunofluorescent staining reactivity of spirochetes in tissues other than midgut in 6 of 8 I. pacificus with generalized infection may reflect adverse physiologic conditions for the development of spirochetes in the hemocele.

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