You automatically accept life with insects when you make the outdoors part of your daily routine. Horses, horse people and outdoor pets are naturally exposed to all kinds of creepy-crawlies, some less harmful than others. One insect in particular is not harmful in itself, rather it’s harmful because of a bacterium that it can harbor and transmit to people and animals: the Ixodes tick, carrying Borrelia burgdorferi, or Lyme disease.
If you live in the northeastern, mid-Atlantic or north-central United States, you probably know someone—a person or a dog, most likely—who’s had Lyme disease. While we often think of this disease as limited to these species, horses are also susceptible. In the worst case, if Lyme disease were to go untreated, a horse could die. It can develop neurological symptoms or crippling arthritis. However, when signs are recognized, a horse should recover with treatment.
Diagnosis Under Cover
Lyme disease in horses is tricky to deal with because symptoms are similar to other diseases—a half-dozen to a dozen others, says David Trachtenberg, DVM, owner of Ledgewood Equine Veterinary Clinic in Ontario, N.Y.
Often, the horse “just isn’t right.” Classical clinical Lyme disease symptoms include: low-grade fever, stiffness, multiple-limb lameness, muscle tenderness, hyperesthesia, multiple swollen joints and behavioral changes. Of these, hyperesthesia, Dr. Trachtenberg says, is pretty Lyme disease-specific: “If you touch the horse, it’s like the horse says, ‘Ouch.’ It’s super sensitive.”
When a horse like this is presented, Dr. Trachtenberg does a physical exam and collects some history. If the horse lives in an area known as a hotbed for Lyme disease—one that has a high density of Ixodes ticks—or if it’s visited one of these areas, that’s another reason to suspect the illness. Blood tests are the next step.
“In this area of the United States, [Lyme disease] is pretty specific to burgdorferi. In other parts of the world, you can see some different organisms,” Dr. Trachtenberg says. Because other organisms can look similar to B. burgdorferi, it’s not uncommon to get false-positive titers with the typical KELA ELISA (kinetics-based, enzyme-linked immunosorbent assay) blood test.
The KELA test returns results in three levels:
1) negative, not enough organisms present to suggest Lyme disease
2) equivocal, meaning there are enough B. burgdorferi to warrant further testing
3) positive, meaning there are enough organisms to confirm Lyme disease.
If an equivocal KELA test is produced, a western immunoblot blood test is performed, testing specifically for Lyme disease organism antibodies.
“When we get a positive [blood test], if the horse fits the clinical signs, we’ll start treatment,” which consists of a several-week-long course of doxycycline antibiotics, Dr. Trachtenberg says. If, however, the blood test comes back positive but the clinical signs aren’t present, he’ll continue testing the horse for other conditions, not convinced that Lyme disease is the issue.
While many owners opt to treat for Lyme disease anyway, Dr. Trachtenberg cautions against this practice: “When we look at response to antibiotics, we’ve assumed that when the horse is doing better, it’s because the horse is responding to the treatment, but we now know that tetracycline antibiotics also have anti-inflammatory properties.” Because of this unintended outcome, if the horse responds positively to the antibiotic treatment, it might not be because the drugs are treating the Lyme disease; it might be because the drugs are easing pain caused by another condition.
If, however, the horse shows the clinical signs, has positive blood tests and responds to the treatment, Dr. Trachtenberg is generally content to assume the horse is fighting Lyme disease.
One final test can identify Lyme disease in horses: the polymerase chain reaction test, which tests for B. burgdorferi DNA. “If you find the DNA, you know the organism has been there. It’s a very sensitive test,” Dr. Trachtenberg says.
The PCR test is performed on synovial-membrane or skin biopsy. It’s effective when the biopsy is performed in the area of the tick bite; however, if the biopsy is taken from an unaffected area of the horse, the test result will be negative.
Just as in humans and dogs, horses can have a relapse of Lyme disease, which is treated by another round of antibiotics.
Reducing the Risk
If your barn is in an area endemic to Lyme disease, it’s difficult to be vigilant about the environment you expose your horses to. (For a map of Lyme disease prevalence in humans around the United States, visit the American Lyme Disease Federation website, www.aldf.com or check out the map on the next page.) In the eastern United States, the Ixodes scapularis is the most prevalent Borrelia carrier. These live in heavily wooded areas, particularly in the leaf litter on the ground. The west’s common carrier, Ixodes pacificus, lives in forested areas, in grassland, around scrub and brush—just about anywhere.
These ticks most commonly pick up the bacterium from white-footed mice and deer—that’s why they’re called deer ticks—and infect their hosts when they feed on the mammal’s blood. Ticks don’t jump or drop out of trees onto animals, rather they meet their hosts when the animal brushes up against them, such as a horse walking through grass in a pasture or along an overgrown trail.
If you can’t keep your horses away from the ticks, you might be able to reduce the ticks around your property. Two mouse-treatment measures the USDA recommends are cotton balls impregnated with permethrin and set out for mice to use as nesting material, or a bait box in which mice contact an acaricide insecticide when they exit or enter. In the process, the mouse’s fur becomes treated with the acaricide, killing the ticks.
Also, the USDA Agricultural Research Service developed a “4-poster” deer self-treatment device. When the deer feed on corn bait, they rub against paint rollers impregnated with insecticide. Because most adult I. scapularis feed on deer, 4-poster devices can have an impact on the tick population over the range of the deer herds that visit the device.
No Lyme disease vaccines are licensed for use in horses, although some have been tested. “There are veterinarians that are vaccinating horses with the canine Lyme disease vaccine. The canine vaccine is very similar to the vaccine they tested in horses. But, it’s not recommended, as a general rule, as you’re taking a lot of risk,” Dr. Trachtenberg says.
There have been problems with vaccine reactions in dogs and in people, and if your horse had a reaction, the drug manufacturer would likely offer no assistance, as you used the product against their recommendations.
Your clients have probably come to you with concern about finding large, blood-filled ticks in the base of their horses’ manes or along their tail bones. These ticks aren’t the Lyme disease carriers, however. “Chances are, if you can see the tick, it’s not Ixodes. They are about the size of a pin head,” Dr. Trachtenberg says.
If you do, by chance, find one of these tiny ticks, he continues, don’t get a blood test immediately. It takes several weeks before antibodies are produced and a positive titer can be recognized. Instead, keep an eye on the horse and communicate your concern to your veterinarian.
Horses most often present with symptoms of Lyme disease in late spring and into summer. A tick has to be attached to its host for 24 hours to transmit the disease-causing organism, although veterinarians are unsure of how long the horse hosts the organism before symptoms appear.
If you’re in a Lyme disease-prone area, Dr. Trachtenberg also suggests using anti-tick shampoos and insect repellents. These may not be 100-percent effective, but they should offer some protection.
As you deal with the mystery lamenesses, sore horses
and horses that “just aren’t right” this summer, keep an open mind while working with the owner and veterinarian. Mention Lyme disease as a concern, if it could be one in your area, but don’t jump to the conclusion that it must be Lyme disease. This is one condition that could be mistaken for others in disguise.