January 2000

Lyme Disease:
A Threat to Horses?

by HEATHER THOMAS

Lyme disease was first reported as a disease in 1975, when 51 people in Connecticut were diagnosed with a unique form of arthritis; the disease was named for one of the towns in which the illness appeared. Research efforts isolated the pathogen causing it (a spirochete) in 1982, when they found the bacterium in the gut of a black-legged deer tick. The next year, this pathogen was recognized as a species that had not been seen before, and was named Borrelia burgdorferi. In the years since then, we've learned that Lyme disease can also occur in dogs, horses, and other animals, as well as people.

Steven Barthold, research scientist at the University of California at Davis, has been studying B. burgdorferi for a number of years. He says that the Borrelia bacteria occur around the world. "Though there are some genetic variations of this bacterium it causes pretty much the same disease in whatever host it infects. For the most part, it is transmitted by the Ixodes ticks, and these ticks are all related; they are similar vectors around the world."

This is a tick and a bacterium that is rather non-selective in the hosts they pick, according to Barthold, capable of infecting mice, birds, people, horses, dogs, etc. "Because of this, we can use mice as laboratory animals, using them as a model to get a better idea of the bigger picture, and how the disease affects other animals," he says. "We don't know a lot about the actual disease in horses. There's not as much clinical or anecdotal speculation as in dogs or people; the horse has not been studied much as a research animal. Symptoms are varied-it's hard to describe an actual symptom-so we use the mouse, in a controlled situation, for studying the disease."

He says, "The bacterium has a number of different proteins on its surface that could be potential candidates for a vaccine, but they are shut off when they enter the host. That's the million dollar question-how can we get around that problem. The bacterium is capable of infecting, and persisting literally forever in a host that is otherwise perfectly normal and should be able to eliminate it. So that's what we are looking at in the mouse's system, looking at how it expresses different proteins in different tissues and how the bacterium evades immunity."

Barthold says researchers have used this system for developing a Lyme disease vaccine for humans and dogs. "The vaccine, called Lymrix, is put out by Smith-Kline Beecham. It is an ospA (outer surface protein A) recombinant vaccine approved for use in humans. It is being used here in California in endemic areas, in people who are most at risk, and several other areas on the West Coast, and a lot of places on the East coast, and it seems to work."

There is no vaccine available at this time for horses, though a research project at Cornell is presently working on one. Barthold says that a vaccine for horses must be tested for safety and efficacy. "There are a couple different vaccines for dogs, and some veterinarians may be using them surreptitiously in horses. The one put out by Fort Dodge is a bacterin, which means it is the whole bacterial product, in which the bacteria are killed and the vaccine uses the whole substance. It works, but has the disadvantage of not being pure. A bacterin usually has other things in it that are not necessary to the vaccine; thus the chances of side reactions or problems are greater."

He says there is a more purified product on the market now for dogs (an ospA vaccine), but the basic question in the veterinary world is whether or not it is appropriate to vaccinate for Lyme disease. "In an area like Connecticut where Lyme disease is endemic, nearly every dog is exposed, and the rate of infection is high, based on antibody tests," he says. "They are probably all infected, and once they are infected they stay infected. The fact that they have antibodies probably means they are infected, but most of them have no clinical disease. There are not that many with lameness or any other symptoms."

He says that even the dogs with symptoms tend to have just a transient illness. "They may be lame, or have a high fever. Some have heart conduction problems. But these problems go away spontaneously, whether or not the dog is treated. The symptoms come and go, and are usually not that severe. So in the East, the veterinary community tends to treat the animal with antibiotics only if it is clinically ill. If not, they leave it alone." Now with the vaccine available, he says most dogs are being vaccinated, "some of them unnecessarily. Now they hardly ever see clinical disease in dogs, so the vaccination may have reduced it some."

Barthold says there are some reports in the veterinary literature of Lyme disease in horses, in which they get arthritis, but a lot more research needs to be done with horses. "We do know that deer get exposed to Lyme disease but don't get infected, and there are other species like rabbits that get infected and then get over it. Some species seem to be more susceptible to the infections, while others get the infection but don't get the actual disease-they have no symptoms. We're not exactly sure how the horse is affected."

He feels that horses probably get infected and occasionally get the disease, and if they do, they tend to show lameness. "But horses are prone to arthritis anyway, since they have so much weight bearing on small legs and joints-so the lameness may be due to something else. There are no easy answers for the horse owner. I think it is probably wise, if you have a horse with high fever and lameness, to have a veterinarian consider Lyme disease as one of the possible list of causes, and treat accordingly."

Whether we should vaccinate horses is questionable, in his view. "If we vaccinate a large population of animals, the rate of adverse reactions might be higher than the actual disease itself. That's probably the case with the dog population too. It doesn't matter how good a drug is, there's a certain percentage of any population that will have an adverse reaction."

THE TICK: The bacterium causing Lyme disease is most commonly transmitted by the bite of an infected Ixodes tick, often called a deer tick or black-legged tick, which is smaller than most other ticks. Not all Ixodes ticks are infected, but in areas where the disease is common (the Northeast, mid-Atlantic coastal states, Minnesota, Wisconsin, and northern California), a much higher percentage of ticks are infected. The tick has three stages in its life cycle. The larvae that hatch from eggs are not infected until they feed on mice and other rodents that are infected. Then they molt and become nymphs, and are still infected.

"The nymphs are rather non-selective as to host, feeding on anything that comes along-mice, deer, birds, frogs, people-and this is the highest risk stage for exposure to the disease, since the nymphs are hungry and will feed on anything. Then they drop off and molt, and the adults depend on the white tail deer population for reproduction. The deer, however, don't become infected. They seem to have some resistance. The deer are important to the life cycle of the tick, but are not involved in maintaining the bacteria that cause Lyme disease," says Barthold.

The life cycle is a little different on the West Coast. "We have Lyme disease here in the West, says Barthold, "but instead of being carried by mice it is carried by pack rats. They have their own little tick, Ixodes neotoma that is somewhat species specific, found on every pack rat. This tick has a pack rat cycle, spreading Lyme disease from pack rat to pack rat. But the rats don't get sick; it's kind of a dead end cycle, rarely being transmitted to people or other animals."

"But we also have another tick, Ixodes pacificus, that is less selective in its feeding. In its early stage it often feeds on reptiles, and they, too are a dead end host. It's when the Ixodes pacificus feeds on pack rats inadvertently that they get infected, and if they also inadvertently feed on horses, dogs, or people, they can transmit bacteria. But this is a complex and inefficient process, having to get the disease first from the pack rat. The tick that bites a horse or human may not even carry B. burgdorferi unless it first fed upon a pack rat. Ixodes pacificus also feeds on lizards and snakes, and this cleanses the tick population of the bacteria because these pathogens don't grow in reptiles."

Barthold says these ticks are rather fragile and do not live in dry climates. "They need a moist environment like a forest floor or damp ground, plus the right types of host. If you live in Southern California or another arid region, you're not apt to get Lyme disease. In the Northwest, however, there is Ixodes augusti, another tick that probably has another type of life cycle. And there are other Borrelia living in different rodent populations at higher elevations, such as in Colorado, and their life cycles never break out in other species. Chances there would be very low of getting Lyme disease; it's mainly a problem in more humid areas."