Lyme disease is an illness caused by the bacterium Borrelia burgdorferi, writes Dr. Sandra Bushmich in the August edition of The Horse. It most commonly is transmitted by the bite of infected Ixodes species ticks, also known as "deer ticks" or "black-legged ticks." Not all such ticks are infected with Borrelia burgdorferi. In areas highly endemic for Lyme disease, including the northeastern and Mid-Atlantic United States, Minnesota, and northern California, a greater proportion of ticks will likely be infected than in areas where Lyme disease is less common.The most common clinical signs of Lyme disease in horses are lameness and behavioral changes. The lameness is usually associated with larger joints (not the foot), and it frequently shifts from limb to limb. The horse may appear to have a generalized stiffness. Fever may or may not be present. Occasionally, laminitis has been associated with Lyme disease. Behavioral changes are difficult to classify. Along with an unwillingness to work (which may be associated with musculoskeletal pain), horse owners frequently observe increased irritability and a change in attitude in their afflicted animals, which quickly return to normal following treatment.Diagnosis of Lyme disease in horses is particularly difficult for two reasons. First, horses, as athletes, are subject to many musculoskeletal injuries and abnormalities that may result in lameness similar to that seen with Lyme disease. Second, the blood tests commonly used for Lyme disease diagnosis detect antibodies to Borrelia burgdorferi, and since subclinical exposure is common, a positive antibody test just indicates that the horse has been exposed to the bacterium, not that his illness is related to Lyme disease.The diagnosis of equine Lyme disease is generally based on the following: history of tick exposure or living in a Lyme disease endemic region, veterinary clinical examination suggestive of Lyme disease, elimination of other possible diagnoses by examination and testing, positive blood tests for Lyme disease to support the clinical diagnosis, and response to appropriate therapy.Lyme disease is treated with antibiotics. The specific drug selected by your veterinarian will depend on his/her experience and preferences for a particular case. Treatment length is often several weeks. Response to therapy is usually seen in the first two to five 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 bacteria 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. Consequently, it is important to monitor the horse for signs of laminitis during the first week of treatment. A veterinarian should be contacted immediately if laminitis is suspected so preventive treatment can be initiated.