Performance of a Borrelia burgdorferi bacterin in borreliosis-endemic areas.

The safety and efficacy of a commercially available Borrelia burgdorferi bacterin were examined under field conditions in 1,969 dogs that received a total of 4,033 doses of bacterin during a 20-month period. For comparison, 4,498 dogs that were not vaccinated also were evaluated. All dogs were examined at 1 of 3 veterinary practices in borreliosis-endemic areas. Owners of all dogs were educated about the benefits of and methods for avoiding infected ticks. Vaccination was found to be safe regardless of previous exposure to B burgdorferi or history of borreliosis. Thirty-eight (1.9%) of the 1,969 dogs had minor reactions; 1 of these dogs had a reaction after the first and second doses of B burgdorferi bacterin. Reactions resolved without complications immediately or within 72 hours after vaccination. Cumulative incidence of borreliosis was 1.0% (20/1,969) in vaccinated dogs and 4.7% (211/4,498) in nonvaccinated dogs. Preventable fraction, a measure of the reduction of incidence of disease in vaccinated dogs vs nonvaccinated dogs, was 78%. Preventable fraction in dogs that were seropositive prior to vaccination (ie, previously exposed to B burgdorferi) was 58% and in dogs that were seronegative prior to vaccination (ie, without evidence of prior exposure to B burgdorferi) was 86%. This suggested that, in borreliosis-endemic areas, dogs may be most effectively protected by vaccinating them when they are young and before they are exposed to infected ticks, and that in areas where borreliosis is not endemic, it may be useful to initiate vaccination programs so that dogs will be protected prior to exposure to infected ticks.