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The Lowdown on Lyme Disease in Horses

Reports of Lyme disease in horses are on the rise. Here’s what you need to know.

Deer TickThe problems started with stiffness in your horse's right hock. Now the hock seems better, but he's off in front. And he's definitely not his normal, perky self—he mopes around in his paddock, and he pins his ears and tries to move away when you groom him.

On-again, off-again lameness, low energy levels, a cranky attitude—those signs could point to something as simple as overwork. But Lyme disease could produce all those problems, and it may be a growing problem for horses in parts of the United States. Lyme disease can have long-term complications that include damage to your horse's joints, skin, nervous system and even vision.

A bacterial disease spread by tick bites, Lyme is a problem for people, dogs and other animals, not just horses. As the ticks that carry this disease slowly expand their range, cases and concern are increasing. Is your horse at risk? If he develops Lyme disease, how will you know, and what should you do? Can you protect him? This article will help you make sense of the threat.

Who's at Risk?
Lyme disease takes its name from Lyme, Connecticut, where it was first identified in the 1970s. It's now the most common vector-borne disease in the United States, according to the US Centers for Disease Control, which tracks human cases. The disease has popped up in ­almost every state, but CDC figures show that most cases occur in the northeastern, mid-Atlantic and north-central states, with pockets in Pacific and southern states.

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While no one collects national statistics on Lyme disease in horses, cases tend to occur in the same areas as human cases, says Thomas J. Divers, DVM, of the Cornell University School of Veterinary Medicine. Those are all places where the disease-carrying blacklegged ticks—mainly deer ticks and Western blacklegged ticks (photo above)—are common. In some areas up to half the blacklegged tick population may harbor the spiral-shaped bacteria, Borrelia ­burgdorferi (Bb), that cause Lyme disease.

The ticks have three life stages (larva, nymph and adult) and need a blood meal ­before they can molt into the next stage.

They pick up the bacteria as larvae and nymphs by feeding on the blood of infected mice, and they transmit the infection to their next hosts—human, horse, dog, deer or any passing mammal or bird. They seem most likely to feed on horses as adults. In cold-winter regions, adults typically appear in early fall, spend the winter dormant in brush and leaf litter and come out again in early spring. This makes early spring and fall prime times for infection. But horses can get Lyme disease whenever infected ticks are active.

CDC statistics show a steady increase in reported cases. In some areas, including parts of Maryland and Virginia and northern New England, human case numbers are up sharply. Maine, for example, went from no cases reported in 1988 to 970 in 2009. Maine state veterinarian Don Hoenig, VMD, says that Lyme disease is turning up in new areas as the vectors—deer ticks—increase their geographic distribution. "We have ticks where we had none six or seven years ago," he says. Several factors likely contribute to the spread:

  • Wildlife populations: Deer and other wildlife hosts, including migratory birds, can carry the ticks and move them to new areas.
  • Changing landscapes: In many areas abandoned farmland is reverting to forest, and the ticks prefer forest habitat to fields. They're often found in the brush of the forest understory and forest edge—and suburban yards, which mimic that habitat.
  • Warming climate: Warmer winters ­allow ticks to expand their range northward and to spread disease for more of the year, as they are active whenever the temperature is above 40 F.

Reports of Lyme disease have also ­increased as people have ­become more aware of it. "We're looking for it more," Dr. Hoenig says. Lyme is now so prevalent in Maine, he adds, that it's no longer on the state department of agriculture's list of reportable diseases.

If your horse is in a region where blacklegged ticks live, he's at risk. But even if he's bitten, he may not get sick. Even if a tick is packing the bacteria, it generally must attach and feed for more than 24 hours before it transmits the infection to its new host. And even when a horse is infected, he may not develop any signs of the disease. "There must be a distinction between infection and disease," says Dr. Divers. Infection is common where the disease-carrying ticks are prevalent—nearly half of adult horses in some areas of the Northeast have been infected. "Clinical disease [disease that produces recognizable signs] does not appear to be common in horses, although we do not have data on this," he says.

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