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 B orrelia burgdorferi-infected adult ticks while being treated with dexametha sone over 5 consecutive days. One SPF pony (pony No. 178) was first exposed to laboratory-reared nymphs without B. burgdorferi infection and 3 weeks l ater was exposed to B. burgdorferi-infected adult ticks with concurrent dex amethasone treatment for 5 consecutive days. Four uninfected ponies treated with dexamethasone, exposed to laboratory-reared ticks without B. burgdorf eri infection served as uninfected controls. Clinical signs, bacteriologic culture, polymerase chain reaction (PCR) for bacterial DNA, immunologic res ponses, and gross lesions and histopathologic changes were investigated dur ing the experiment or at necropsy 9 months after tick exposure. In all of t he seven challenged Denies, infection with B. burgdorferi was detected from monthly skin biopsies and various tissues at postmortem examination by cul ture and by PCR. However, pony No. 178 exposed to laboratory-reared nymphs (without B. burgdorferi infection) and challenged with B. burgdorferi-infec ted adult ticks 2 months later did not develop a B. burgdorferi infection. All of the infected ponies seroconverted. Control ponies and pony No. 178 w ere negative by culture, PCR, and serology. Except for skin lesions, we fai led to induce any significant histopathologic changes in this study. This i s the first report of successful tick-induced experimental infection in pon ies by exposure to B. burgdorferi-infected ticks. This Lyme disease model w ill be very useful to evaluate efficacy of vaccines against the Lyme agent and the effect of antibiotic therapy on horses infected with B. burgdorferi .