Efficacy of a nonadjuvanted, outer surface protein A, recombinant vaccine in dogs after challenge by ticks naturally infected with Borrelia burgdorferi.

In a blinded, controlled study, thirty purpose-bred, Borrelia burgdorferi negative, mixed-breed dogs 10 to 12 weeks of age were randomly divided into three groups of ten animals each for the purpose of evaluating a recombinant nonadjuvanted B. burgdorferi OspA vaccine (Recombitek Lyme [Merial Limited]) for efficacy and safety. Two groups received two doses of two different lots ofa nonadjuvanted, OspA, recombinant vaccine; the third group served as nonvaccinated controls. All dogs were challenged 3 weeks after the second vaccination with blacklegged deer ticks (Ixodes scapularis) harvested from a B. burgdorferi endemic area in Rhode Island. Clinical signs, antibody titers by ELISA, Western blot assays, and isolation and polymerase chain reaction analyses of spirochetes from biopsy specimens were used to evaluate vaccine efficacy. Direct fluorescent antibody assay was used to evaluate the infection rate in the challenge ticks and in naïve ticks allowed to feed on study dogs after the dogs were infected (xenodiagnosis). Vaccinates responded with high levels of antibodies (determined by ELISA and measured by optical density [OD]), which did not rise after challenge. Vaccinates demonstrated no clinical signs, negative isolation of spirochetes on biopsy, only an OspA antibody pattern on Western blot assay, and negative isolation of spirochetes on biopsy, confirming that the vaccine blocked infection with B. burgdorferi in all vaccinated dogs (20/20). Control dogs demonstrated clinical signs (2/10), antibodies characteristic of infection with B. burgdoferi (10/10), isolation of spirochetes (10/10), and polymerase chain reaction (PCR) detection of spirochetes (9/10). The recombinant, nonadjuvanted B. burgdoferi vaccine protected 100% of vaccinates against infection after a severe challenge that infected 100% of control dogs. The OspA vaccine proved to be highly safe and effective in this study.