Lyme Disease in Horses
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Lyme Disease in Horses

by Sandra L. Bushmich, MS, DVM
Associate Professor of Pathobiology
University of Connecticut, Storrs, CT 06269

What is Lyme disease? Lyme disease is a bacterial illness caused by the spirochetal (corkscrew shaped) bacterium Borrelia burgdorferi. It is most commonly transmitted by the bite of infected Ixodes spp ticks, commonly referred to as "deer ticks" or "black legged ticks". Not all Ixodes spp ticks are infected with Borrelia burgdorferi. In areas highly endemic for Lyme disease, such as the northeastern and mid-Atlantic USA, Minnesota, Wisconsin, and northern California, a greater proportion of ticks will likely be infected than in areas where Lyme disease is less common.

How do ticks transmit the Lyme disease bacteria? Immature (larval) deer ticks hatch from eggs and can become infected with Borrelia burgdorferi (the bacterium that causes Lyme disease) if they take a blood meal on a rodent which is already infected with Borrelia burgdorferi. The tick may then transmit the infection to new hosts when it takes its subsequent blood meals. These ticks have 3 developmental stages (larvae, nymph and adult) and must have a blood meal before they can molt (change) into the next stage. They live for 2 years, overwintering even in cold climates. The ticks must attach to the new animal host and feed for 12-24 hours before they can transmit the bacteria to infect the new host. Rodents, especially the white-footed mouse, serve as the natural wildlife hosts of the tiny larval Ixodes spp ticks. Middle stage ticks (nymphs) feed on a variety of wild and domestic species, including humans, rodents, horses, cats, birds, etc. Deer are the predominant wildlife host for adult ticks, but domestic animals, such as horses, cattle, horses and even cats may serve as substitute hosts.

What effect does Borrelia burgdorferi infection have on domestic animals? The disease caused by Borrelia burgdorferi (Lyme disease) affects humans and many domestic animal species. It is a multisystemic disease, affecting primarily the joints, the musculoskeletal system and the neurological system. It is important to know that in many domestic animals, including horses, cats, cattle and horses, subclinical infection (e.g. infection without the development of clinical disease) is common.Thus, a domestic animal living outdoors in a region where Lyme disease is common may be bitten by Borrelia burgdorferi infected ticks and become infected with Borrelia burgdorferi. This animal may go on to develop symptoms of Lyme disease (which happens less than 10% of the time) or may remain clinically healthy. Subclinical infection makes diagnosis difficult, as the common blood tests for Borrelia burgdorferi infection measure antibodies (part of the body's response to infection, and would be positive in an animal which had been exposed to Borrelia burgdorferi regardless of whether the animal became clinically ill from the infection or not.

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 tick adjacent to the skin with tweezers and pulling gently back. Prompt removal of the tick minimizes risk of Borrelia burgdorferi transmission.

What are the clinical signs of Lyme disease in horses? 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 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.

How is Lyme disease diagnosed in horses? 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 following: i- History of tick exposure, or living in a Lyme disease endemic region. ii- 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.) iii- Positive blood tests for Lyme disease to support the clinical diagnosis. iv-Response to appropriate therapy.

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.

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 horses, 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 haircoat (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 spp ticks are active. Keeping pastures mown will make the environment less hospitable for ticks, and removing brush, wood piles, 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.

From the Association of Equine Practitioners

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