September brings the cool whiff of autumn to the northern states. With the heat and humidity of summer on the wane, the weather is perfect for lazy afternoon rides deep into the woods, evenings spent around crackling campfires, and nights of gazing at the stars. If you're planning any of these activities, however, it's wise to remember that these halcyon days also go by a less-than-idyllic name--tick season.
Tiny arthropods who feast on the blood of mammals, ticks usually do little harm to their hosts, but they can carry the agents of disease. Especially in New England, the northern Midwest and northern California, species known variously as deer ticks, bear ticks or black-legged ticks sometimes carry a bacterium called Borrelia burgdorferi
, which is responsible for Lyme disease.
Since it was first recorded in 1975 near Lyme, Conn., Lyme disease has become a widely discussed aspect of outdoor life. Prompt treatment with standard antibiotics usually resolves the disease, but the physical problems it causes are often easy to miss or to misinterpret. In people, Lyme is characterized by sometimes vague symptoms such as fatigue, chills, fever, joint pain, swollen lymph nodes and a distinct "bull's-eye" rash at the site of the tick's bite. In horses, the disease causes a host of signs including irritability, mild lameness, stiffness, low-grade fever and reluctance to work.
The good news is that few horses develop Lyme disease. Even those who live where the incidence of the disease is high among people have only about a 50 percent likelihood of becoming infected with B. burgdorferi
, and among those horses, less than one in 10 ever develops clinical signs of disease. The remaining horses either have a subclinical infection--that is, they carry the bacteria antibodies against B. burgdorferi
but remain clinically healthy--or their immune systems fight off the bacteria (in which case they may carry the antibodies to it for up to a year).
Despite those favorable odds, it's wise to take seriously the Lyme disease risk. After all, the ticks that transmit the disease lurk in many common horsekeeping and riding settings, and Lyme disease responds much more readily when treatment is initiated in its early stages. "The sooner you treat Lyme disease, the more likely you are to eradicate it," says Sandra Bushmich, DVM, a University of Connecticut pathobiologist who studies the disease in horses. Indeed, says Bushmich, the longer a horse with Lyme disease goes without treatment, the more likely he is to experience continuing flare-ups of illness throughout his life.
Fortunately it's relatively easy to protect your horse against Lyme disease. All you need to do is become familiar with the disease's modes of transmission and take a few simple, precautionary steps.
The incidence of Lyme disease in people is highest in a few concentrated areas, but statistics indicate that it can appear almost anywhere in the United States. In fact, although 92 percent of Lyme cases occurred in only 10 Midwestern and Northeastern states, at least one case has been reported in each of the 49 states plus the District of Columbia in the past two decades. Several factors influence an individual's risk of contracting Lyme disease:
If you'd like to estimate your horse's likelihood of developing Lyme disease, take a look at the statistics indicating the incidence of the disease among people, says Bushmich. Horses are most at risk where Lyme disease is highly endemic, meaning that it occurs most often within tightly drawn geographic areas. For example, a Centers for Disease Control survey of Lyme cases in people from 1982 through 1998 indicated that nearly all the cases occurred on the Eastern seaboard as well as in western New York and Pennsylvania, central Minnesota and Wisconsin, and northern coastal California. "The areas that are endemic for people are also endemic for horses and other animals," says Bushmich.
The two species of Ixodes ticks--I. scapularis
in the East and I. pacificus
on the West Coast--that carry B. burgdorferi
are most likely to be found near sources of water. "Ticks like moisture," says Bushmich. "They love to be in grassy or wooded areas that stay damp." Therefore, horses kept on pasture are more likely to encounter the ticks. In any area, however, different microclimates, created by vegetation, topography and other factors, cause variations in tick numbers. For instance, pastures located along waterways or in marshy areas are more likely to harbor ticks, and some seasonal weather patterns, such as an especially rainy summer or snowy winter, are likely to encourage growth in tick populations.
Ticks are also more likely to thrive in areas where their normal host species--deer as well as small rodents, usually white-footed mice--also flourish. Ticks go through three major life stages: larva, nymph, adult. At each stage, the tick must find a bloodmeal; the larva and nymph cannot pass into their next stages, and an adult cannot lay eggs, without one. If the larva or nymph feeds from a host that is carrying B. burgdorferi
, then it can carry the spirochete0 in its gut and infect the next host.
In endemic areas, as many as 50 percent of the Ixodes ticks may be carrying the spirochete. Hunterdon County, New Jersey, in 1996 reported the third highest caseload of Lyme disease of all counties in the United States, with 524 cases per 100,000 people. That year, researchers gathered Ixodes ticks to test them for several pathogens; they found that 43 percent carried B. burgdorferi
. Similar counts on Nantucket Island, Mass., found that 36 percent of Ixodes ticks carried B. burgdorferi
, and in Westchester County, N.Y., the rate was 53 percent.
Once B. burgdorferi
is established within the tick population, it becomes entrenched in the local wildlife as well. When researchers from Georgia Southern University surveyed 14 species of wild animals--including several species of mice, foxes, shrews, feral cats, opossums, deer, raccoons, rats and voles as well as feral horses from Assateague Island National Seashore in Maryland and Virginia and Cape Hatteras National Seashore in North Carolina--they found B. burgdorferi
antibodies in every species they tested from each locale.
Time of year.
Although ticks can feed from any host animal in any life stage, it is the adults that are more likely to affect animals as large as horses. In their earlier life stages, ticks are more likely to attach themselves to smaller hosts, usually mice or other small rodents. As larvae, the Ixodes ticks are barely larger than the period at the end of this sentence, and they grow to about the size of a sesame seed in adulthood. An adult female that has fully engorged herself on blood and is carrying eggs can be the size of a small pea.
Fortunately, the adults are most likely to be present only at certain times of the year. "They are out in fall in the Northeast--September, October, November, until it freezes," says Bushmich. "The ones who don't get their bloodmeal will overwinter and come out again in early spring, usually March or April--as soon as it thaws for a few days--and that's another opportunity for infection." On the West Coast, the peak period for infection runs from November to April. "Clinical disease can appear quite a few months later," she says.
Even when there's good reason to suspect that a horse has Lyme disease, making a definitive diagnosis can be a long and frustrating process. For one thing, the signs of the disease in horses, which include uncooperative behavior, mild lameness, stiffness and low-grade fever, are virtually indistinguishable from many of the minor complaints that come and go in working horses as they periodically recover from taxing workouts or minor infections.
What's more, because so many horses are exposed to B. burgdorferi
without developing disease, laboratory tests looking for the antibodies to the bacterium in a horse's bloodstream offer no definitive answers. "It's a diagnosis you have to back into," says Bushmich. "You must first do an extensive workup to rule out all other common causes of lameness, including bloodwork, radiology and neurologic testing. If all those tests are negative, and the horse is seropositive for B. burgdorferi
, then you can consider Lyme disease."
However, says Bushmich, equine Lyme disease does have some hallmarks. Because B. burgdorferi
spirochetes tend to cluster within joints and connective tissue, causing an inflammatory response, most horses with Lyme disease experience joint troubles. "They will show lameness in the large joints, usually the hocks, knees, stifles, elbows and sometimes the fetlocks," she says, "but it usually shifts from joint to joint," unlike an injured horse, who favors only the affected limb.
Another distinctive sign of Lyme disease is sudden, abrupt behavioral change. "It's the second most common sign and it's pretty remarkable," says Bushmich. "A quiet, well-mannered horse will suddenly become crabby, irritable and unwilling to work." The attitude change is probably related to another Lyme disease sign--hyperesthesia, a heightened sensitivity to pain and other stimuli: "They'll seem generally sore all over, especially in the back," she says.
Horses diagnosed with Lyme disease are typically put on a regimen of antibiotics, usually tetracycline and doxycycline. "Some people will also give anti-inflammatories to make the horse feel better, but then you can't see his response to antibiotic therapy," says Bushmich. "If it's Lyme disease, antibiotics alone will make the horse feel immensely better very quickly. If the antibiotics don't produce that relief, then you've probably got a different problem. But anti-inflammatories can mask that response."
The latter is an important consideration, because laminitis is a rare but possible side effect of Lyme disease and treatment. "When the antibiotics are first started, the spirochetes die off in bulk," says Bushmich. "The toxins released by all those dying bacteria can cause laminitis. It only happens in less than 1 percent of horses treated, but you still must watch out for it. Feel those hooves for heat several times daily for the first few days of treatment, and if he shows any pain or lameness there, begin treatment for laminitis immediately."
Bushmich advises that horses be treated for Lyme disease only if they are showing signs of clinical illness. "You could take a perfectly fine, sound horse and give him laminitis and end up with a case of founder," she says, adding that horses who are positive for B. burgdorferi but showing no signs of illness generally do not need treatment.
No one is quite sure why only a few horses develop clinical Lyme disease after exposure to B. burgdorferi
while most do not. The phenomenon could simply reflect variations in individual immune system functions. Or it may be related to the level of exposure to the pathogen. "A lot of us feel that repeated exposures might make a horse more likely to develop clinical Lyme disease," says Bushmich, who explains that a horse who is bitten more often may be more likely to develop the disease than one who is bitten only once or twice. "But no studies have looked into that," she says. "It's just a hunch."
How about boosting a horse's immunity through a vaccine? Currently, that's not an option. A vaccine protecting dogs against Lyme disease has been on the market for several years, and a vaccine
for people (Lymerix) was approved in December 1998. However, no Lyme vaccine is approved for horses, and it doesn't seem likely that one will become available for quite some time.
For now, the most effective way to reduce a horse's risk of Lyme disease is to limit his exposure to ticks. Here are some simple ways to protect him:
- Do a daily tick examination. Because it takes 12 to 24 hours for the Lyme-causing bacteria to migrate from the tick into the host, removing ticks every day vastly reduces the chances of infection. Use a flea comb to sift through the longer hairs found on or near the head, mane, throatlatch, belly, tail and dock, since these are the areas where ticks are most likely to bite. If you find any implanted ticks, remove them with tweezers by grasping their mouthparts just above the horse's skin and pulling gently.
- Spray insect repellent on turned-out horses. The most effective repellent against ticks are permethrin-based products, which are labeled for that use. "Most people spray during fly season, but they also need to know that tick season is not the same as fly season," says Bushmich. Treat turned-out horses during peak tick seasons: early spring, late summer and fall in the East and Midwest and November through April on the West Coast.
- Reduce habitat for ticks and their hosts. Ticks attach to new hosts by climbing onto long grass and waiting for a passerby. Keeping pastures mown short will help to reduce ticks' opportunities for reaching horses. Also, keep pasture areas free of brush and woodpiles--which are attractive nesting areas for mice and other rodents that can carry tick larvae and nymphs. If possible, keep deer out of your pastures, too.
This article originally appeared in the September 2001 issue of EQUUS magazine.