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.

Cornell researchers have found in their treatment of Lyme disease that clinical signs are most commonly reported in performance horses used for eventing, probably because mild lameness/stiffness and hyperaesthesia are more likely to be noticed in these closely monitored animals. Just before seroconversion (the change of a serologic test from negative to positive, indicating the development of antibodies in response to infection or immunisation) to Lyme disease, fever and edema have been reported in some horses, but these signs are more likely a result of Anaplasma phagocytophilia infection, which can be found concurrently with B. burgdorferi in many Ixodes ticks and cause dual infection in the horse.  

Treatment and Vaccination

In a second study, Cornell researchers found that tetracycline given intravenously once daily is superior to orally administered doxycycline once daily or intramuscular ceftiofur (Naxcel) twice daily, which both have been used in Lyme disease cases. They think this might be because tetracycline achieves higher tissue concentrations compared to relatively low levels of doxycycline given orally. "Some of the ponies treated with doxycycline or ceftiofur had a significant decline in antibody level during treatment, but antibody level increased after treatment was discontinued in three of four ponies in each group," wrote the researchers. So it appears that the drugs might inhibit reproduction of, but will not always eradicate B. burgdorferi. The efficacy of tetracycline is unproven in horses which have not been previously treated and have been infected for longer than five months.

A final study showed that the rOspA vaccine is effective in preventing Lyme disease in ponies challenged with infected ticks three weeks after a three-dose vaccination series. The vaccine seems to inhibit the spirochete within the tick since there was no serologic conversion in vaccinated ponies. This is probably because when the tick feeds on the blood of vaccinated horses, the spirochete, which inhabits the gut lumen of the tick, is exposed to antibodies in the horse's plasma. 

"We do not know how frequently to vaccinate" in order to maintain protection against B. burgdorferi, Divers said. The vaccine would be of questionable value once the animal is infected, and safety studies need to be completed on it before it becomes widely used. 

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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