A well-attended Table Topic session on equine Lyme disease was held at the 2010 American Association of Equine Practitioners Annual Meeting, held Dec. 4-8 in Baltimore, Md. The audience agreed that the disease is more common in certain parts of the United States than in others, with most cases occurring in the Northeast. Diagnosis is often subjective rather than definitive, and it is important to include both ELISA and Western blot testing in the testing arsenal. Other diagnostic elements include the horse's clinical signs, medical history, degree of exposure to ticks, and retrospectively response  to appropriate therapy for Lyme disease. Testing repeated after successful treatment is more likely to show a drop in the severity of the Western blot test than in the ELISA titer; many horses will carry a persistently elevated ELISA titer after an initial infection.

Regarding the vaccination of horses against Borrelia (the disease-causing organism), attendees were reminded that there is no vaccine licensed for use in horses. As such, no safety or efficacy data exist. However, in one experimental trial, ponies vaccinated with a canine Borrelia vaccine proved to have effective immunity against a subsequent challenge with the Borrelia bacterium. Treatment of the disease should be for horses with actual clinical signs and not simply a positive antibody test; the majority of horses in endemic areas have been infected but might never exhibit clinical signs of disease. This becomes especially relevant when a buyer requests testing for Borrelia infection as part of a pre-purchase examination. Veterinarians recommend that such testing be considered as part of the medical history but, in the absence of clinical abnormalities, not be considered as a reason to purchase or not purchase the horse. While different antibiotics have been employed against Lyme disease, oxytetracycline intravenous and oral doxycycline remain the most commonly used.

The audience agreed that complete protection of the horse from exposure to tick bites is rarely possible. However, owners can take some measures to minimize the risk of tick-borne diseases:

• Twice daily use of fly repellants;

• Use of extended-efficacy 45% topical permethrin insecticide;

• Daily inspection for and removal of ticks; and

• Keeping brush and grass trimmed in turn-out areas.

This session was moderated by AAEP past president Harry Werner, VMD, and Tom Divers, DVM, Dipl. ACECC, ACVIM, a professor of medicine at Cornell University's College of Veterinary Medicine