How do horses become infected with Borrelia burgdorferi? Horses living in Lyme disease endemic regions may become infected with Borrelia burgdorferi via the bite of infected Ixodes species ticks. The adult ticks, which are present in the fall and early spring, are the stage most likely to feed on horses. This stage of tick is usually large enough to be detected during grooming; the ticks are often found around the head, throatlatch area, belly, and under the tail. Removal of the tick can be accomplished by grasping the mouth parts of the tick adjacent to the skin with tweezers and pulling gently back. Prompt removal of the tick minimizes risk of Borrelia burgdorferi transmission.
Clinical Signs
Clinical signs (symptoms) of Lyme disease appear in less than 10% of horses that are infected with Borrelia burgdorferi. The most common symptoms are lameness and behavioral changes. The lameness is usually associated with larger joints (not the foot), and the horse frequently shifts from limb to limb. The horse may appear to have a generalized stiffness. Fever may or may not be present. Occasionally, laminitis (an inflammation of the tissues inside the hoof wall) has been associated with Lyme disease. Behavioral changes associated with Lyme disease are difficult to categorize. As well as an unwillingness to work (which may be associated with musculoskeletal pain), owners frequently observe increased irritability and a changed attitude in these horses, which quickly return to normal following treatment.
Diagnosis
Diagnosis of Lyme disease in horses is particularly difficult for two reasons: horses, as athletes, are subject to many musculoskeletal injuries and abnormalities which may result in lameness similar to that seen with Lyme disease, and the blood tests commonly used for Lyme disease diagnosis detect antibodies to Borrelia burgdorferi; since subclinical exposure (see above) is common, a positive antibody test just indicates that the horse has been exposed to Borrelia burgdorferi, not that his illness is related to Lyme disease. The diagnosis of equine Lyme disease is generally based on the history of tick exposure, or living in a Lyme disease endemic region. A veterinary clinical examination suggestive of Lyme disease, elimination of other possible diagnoses by examination and testing (lameness exams, X rays, blood work for other diseases, etc.), positive blood tests for Lyme disease to support the clinical diagnosis, and response to appropriate therapy are confirmation of the disease.
Treatment
How is Lyme disease treated in horses? Lyme disease is treated with antibiotics; the specific drug selected by your veterinarian will depend on his/her experience and preferences for the particular case. Treatment length is often several weeks. Your veterinarian may choose oral, intramuscular, or intravenous routes of administration for antibiotic therapy. Response to therapy is usually seen in the first 2-5 days following treatment. In addition to antibiotics, some veterinarians will administer anti-inflammatory drugs and/or medicines to help replace the normal intestinal bacteria killed by the antibiotics. A side effect of treatment in a small number of animals with Lyme disease is a reaction to toxins released by Borrelia burgdorferi that are killed during the first few days of treatment. This can result in worsening symptoms for a day or two, and in the horse this reaction may precipitate laminitis. It is therefore important to monitor the horse for signs of laminitis (reluctance to move, "walking on eggshells" gait, increased warmth in hooves) during the first week of treatment. The veterinarian should be contacted immediately if laminitis is suspected, so preventive treatment may be initiated.
Prevention
What can be done to prevent equine Lyme disease? There is no vaccine for Lyme disease currently licensed for horses. There are several Lyme disease vaccines available for dogs, and a human vaccine should be available in the near future. One would expect an equine vaccine to follow. Until then, prevention of Lyme disease in horses is based primarily on tick control. Daily grooming and removal of ticks is one of the best ways to prevent Borrelia burgdorferi infection. (Remember, the ticks need to attach and feed for 12-24 hours before they can transmit the bacteria). Tick repellents may be applied to the hair coat (particularly head, neck, legs, belly, and under the tail) when horses are turned out. Tick repellents containing the chemical permethrin are especially effective, and several products containing this chemical are approved for use on horses and other domestic animals. Remember to apply these products in early spring and fall, when adult Ixodes species ticks are active. Keeping pastures mown will make the environment less hospitable for ticks, and removing brush, woodpiles, etc. from pasture areas will decrease rodent nesting areas, which also helps decrease tick populations. As always, if you suspect Lyme disease or any illness in your horse, consult your veterinarian for early diagnosis and treatment, as well as for advice on disease prevention.
