Lyme Disease in Horses

Ever had a horse with a odd lameness that just didn't seem to fit with the typical leg lamenesses. One week you think the lameness is in the left rear and you swear the next week the lameness is in the right front.

Shifting leg lameness is one of the clinical signs of Lyme disease that makes it so hard to isolate the etiology of such a lameness.

Lyme disease is well known as a human ailment caused by the bite of infected ticks. This disease has been identified in 49 states and the District of Columbia with 95% of human cases identified from the states of Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Jersey, New Hampshire, New York, Pennsylvania, Rhode Island and Wisconsin.

Statistics published by the Center for Disease Control indicate that well over 23,000 people were diagnosed with Lyme disease in 2002. Knowing its prevalence, it seems logical that our equine companions are at risk as well.

The species of tick most commonly associated with Lyme disease is the Ixodes species. Known as deer ticks and black-legged ticks, this species often harbors the bacteria (Borrelia burgdoferi) responsible for Lyme disease. These bacteria may be transmitted during a tick's feeding session. It should be noted that all ticks do not carry the bacteria that cause Lyme disease.

The tick's two-year life cycle is generally maintained through two host mammals: the white-footed mouse and deer. The tick initially becomes infected when it feeds upon an infected white-footed mouse. The tick then transmits the bacteria to others during its nymphal and adult stages as it feeds in the summer and fall.

The tick's second natural host, the common deer, doesn't appear to be affected by the disease at all. However, if an infected tick finds another animal to feed upon (i.e. human, horse, dog, etc), infection and symptoms of Lyme disease may follow.

It is believed that although a large percentage of adult horses in endemic areas may be seropositive (showing production of antibodies) to Lyme disease, only approximately 10% exhibit clinical signs of illness. These signs may vary widely, and diagnosis of Lyme disease as a cause of illness in horses can be a long and difficult process.

A characteristic red concentric rash expanding outward from the tick bite is often the first sign of Lyme disease in humans, however in horses the clinical signs are much more subtle. Symptoms of the disease can include muscle tenderness, stiffness and multiple limb lameness, low-grade fever, strange behavior, panuveitis, extreme sensitivity to touch, and swelling of the joints.

Because of the wide range of symptoms that can be exhibited in your equine, many other similar conditions must be ruled out before a diagnosis of Lyme disease can be made. Other ailments that may mimic the signs of Lyme disease include arthritis, rhabdomyolysis, polysaccharide storage myopathy (PSSM), and equine protozoal myelitis (EPM) to name a few.

So how does one positively diagnose Lyme disease? Diagnosis is generally based upon a history of tick exposure in an endemic area, laboratory evidence of exposure, clinical signs, and exclusion of all other diseases.

Recently, there has been discussion of the use of instant "snap" C6 peptide tests (utilized by veterinarians in the clinic for assessing Lyme disease in dogs) as a means of quickly diagnosing Lyme in horses. This assay, although currently labeled for use in assessing Lyme disease in dogs, appears to conveniently and reliably indicate infection in horses.

The result of this serological test has great supportive value when establishing a diagnosis of Lyme infection, and offers the advantage of producing a rapid result (approximately 8 minutes). Two drops of serum, plasma, or whole blood are required for testing.

Perhaps the greatest value of utilizing this instant test is in ruling out the possibility of Lyme disease in horses exhibiting clinical signs of disease. I have found no data indicating that positively tested horses without overt symptoms will benefit from antibiotic therapy. However, horses that do test positive for Lyme disease with corresponding clinical signs of illness can and should be treated. The two most commonly prescribed drugs are oral doxycycline and intravenous tetracycline.

Currently, there are no available vaccines labeled for prevention of Lyme disease in horses. If you live in an area with a high incidence of Lyme disease, the best means of avoiding an infection in your horse is through preventative measures. Liberal use of repellents, routine tick checks and instant removal of ticks are called for! Bacterial transmission by the tick often occurs sometime after the first 24 hours of attachment, so immediate action can be an effective means of prevention.

Landscaping practices such as removal of brush and providing a buffer between common areas and overgrown, less-manicured areas can decrease the likelihood of tick infestation as well. There may be some benefit in a general pesticide application of these buffers as this can be helpful in decreasing the local tick population.

Lyme disease is a debilitating disease in man or beast, so be watchful for the symptoms, be aware of the incidence of Lyme in your area, and be prepared to limit the chances of your beloved equine contracting this disease.