06/28/2025
Lyme Disease in Horses
Brian S. Burks DVM
Diplomate, ABVP
Board-Certified in Equine Practice
Lyme disease (please, not lymes) is caused by the Gram-negative spirochete bacterium Borrelia burgdorferi. The organism was first identified in Lyme, Connecticut, and is common in areas endemic for the Ixodes spp. ticks. It is frequently the cause of disease in humans and animals, especially dogs and cats, and less commonly in horses and cattle. Large mammalian hosts such as deer are important in maintaining the tick population while natural reservoirs of B. burgdorferi such as white-footed mice are important in infecting the ticks. While many horses will become infected with B. burgdorferi following a tick bite, the percentage of horses that will go on to develop Lyme disease is unknown.
Lyme disease is one of many tick-borne diseases, including Anaplasmosis, Rocky Mountain Spotted Fever, Babesia, and Theileria (the latter two are not found in the United States, except along the Mexican border- I guess the ticks don’t stop for customs). Tick nymphs and larvae are very small and easily overlooked on both humans and animals.
Transmission is by ticks alone, not from horse to horse, and there is no evidence of in utero transmission to the fetus. The disease is not transmitted from horses to humans.
Documented syndromes attributed to B. burgdorferi include neuroborreliosis, cutaneous pseudolymphoma (dermal masses at the site of tick bite), and uveitis (blepharospasm, epiphora, yellow-green fibrinous aqueous flare). It causes a fever, often an isolated, non-specific clinical sign. Limited evidence exists to document stiffness, lameness, and malaise in horses because of B. burgdorferi infection, although these signs are possible.
Definitive diagnosis is difficult in horses, and is based upon history, clinical signs, and serological testing. Other causes of the clinical signs must be ruled out. The classic ‘bulls’ eye’ rash in humans is seldom noted in horses. A history of tick exposure and elimination of other diseases is helpful. There is often (but not always) shifting limb lameness, along with polyarthritis, stiffness, laminitis, muscle tenderness, hyperesthesia, lethargy, anorexia, weight loss, chronic poor performance, and fever. There may be signs of cardiac and renal infection. Limb edema is possible, but less common. There may also be uveitis (inflammation of the uveal tract in the eye) and sometimes neurologic symptoms. Other signs are organ specific, and depend upon which ones become infected with the bacterium. The symptoms may wax and wane, or disappear only to recur at a later date. Recurring signs are responsible for chronic inflammatory arthritis. Many, if not all, of these symptoms are non-specific, and can be caused by a myriad of other bacteria, protozoa, and viruses.
Neuroborreliosis, the neurologic form of the disease, may cause behavioral changes, hyperesthesia, ataxia, dysphagia, respiratory distress due to laryngeal dysfunction, cranial nerve deficits, muscle atrophy and neck stiffness. Specific neurologic examination may reveal limbs that are inconsistently or bizarrely placed. The horse may stumble or catch the feet due to hypometria- the lack of picking up the feet normally. Alternatively, there may be hypermetria where the limb is picked up more than necessary or normal. The clinical signs may be asymmetrical; meaning not the same on both sides, and may affect only the hind limbs, or may affect all four limbs. In severe cases, the horse may fall due to severe ataxia, and/or may not be able to rise. There are usually other neurologic signs before this occurs, helping to rule out other neurologic diseases. Cerebrospinal (CSF) fluid cytology is frequently abnormal with neutrophilic or lymphocytic pleocytosis. Antigen testing with PCR on CSF can confirm disease.
The bacteria may hide in cysts within tissues of the body, causing recrudescence of disease. Researchers may finally have an explanation: The tiny, spiral-shaped bacterium called Borrelia burgdorferi can quickly grapple along the inner surfaces of blood vessels to get to vulnerable tissues or to hiding places where it can hole up beyond the reach of drugs. The bacteria rely on an adhesive protein called BBK32 to tether themselves to the endothelial cells like an exceptionally strong bungee cord, helping the bacteria accelerate through the vessels or decelerate when they needed to get out of the bloodstream and into surrounding tissue. This type of attaching and rolling is very similar to how leucocytes (white blood cells) travel to sites of infection and injury, although they are very different both physiologically and genetically. Researchers hope to sequence the protein and identify its configuration to develop medication that target BBK32 protein or its endothelial receptors, helping to prevent or slow the spread of Lyme disease.
In addition to clinical signs, diagnostic testing is available to help give a definitive diagnosis. Testing may not be positive until three or more weeks following exposure. It should be noted that many asymptomatic horses are seropositive to Lyme disease, indicating exposure, but not current infection. This involves testing blood, joint fluid, or tissue that may be involved. Routine testing of healthy horses is not recommended. There may be anemia and an elevated white blood cell count, along with fibrinogen, or the blood may be unremarkable. Culture of the organism is very difficult, requiring special media. Like other testing, elevated blood titers of antibody indicates exposure, not necessarily infection. Re-testing in 14 days is recommended to look for rising titers. False negatives may be apparent initially, as the body has not had time to respond before blood samples are taken.
In humans, a simple IgG test is used. There are many false negatives with this test. In animals, a multiplex assay is used for diagnosis. This test looks for several outer surface proteins, which can help differentiate vaccination from disease, and acute vs. chronic infection.
The organism has a life cycle that involves small mammals (mice) and deer. Small mammals are the reservoir for Lyme disease. Ticks are the vector travelling between the two, or to the horse, primarily the deer tick in the eastern portions of the country. Transmission may take as little as 15 minutes after the tick bites the animal, so prompt removal prevents disease. Most ticks crawl around the body for a while, looking for the perfect place to bite. Once inside the host, it may travel to many different tissues and organs, causing disease.
Treatment is both specific and symptomatic. Intravenous oxytetracycline is the treatment of choice, and is usually given for the first 7-14 days. If this drug is given outside of a vein, there can be severe reactions in the surrounding tissue, thus the horse should be hospitalized with an IV catheter, or at least a veterinarian should come to the farm daily to give this medication. A response is usually noted within 48 hours, but sometimes it can take seven days. Treatment should be continued with doxycycline or minocycline for up to six weeks. This is because the organism is hard to completely clear. Failure to completely clear the organism is responsible for recurrence and for inflammatory arthritis. Tetracyclines can cause diarrhea or renal disease in a small number of cases. Neuroborreliosis requires a second antibiotic, which will pe*****te the blood-brain-barrier, usually a cephalosporin antibiotic or intravenous penicillin. Metronidazole has reported efficacy against encysted Borrelia organisms, though not against the free forms in skin.
Symptomatic treatment involves the use of anti-inflammatory drugs such as phenylbutazone, Banamine or dexamethasone to reduce fever and inflammation. It may also be necessary, in some cases, to lavage an infected joint to remove the bacteria and inflammatory mediators. Medications such as Adequan may also be advised to help protect the cartilage of the joint.
The prognosis for B. burgdorferi infection is generally good as most exposed horses do not show clinical disease. The prognosis for neuroborreliosis and B. burgdorferi uveitis is guarded to poor.
Prevention is aimed at reducing tick populations and preventing infestation of the horse, dog, or cat (or person). This involves reducing brush in pastures and keeping wood piles away and horses away from the woods. Multiple spray and spot-on tick repellent products are available. These products may contain a combination of cypermethrin, permethrin, pyrethrins, or piperonyl butoxide, and demonstrate variable efficacies and durations of action. Spray the limbs, under the tail and jaw, and under the belly help to reduce or prevent tick bites. Daily grooming to find and remove ticks as soon as possible also greatly reduces the risk of transmission of Lyme disease. Contrary to popular belief, you should not smother it with petroleum jelly as this can lead to the tick regurgitating the blood into the horse, thus increasing the possibility of inoculating a disease. Special tweezers can be used to firmly remove the tick. You can then dispose of the tick by immersing it in a strong alcohol and then washing your hands to remove any organism that may have been regurgitated or leak from the tick during removal.
There is a vaccine licensed for dogs that is used, off label, for horses. The vaccine appears safe and induces antibody production in the horse. It is given on day one, again in three weeks, at three months, and then every six months. Remember that no vaccine is perfect, and that some animals may not be fully protected, especially if the vaccine protocol is not followed.
Although Borelliosis is classified as a zoonotic disease, meaning transferred from animals to humans, domestic animals have not been shown to be involved in human acquired cases. Rather, people are bitten by an infected tick (an animal) that transmits the disease.
Dr. Brian Burks, Dipl. ABVP is the owner/veterinarian at Fox Run Equine Center, a 24-hour medical-surgical center near Pittsburgh, Pennsylvania. He is board certified by the American Board of Veterinary Practitioners (Equine Practice). This certifies him as an expert in all categories of equine practice. He enjoys the diagnostic and treatment challenges of internal medicine.
www.foxrunrequine.com
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(724) 727-3481
Fox Run Equine Center
