Geographic survey of vector ticks (Ixodes scapularis and Ixodes pacificus)for infection with the Lyme disease spirochete, Borrelia burgdorferi

Populations of adult Ixodes scapularis and Ixodes pacificus, the two princi pal vectors of Lyme disease spirochetes in the United States, were collecte d from 17 sites in 12 states. Female ticks were fed on experimental rabbits ; ticks and rabbits were subsequently examined for infection with Borrelia burgdorferi. Fourteen rabbits were exposed to I. scapularis ticks from the northeastern states of Connecticut, New York, New Jersey, and Maryland; all 14 rabbits became infected with B. burgdorferi. A total of 165/226 (73%) o f these northeastern ticks was infected. Similarly, ticks from the midweste rn states of Michigan, Wisconsin, and Minnesota transmitted infection to al l three exposed rabbits; 29/51 (57%) of these midwestern I. scapularis were infected. In marked contrast, none of the 12 rabbi ts exposed to I. scapul aris ticks from the southeastern states of South Carolina, Georgia, Florida , and Mississippi acquired infection with B. burgdorferi, and 0/284 (0%) of these ticks contained spirochetes. Four rabbits were exposed to I. pacific us collected from one location in California; 2/4 of these rabbits acquired infection and 2/57 (4%) of the I. pacificus were infected with B. burgdorf eri. The antigenic profiles of all 58 strains tested were consistent with a n identity of B. burgdorferi sensu late. The availability of a human Lyme d isease vaccine adds urgency to our efforts to calculate the ecological tran smission risk throughout the United States, as an aid to the judicious use of such a vaccine.