As many as 20% of adult horses in certain areas of the United States are infected with Borrelia burgdorferi, the spirochete (a type of spiral-shaped bacterium) that causes Lyme disease, according to Cornell University researcher and clinician Tom Divers, DVM, Dipl. ACVIM. Horses are infected through Ixodes tick bites. Divers said that many horses in endemic areas are, or have been, infected, which is evidenced by the fact that 75% of horses in the Northeast and Mid-Atlantic states already have antibodies against the organism.

What is less clear is the association between finding those antibodies and concluding that the horse has Lyme disease. A spectrum of clinical signs are linked with Borrelia infection, but many--or even most--horses with active infections don't have clinical signs of disease. Therefore, the finding of an antibody titer is not necessarily an indication that the horse should be treated for Lyme disease.

The clinical signs that are typically attributed to Lyme disease (so named because early investigations centered around clusters of the disease in humans around Lyme, Conn.) include fever, stiffness and lameness in multiple limbs, painful muscles, hypersensitivity of the body surface and resentment of touch or pressure, joint swelling, and dullness or other changes in behavior. Signs of Lyme disease have been confused with osteochondritis dissecans (OCD), degenerative joint disease (DJD), tying-up, and equine protozoal myeloencephalitis (EPM), among others.

In a study led by Divers, none of the 27 ponies experimentally infected with B. burgdorferi developed overt clinical signs of disease even though when the ponies' tissues were examined, the organism was found in several sites. The tissues most likely to harbor B. burgdorferi were lymph nodes (especially in the region of the infecting tick bite), muscle, fascia (the whitish tissue that connects muscles to subcutaneous tissue or binds muscles together in bundles), and synovial (joint) membranes in all limbs. Less frequently, the spirochete ended up in the heart, kidney, or meninges (membranes covering the brain and spinal cord).

Vaccinated horses generate serum antibodies as they do in response to natural infection, but tests are available that enable veterinarians to distinguish between vaccine-induced antibodies and those induced by natural exposure to the organism. Divers said vaccination is likely to be of limited value once a horse is infected. Natural infection doesn't do much better at stimulating a protective immune response; persistent infection and re-infection, even after a bout of naturally acquired disease, are common.

The mainstay of treatment is administration of antimicrobials such as oxytetracycline (which must be given intravenously), doxycycline, and ceftiofur. Non-steroidal anti-inflammatory drugs can be administered to control muscle and joint pain, and chondroprotective agents and acupuncture can be helpful adjunctive treatments.

Evidence suggests that infected ticks must remain attached to a horse for at least 24 hours for transmission of the B. burgdorferi organism. For this reason, aggressive tick control measures such as insecticidal sprays or wipes might be the most effective measure of all for preventing the disease.

About the Author

Kim A. Sprayberry, DVM, Dipl. ACVIM

Dr. Kim A. Sprayberry, DVM, Dipl. ACVIM, is an internal medicine specialist at Hagyard Equine Medical Institute in Lexington, Ky. When not working with horses, she enjoys pursuits in medical journalism and editing as well as kayaking and American southwest archaeology.

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