Lyme disease in humans has been called the “great imitator” because of its ability to cause a variety of symptoms, including skin lesions, muscle pain, weakness, arthritis and even psychiatric disorders. Since there is no fever at the time, physicians don’t usually look for an infectious cause.
It’s just as difficult to diagnose in horses. Plus, for most infected people and animals, symptoms are mild and resolve spontaneously when the immune system clears the infection. Still, this spiral-shaped bacterium (spirochete), carried by ticks, can cause a lot of trouble.
Deer ticks transfer Lyme disease when they feed on the horse. Deer ticks are tiny, with the larvae and nymph stages the size of a pin head. Adult ticks measure around 2.5 mm and are oval-shaped. Some people compare an adult tick’s size to a sesame seed. Suffice it to say, deer ticks aren’t easy to see or to grasp with tweezers.
While tick larvae can also transmit the disease, the nymphs and adults, which feed actively in the summer and fall, are most likely involved in transmission.
Ticks don’t jump or fly. They only crawl. They like areas of high humidity at the ground level, such as in high grasses, wooded areas, dense shrubs and areas with a thick cushion of organic material. They climb up blades of grass, shrubs and trees to wait for their next meal.
The horse picks up the tick either by brushing against vegetation or having ticks come on board at hoof level and then crawl to a more secluded spot on the body to feed.
Lyme is heaviest in the Northeast, Mid-Atlantic, and upper Midwestern/central states, although it’s found in almost every state.
Several serological studies on horses living in known Lyme areas to determine the level of exposure show, on average, about 10% of horses in endemic areas have blood antibodies indicating exposure. About 10% of those exposed may develop symptoms, for an overall risk of Lyme disease of about 1%.
This figure includes everything from a mild, self-limiting illness to arthritis. In other words, the estimated risk is about the same as for EPM and much lower than for common infectious diseases, like flu.
The hallmark of Lyme in people is an expanding red rash around the tick- bite site seven to 14 days after the infectious bite. If the horse gets this, we would miss it because of the hair and dark skin. Lyme isn’t even suspected in most horses until long past this stage anyway.
However, a 1988 University of Pennsylvania report described symptoms in foals on a central New Jersey farm with a high number of Lyme-positive horses (by blood tests) that may represent an early stage of Lyme. The main symptom was an unexplained edema of the legs, sometimes with dermatitis.
The dermatitis certainly fits, but the edema might have been caused by infection with another organisms carried by ticks (see sidebar at end of story). Except for dermatitis and edema, early signs close to the time of the bite are likely to be so mild — low-grade fever and transient achiness — that they are missed entirely.
Most horses likely successfully clear the infection themselves with no outstanding symptoms. Those that don’t may progress to a chronic infection, showing arthritis and muscular pain.
Joint pain may be severe and often changes in location from day to day — a key to differentiating this from other causes of joint and muscle pain. The onset and severity of symptoms often has no relationship to the level of exercise, another clue. The shifting lameness involving the large joints can lead to gait abnormalities that are suspicious for EPM when they involve the hind end.
Horses have not been noted to get the neurological symptoms, like facial paralysis, that humans do. If they did, you would likely consider EPM. And, except for lethargy and irritibility, horses don’t get the personality/psychiatric changes some people experience.
A positive Lyme diagnosis is even tougher to confirm than for EPM. The main reason is the same as with EPM: Positive blood tests for antibodies only mean the horse has been exposed, not that the organism ever did or will cause illness.
However, it’s even worse than with EPM because a negative blood test doesn’t rule out Lyme disease. Some people and animals never mount a good immune response, which means it doesn’t show up in the blood tests.
A positive PCR test on joint fluid from suspected cases would likely clinch the diagnosis, but this test is not widely available to veterinarians.
In most cases, the veterinary diagnosis is based on a combination of positive blood test results, residence in a known Lyme area, a high likelihood of exposure to ticks, and the development of the ill-defined, shifting lameness. Rapid improvement after the antibiotics start confirms the diagnosis.
Fortunately, the Lyme organism is susceptible to antibiotics. Therapy is usually continued for a few weeks to make sure all organisms are eliminated. The Lyme organism can be killed by tetracyclines, penicillin and penicillin derivatives.
The choice of antibiotic is up to the treating veterinarian. However, given that there is always the chance of a concurrent infection with Ehrlichia (see sidebar “More Tick Trouble”), tetracycline is probably the drug of choice in most cases.
Prevention is a matter of avoiding exposure to ticks. Keep grass mowed regularly, including pastures. Remove piles of brush and leaves that may harbor ticks. Clear thickets. Remember, ticks dislike sunlight and fresh air. They like moist, humid environments.
Avoid turnout in wooded areas, and keep the ground under shade trees free of buildup of leaves or other organic matter that would encourage ticks. Dogs and cats may also harbor hungry ticks, so use safe small-animal tick-control practices.
Of the insect repellents safe for horses, permethrins are the most effective against ticks. Apply liberally from head to toe, including the undersurface of the tail, under and in the mane, around the ears. DEET is probably the most effective repellent for humans, but it is not proven safe for use on horses.
After riding through wooded or high-grass areas, check the horse meticulously for ticks after your ride and again the next day in case they were too small to detect initially. These should be grasped near the head and pulled out.
It is believed the ticks must remain attached for a minimum of 12 hours, possibly up to 48, before they can actually transmit the infection, so early detection and removal cuts the risk. Ticks may attach anywhere on the horse, but the mane, tail and ears are favorite spots.
If you suspect Lyme disease, contact your veterinarian immediately. The decision on a blood test is debatable, but the sooner treatment begins the less likely your horse will experience long-term effects.