A veterinarian shares the latest research findings regarding Lyme disease in horses and her practical experience as a veterinary internal medicine specialist in the heart of Lyme country.
A refresher on this difficult-to-diagnose tick-borne disease
A horse that’s underperforming, lethargic, and repeatedly yet subtly lame—it’s one of the most frustrating scenarios for an owner. At some point in your riding career, you’ve likely encountered a horse with lameness that was either nonspecific or undiagnosed. Maybe you noticed it in one limb one day and another the next.
If this sounds familiar and you live in a Lyme-endemic region (see the map on the next page), someone has likely mentioned the possibility that your horse has Lyme disease. Our knowledge of equine Lyme disease, otherwise known as Borrelia burgdorferi infection, is burgeoning and nascent. Currently, there are more questions than answers in the scientific literature. So how do you and your veterinarian determine if your horse is affected? Read on to find out, based on the latest research findings and consensus statements, as well as my practical experience as a veterinary internal medicine specialist in the heart of Lyme country.
What Is It?
Lyme disease is a tick-borne infectious disease caused by the bacterium B. burgdorferi. In humans infection can lead to a wide range of clinical signs, including rashes; arthritis; skin, neurologic, and cardiac issues; and possibly prolonged fatigue. In dogs Lyme disease can also cause kidney issues. The disease is not as well-characterized in horses, however, due to the limited equine studies that exist, but researchers believe its signs include neurologic issues, uveitis (eye inflammation), cutaneous pseudolymphoma (a nodular skin disease around the tick bite), muscle atrophy (wasting), behavioral changes, hyperesthesia (excessive sensitivity to stimuli), and heart rhythm abnormalities, among others.
How Does a Horse Get It?
In North America hard-bodied (ixodid, from the family Ixodidae) ticks carry B. burgdorferi. In the East the common culprit is the deer tick or black-legged tick (Ixodes scapularis); in the West it’s the Western black-legged tick (Ixodes pacificus). These ticks have a two-year life cycle and must feed on small and large mammals’ blood to survive. Adult ticks tend to be active in the spring and fall, climbing to grass tips when temperatures rise above 40°F and attaching to animals that brush against the grass. This is typically how a tick attaches to a horse, which is a dead-end host—it cannot pass the bacteria on to humans or other animals via contact, needles, or ticks or other bloodborne vectors (intermediate hosts such as mice, squirrels, and deer, however, can serve as reservoirs for B. burgdorferi, which can be transmitted via ticks). Researchers know that after several hours of feeding, the tick will have infected that horse with B. burgdorferi. How quickly clinical signs appear, if at all, however, continues to stump them.
“Borrelia burgdorferi infection is common in horses living in Lyme-endemic areas,” says Rachel Gardner, DVM, Dipl. ACVIM, an equine internal medicine specialist at B.W. Furlong & Associates, in Oldwick, New Jersey. “However, the frequency with which horses develop clinical infection is unknown.”
According to the Centers for Disease Control and Prevention, ixodid ticks are endemic in 14 states in the Midwest and along the East Coast, as well as in six Canadian provinces. Their geographical range appears to be expanding. Because horses live primarily outdoors, they’re naturally at a higher risk of contracting Lyme disease than humans and pets. In fact, horses might be more widely exposed to the disease than we realize. Scientists at the Connecticut Agricultural Experiment Station, for instance, showed that as many as 59% of healthy horses in Connecticut and New York tested had antibodies against B. burgdorferi in their blood—indicating exposure—but did not show clinical signs of Lyme disease. Similarly, Virginia-Maryland College of Veterinary Medicine researchers found that 33% of healthy horses in Southwest Virginia tested positive for the antibodies.
Testing for Lyme Disease
Due to the sparse scientific literature, nonspecific clinical signs, and lack of an experimental model for equine infection, diagnosing equine Lyme disease is both challenging and controversial. Because a positive lab test does not on its own lead to a positive Lyme disease diagnosis, the condition is often overdiagnosed. Indeed, a positive result only means the horse has been exposed to the disease at some point and has produced antibodies against it. Previously vaccinating a horse for Lyme disease can also result in positive testing. So with the high number of normal horses that test positive for the disease, how can a veterinarian confirm that an abnormal horse is actually infected?
Currently, diagnosis consists of four steps:
- Confirming probable tick exposure, meaning the horse is located in an area where Lyme disease is prevalent.
- Observing clinical signs consistent with Lyme disease. “Neurologic signs, uveitis, and specific skin disorders are possible clinical signs of Lyme disease,” says Gardner. “While commonly implicated as clinical signs of Lyme disease, stiffness and lameness have not been documented as sequelae (a consequence) to infection.”
- Ruling out other causes of clinical signs, because these signs overlap with many other diseases’.
- Positive laboratory testing.
Laboratory tests consist of blood and tissue testing from the affected organ (via biopsy or fluid sampling). Blood testing can be performed through the indirect fluorescent antibody test (IFAT), Western blot (WB), whole cell enzyme-linked immunoassay (ELISA), or multiple antigen ELISA (Multiplex). The WB and Multiplex tests can help distinguish between acute exposure to the agent, chronic exposure, and vaccination. However, blood testing on its own can be inconclusive or difficult to interpret without tissue testing.
For example, if a horse is positive on blood testing and has musculoskeletal or skin-related clinical signs, a veterinarian at a referral clinic might biopsy the affected joint capsule or skin area to confirm a Lyme disease diagnosis. It’s more difficult to accurately diagnose horses with neurologic signs or eye inflammation, as blood test results can be positive or negative. In these cases the referral clinic veterinarian might test cerebrospinal (spinal fluid or fluid surrounding the brain) or eye fluid and repeat blood tests as needed to confirm the diagnosis.
As we continue to learn more about this disease, diagnostic recommendations are likely to change. But the main take-home here is that the veterinarian must consider both test results and clinical signs when making a diagnosis.
Treating Affected Horses
Lyme disease treatment isn’t any more straightforward than diagnosis. There is no “golden bullet” that can rid a horse of clinical signs and, due to the high antibody levels that can persist during and after treatment, there’s no good method for monitoring a horse’s treatment response.
In a consensus statement Divers et al. released this year, veterinarians wrote that the broad-spectum tetracycline and β-lactam antibiotics are the most commonly used drugs to treat Lyme disease. Study results don’t yet support the use of one particular drug over another, though. Divers says veterinarians should treat horses with the neurologic form of disease, in particular, with injectable β-lactams penicillin or cefotaxime or the tetracycline oral minocycline.
Faced with the complicated and potentially lengthy diagnostic testing process, as a horse owner you might be tempted to jump straight to treatment or a treatment trial (administering antibiotics to see if they solve the issue). However, researchers don’t recommend this in horses, as it can be expensive, can expose the horse to unnecessary side effects, and can contribute to antibiotic resistance in the microbial populations that affect our horses. Furthermore, some of the antibiotics used to treat Lyme disease have secondary anti-inflammatory effects and might seem to resolve the clinical signs simply because they improve the horse’s comfort level (similar to the anti-inflammatories flunixin meglumine or phenylbutazone).
What’s the Prognosis?
Although people with Lyme disease typically have good prognoses, horses aren’t always so fortunate. Many horses appear to improve clinically, despite persistently positive laboratory test results. However, those with neurologic signs and uveitis tend to have poor prognoses for recovery. Only one horse has reportedly been treated successfully for neurologic signs, and most horses with Lyme-associated uveitis lose their vision.
Can We Prevent It?
The basis of Lyme disease prevention is environmental management and controlling horses’ risk of exposure. You can lower your horses’ risk by mowing tall grasses, clearing shrubs and bushes, and keeping them out of forests and woodland. You can target the small mammal reservoirs by using permethrin rodent bait boxes and ward off the large mammal (deer) with high fencing. Some states use feeding stations with insecticide-laden rubbing posts to treat deer for ticks. Keeping an area of mulch between the woods and your pasture can also help by creating a buffer area against ticks.
Regular grooming and careful tick removal can help prevent ticks from staying attached to horses long enough to transmit the disease. You can also apply a tick preventive such as a permethrin spray or spot-on to deter ticks from latching onto horses.
Finally, administering vaccines in the spring and fall might help prevent infection. A Lyme vaccine for horses is currently in development. Some veterinarians do administer a canine Lyme disease vaccine off-label, but its duration of protection might only be four months, and because it’s not FDA-approved for equids, scientists have not fully explored its safety and efficacy in horses.
“Many questions remain to be answered regarding best practices for confirmation of clinical infection, definitive diagnosis, and treatment,” says Gardner.
Although we still have much to learn about diagnosing and treating Lyme disease in horses, here’s what we do know:
- It’s transmitted through ticks found on small and large mammals;
- Diagnosis should be a four-step process, due to the high levels of equine exposure in certain regions (blood antibodies don’t necessarily signify infection);
- Veterinarians have three to four antibiotics at their disposal for treating Lyme disease; and
- Owners should take environmental and individual measures to reduce horses’ tick exposure.
Armed with this information, you now know Lyme disease is likely not the straightforward diagnosis for your horse with nonspecific lameness, and your veterinarian needs to explore many other lameness causes before venturing down this diagnostic path.