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May 1, 2011

Insidious Lyme Disease Has Many Disguises

The little town of Lyme, Conn., has a storied history, dating back to the 1640s. But since 1975, its rather dubious claim to fame has been a nasty tick-borne disease first noted in the area.

Lyme disease, as it soon came to be called, is caused by a spiral-shaped bacterium called Borrelia bergdorferi, carried by deer ticks. Though it prefers deer and white-footed mice as hosts, the deer tick, an eight-legged creepy-crawly more closely related to spiders than to insects, is a fairly democratic critter. It will readily latch on to a passing horse, cow, human, dog or cat with its grasping mouth parts and transmit the infection it’s carrying as it feeds on its host’s blood.

Ticks usually attach themselves for one to several days, then let go and drop off into forest foliage or tall grass. The longer a tick is attached, the better its chances of transmitting the Lyme disease bacteria; researchers estimate a nymph-staged tick has to be attached for at least 24 hours to transmit the infection, and an adult needs to feed for 48 hours or more to infect its host.

Lyme disease is now a continent-wide problem. It’s been diagnosed in all 50 states and in most Canadian provinces—and although humans were the first official “victims,” it didn’t take long for veterinarians to realize that it was also affecting horses and pets. Horses, after all, live and work in prime tick territory, and their legs and tails are perfect “tick ladders,” allowing the tiny critters to make their way to thin-skinned areas (like the throatlatch, the inside of the stifle, the belly and the underside of the tail) where they can sink in their sharp mouth parts for a blood meal.

Because tick saliva contains an anaesthetic-like substance, their bites don’t hurt, and their small size makes them inconspicuous—at least until they’re gorged on your horse’s blood, which can pump them up to 10 times their original dimensions. It’s easy to miss a tick that has latched on to your horse (especially the nymphs, which are about the size of a poppy seed), but missing them can potentially set your equine friend up for an infection that can be devilishly difficult to diagnose.

Both horses and deer are “reproductive hosts” for deer ticks, meaning that the adult ticks will take a blood meal from the host, then drop off to lay eggs and die. (The larval ticks target mice in the leaf litter, which is where they pick up the Lyme organism.) If a deer or horse contracts Lyme disease from the tick contact, it can’t pass it along to another mammal. Nor can a horse contract Lyme from an infected dog, cat, other wild animal or human.

Lyme Disease Sort-Of Explained

The trouble with Lyme disease is that its symptoms mimic about a dozen other conditions. Muscle soreness is common, as is lameness, which can shift from limb to limb or play a game of on-again, off-again. There’s often a low-grade fever, which may go unnoticed. There can be joint swelling and heat, especially in the front limbs. Subtle behavioral changes, such as an unwillingness to work (likely a response to low-grade musculoskeletal pain), can also occur. In some horses, Lyme can trigger laminitis.

Most of these symptoms are vague enough that Lyme can be easily mistaken for arthritis, mechanical lameness, sourness or neurological disease. Though Lyme is not considered fatal, it can easily lead to a performance horse deteriorating into an unsound, unhappy pasture potato, with symptoms worsening the longer they go untreated.
Even more baffling is that not all horses infected by the Lyme bacterium develop symptoms. In fact, it’s estimated that only 10 percent of infected animals exhibit any outward signs. These subclinical infections mean that an affected horse produces antibodies to the Lyme bacterium and will test positive for the disease; no one knows whether symptoms can crop up months or years after exposure, or whether those horses who are asymptomatic remain so.

Diagnosing Lyme disease can be a tall order, because horses who live in regions where the disease is common will produce antibodies once they are exposed; this causes them to test positive for the causative organism whether they are exhibiting symptoms or not.

False negatives can also occur if the diagnostic blood test is performed in the first few weeks after exposure, because it takes some time for the antibody levels to build up. To clarify the diagnostic picture, your veterinarian also has to take into account your horse’s history of tick exposure (if you know it), the likelihood that he’s been exposed given the incidence of Lyme in your area, and the elimination of all sorts of other possibilities, through things like lameness exams, X-rays, ultrasound, and blood work which eliminates other diseases.

Suffice it to say, Lyme disease is rarely the first thing anyone proposes; it’s a diagnosis often arrived at only when everything else has been ruled out. Linda Mittel MSPH, DVM, of the Animal Health Diagnostic Center at Cornell University’s College of Veterinary Medicine (N.Y.), said, “You often have to go through extensive lameness diagnostics before you arrive at Lyme. Here in New England, Lyme is probably higher on the list [of possibilities] than it might be out west. It depends on the tick populations; the higher the tick intensity, the higher the number of symptomatic horses.”

Laboratory tests, including ELISA and Western blot tests to detect antibodies to the Lyme organism in equine blood, are “good, but not great,” Mittel said. “They’re the best we have at the moment. We’re working on another platform which will have the ability to measure certain outer surface proteins of the Borrelia bacterium, to help us better define whether the disease is acute or chronic, but it’s not available yet.”

Mittel said there’s also a quick screening test that can be used in the field, which detects the presence of antibodies to a particular marker called C6. “It’s not 100 percent accurate, but a positive C6 combined with the presence of symptoms can give you an initial diagnosis that could be confirmed with the ELISA and/or Western blot,” she said.

Treating Lyme

The good news is that most cases of Lyme disease respond well to being bombarded by antibiotics, sometimes bolstered by anti-inflammatory drugs. Horses diagnosed early respond best to the treatment, but it still may take several weeks of anti-biotic therapy before their symptoms subside completely. Still, improvement is usually seen in the first few days after antibiotics are started—so much so that the response is often the best confirmation that Lyme disease is, indeed, the problem that needs treating.

No drug therapy is without its hazards, of course, and one of the difficulties with administering antibiotics long-term is that these drugs can be indiscriminate, killing both good and bad bacteria alike. Clearing out the beneficial “gut bugs” in the intestinal system can result in compromised digestion or colic. And when the antibiotics start to kill off the Lyme organisms in the first few days, toxins can sometimes be released which result in the horse’s symptoms temporarily getting worse, rather than better—and potentially triggering laminitis. Careful monitoring is necessary during the first week of treatment especially.

How long to continue antibiotic therapy is yet another bone of contention in tackling Lyme disease. Some horses seem to have a recurrence of symptoms weeks or months after antibiotics are discontinued, suggesting that the patho-gen may be able to “take refuge” in tissues which are hard to affect with antibiotics, such as connective tissues, only to re-emerge when the coast is clear.

Said Mittel, “The drugs most people fall back on in New England, where we see a lot of Lyme cases, are oxytetracycline, which is given intravenously, and doxycycline, which is given orally. There are various protocols, including starting with two weeks of the IV drug and then following up with another two weeks of the oral ‘doxy’, but you’re looking at 30 to 60 days of antibiotic therapy at least.”

“There’s a problem with doxycycline,” she added. “It has a marked anti-inflammatory effect, which can really muddy the treatment picture. Because it can reduce muscle aches and pains, it can help alleviate symptoms, but then it can be difficult to tell whether you’ve just treated Lyme disease or another inflammatory process such as arthritis.

“This was an absolute nemesis for me when I was treating possible Lyme cases. With some other diseases, if the horse responds to therapy you can say, ‘That’s as good as a diagnosis,’ but with Lyme that’s not necessarily the case. I think it’s part of the reason that Lyme is over diagnosed.”

Tick Defense

It’s certainly preferable to prevent your horse from contracting Lyme disease in the first place. But warding off equine contact with ticks is a constant struggle, especially in the Northeast, where they are so prevalent.
Tom Mather, Ph.D., of the Tick Encounter Resource Center at the University of Rhode Island, explained,

“The nymphal stage of the deer tick prefers high humidity when it’s active in the summer months. They need high atmospheric moisture conditions; moisture in the soil alone won’t do it. That’s why these ticks aren’t distributed everywhere in North America. The highest rates of infection are along the coastlines where the humidity is optimal and in the Great Lakes region.”

In the eastern United States, the nymphal deer ticks are most active in the summer months, down in the leaf litter in wooded areas. Lyme disease transmission tends to be highest in May and June, when these nymphs are at their peak.

“In the fall, you should be most concerned with the adult-stage deer ticks, which will sometimes crawl two to three feet up on foliage so they can latch on to a passing mammal,” said Mather. “You can avoid some of them by keeping to the middle of wooded trails and not striking off into the denser vegetation.”

There are two chemical lines of defense against ticks. The first is familiar to most horsepeople: ivermectin, which bumps off external parasites as well as the internal kind. Though its action against ticks will be strongest immediately after you deworm, it should have some activity for a few weeks afterwards. Moxidectin (a.k.a. Quest) has the same effect.

The second is permethrin, a synthetic pyrethroid found in a number of commercial equine fly sprays. Spray or wipe a permethrin-based product liberally over your horse, especially on the lower legs, if you plan to be heading into tick territory such as a wooded trail or long-grass meadow.  

Even with chemical help, it’s wise to do a daily “tick inspection” on your horse when you return from a hack or after he’s been out on pasture. Look on the muzzle, the ears, under the mane, inside the stifles, around the groin area, on the underside of the tail and around the anus—anywhere the skin is thin and easy to pierce. Look on the coronet area on your horse as well.

Don’t forget to protect yourself against ticks when you go hacking. Wear long sleeves and use a permethrin-based insect repellent on yourself; DEET-based repellents aren’t effective against ticks. Better yet, purchase some tick-repellent clothing, such as those with “Insect Shield” (carried by outfitters for outdoor and hiking clothing).

Effective rodent control also contributes to tick control. Keep pastures mown and remove brush and woodpiles that could serve as rodent nesting areas.

 
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