Infection with Borrelia burgdorferi, the causative agent of Lyme disease, is widespread in the northeastern United States, with nearly 50% of adult horses in some areas infected or with a history of infection. Thomas Divers, DVM, Dipl. ACVIM, Dipl. ACVECC, and others at Cornell University recently completed research that confirmed the consistency and predictability of how B. burgdorferi infects ponies, the value of tetracycline treatment, and efficacy of a recombinant outer-surface protein A (rOspA) vaccine against B. burgdorferi. Divers presented the findings at the 2003 American Association of Equine Practitioners' Convention.

B. burgdorferi, a spirochete (a spiral-shaped bacteria), is maintained in a two-year cycle that involves mammals and ticks (specifically Ixodes species, of which 55-60% in certain geographic areas are infected with B. burgdorferi). Mammals generally are infected by larval, nymph, or adult ticks, but researchers aren't sure whether larval or nymph bites play a role in equine infection. 

"There must be a higher seroprevalence of Lyme disease in horses vs. humans because ticks are more likely to attach to and stay on horses longer," said Divers. For a mammal to get Lyme disease, the ticks are generally attached for more than 24 hours. 

"Previously reported clinical signs in horses have included stiffness and lameness (in more than one limb), muscle tenderness, hyperaesthesia (increased or altered sensitivity to sensory stimuli), swollen joints (rarely), and behavioral changes. Experimentally infected ponies did not demonstrate obvious clinical signs, but subtle signs may have not been noticed," said Divers. Ponies experimentally infected with B. burgdorferi (infected ticks were placed on the ponies for seven days) had detectable antibodies in their blood at five to six weeks post-infection. They showed high antibody titer levels for B. burgdorferi in their blood for nine months (the duration of the study) after tick exposure, and Divers said he and colleagues believe that the organism might migrate through the connective, perineural (around the nerves), and perivascular (around blood vessels) tissue in the skin, fascia (flat layers of fibrous tissue that separate different layers of tissue), muscle, and synovial membrane because of pathology seen in these parts of the ponies after euthanasia. Some of the previously reported clinical signs make sense, because pathology in those areas "could certainly cause hyperaesthesia and lameness," according to Divers' paper on the subject