Q I’m worried about Lyme disease. How does it affect horses? Can I protect my gelding?

ELLEN MUNSON, Amherst, Massachusetts

A Lyme disease, the most common tick-borne infection in North America, can cause severe illness in people, dogs, and horses. It’s been reported in many mammals, but doesn’t always cause disease. Many individuals of various species have antibodies to Lyme disease—meaning they’ve been exposed, mounted an immune response, and successfully fended off the bacterium, Borrelia burgdorferi—without ever having developed a recognizable disease.

It’s easy to detect whether a horse has been exposed, using serologic testing (looking for antibodies in the blood). Such tests only reveal exposure, however, not whether the bacterium is still present. Hard evidence of infection—detection of the actual bacterium in the horse’s body—is rare and difficult to find.

Owners often report non-specific signs of the disease, such as a mild and shifting lameness, poor performance, muscle soreness, fatigue, behavioral changes, and poor body condition. Rarer presentations include neuroborreliosis (infection of the central nervous system), pseudolymphoma (a dermatologic condition), uveitis (infection of the back of the eye), and polyarthritis and synovitis (inflammation and swelling of multiple joints).

With the non-specific signs, it’s difficult to pinpoint cause. Even with a positive serological test for antibodies, a horse with poor performance and weight loss may be showing these signs because of gastric ulcers that have nothing to do with Lyme disease. So it’s always crucial to look for other common causes of non-specific signs to give the horse his best chance for recovery.

If a high antibody response to the bacterium is detected, Lyme disease is treatable with tetracycline-type antibiotics, including doxycycline and minocycline. These antibiotics come with concerns, however (see box).

We in the Northeast are probably most at risk for Lyme disease. We’re not far from the original epicenter—Old Lyme, Connecticut—plus have plentiful deer and mice to carry the ticks. To protect your horse, groom him often and well. A tick must be attached and feeding for 24 to 48 hours before it can transmit the disease, so if you find and remove ticks early, you’ll decrease the chances of infection. Ticks that carry Lyme disease like bushy, wooded areas with long grass, so modifying your pasture to avoid these conditions may help. Ticks often cause itching and discomfort, so if you see your horse rubbing or shaking his head (tick in the ear, perhaps), try to find the culprit. Finally, there are spot-on tick repellents that seem to help. Ask your vet about them.

MELISSA MAZAN, DVM, DACVIM
Director, Equine Sports Medicine
Cummings School of Veterinary Medicine
Tufts University

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