CHAPTER 32 Lyme Disease
Lyme disease in North America is caused by the spirochete Borrelia burgdorferi. B. burgdorferi is maintained in a 2-year enzootic cycle involving Ixodes spp. ticks and mammals. Deer and the white-footed mouse (Peromyscus leucopus) are the most common mammals involved in maintaining the life cycle. Infection in mammals generally results from larval or nymph bites in the spring and summer or adult female tick feeding in summer, fall, or winter. In horses, it is not known whether larval and nymph bites play an important role in Lyme infection. In most instances, the ticks must be attached to the mammal for at least 24 hours for B. burgdorferi transmission. A large percentage of adult horses in the more eastern parts of the Northeast and Mid-Atlantic states are or have been infected with B. burgdorferi. Infection is also common in Wisconsin and Minnesota. Prevalence is confirmed by serologic surveys, which reveal that up to 75% of adult horses in some of these areas are seropositive. Seroprevalence in other parts of the United States has not been reported but would be expected to fluctuate in a manner similar to that seen with the human form of the disease (Figure 32-1).
Figure 32-1 High-risk areas for Lyme disease in the United States. Adapted from CDC National Lyme disease risk map 2003.
