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.”