Experimental infection of ponies with Borrelia burgdorferi by exposure to Ixodid ticks.

Seven specific-pathogen-free (SPF) ponies, 1-5 years old, were exposed to Borrelia burgdorferi-infected adult ticks while being treated with dexamethasone over 5 consecutive days. One SPF pony (pony No. 178) was first exposed to laboratory-reared nymphs without B. burgdorferi infection and 3 weeks later was exposed to B. burgdorferi-infected adult ticks with concurrent dexamethasone treatment for 5 consecutive days. Four uninfected ponies treated with dexamethasone, exposed to laboratory-reared ticks without B. burgdorferi infection served as uninfected controls. Clinical signs, bacteriologic culture, polymerase chain reaction (PCR) for bacterial DNA, immunologic responses, and gross lesions and histopathologic changes were investigated during the experiment or at necropsy 9 months after tick exposure. In all of the seven challenged ponies, infection with B. burgdorferi was detected from monthly skin biopsies and various tissues at postmortem examination by culture and by PCR. However, pony No. 178 exposed to laboratory-reared nymphs (without B. burgdorferi infection) and challenged with B. burgdorferi-infected adult ticks 2 months later did not develop a B. burgdorferi infection. All of the infected ponies seroconverted. Control ponies and pony No. 178 were negative by culture, PCR, and serology. Except for skin lesions, we failed to induce any significant histopathologic changes in this study. This is the first report of successful tick-induced experimental infection in ponies by exposure to B. burgdorferi-infected ticks. This Lyme disease model will be very useful to evaluate efficacy of vaccines against the Lyme agent and the effect of antibiotic therapy on horses infected with B. burgdorferi.

Vaccination against lyme disease with recombinant Borrelia burgdorferi outer-surface protein A (rOspA) in horses.

Eight 1-year-old ponies were vaccinated with recombinant OspA (ospA gene derived from B. burgdorferi B31) with adjuvant (aluminium hydroxide). Four ponies were used as non-vaccinated controls with adjuvant. One hundred and twelve days after the first vaccination, the vaccinated and non-vaccinated ponies were challenged by exposure to B. burgdorferi-infected adults tick (Ixodes scapularis) collected from Westchester County, New York (tick infection rate >/=60%). Protection from infection was evaluated by culture for B. burgdorferi from three monthly skin biopsies taken near the site of tick bites. B. burgdorferi was not isolated from any of the vaccinated ponies. In contrast, three of four control ponies challenged by tick exposure were skin culture positive. At the time of tick exposure, vaccinated ponies had antibody to B. burgdorferi demonstrable by KELA (kinetic-ELISA), western blot and a serum growth inhibition assay. Antibodies in the challenge control ponies were only detectable by two to three months after tick exposure and remained at intermediate levels until termination of the study. By western blot analysis, antibodies to OspA first appeared in the sera of vaccinated ponies three weeks after the first vaccination. The absence of additional bands, known to develop when the animal is infected, suggests that infection was blocked after tick exposure of vaccinated ponies. Results from this study show that vaccination with recombinant OspA protected ponies against infection after experimental challenge with B. burgdorferi-infected ticks.