Borrelia Burgdorferi Infection and Lyme Disease Guidelines

Infection with Borrelia burgdorferi, the Gram-negative spirochete bacterium that causes Lyme disease, is common in horses residing in regions endemic for Ixodes spp. ticks, and infection prevalence is likely on the rise as tick vectors undergo range expansion. While many horses will become infected with B. burgdorferi following a tick bite, the percentage of horses that will go on to develop clinical signs of illness, i.e., Lyme disease, is unknown. Many horses infected with B. burgdorferi will remain asymptomatic and never develop clinical disease, and it is generally considered that infection is common, but clinical Lyme disease is rare. Documented syndromes attributed to B. burgdorferi and Lyme disease include neuroborreliosis, cutaneous pseudolymphoma, nuchal bursitis, and uveitis. Limited evidence exists to document stiffness, lameness, and malaise in horses as a result of B. burgdorferi infection, although these signs might be possible.

Diagnosis of clinical Lyme disease in horses can be challenging. Ruling out other diseases which present with similar signs should be undertaken. A variety of serological tests exist to assess antibodies to B. burgdorferi in horses. These tests must be interpreted cautiously because a positive test result: (1) only indicates exposure, (2) does not prove that any clinical signs are due to B. burgdorferi infection, and (3) does not mean that the horse will experience disease in the future. Routine serological testing of healthy horses is not recommended, nor is the treatment of seropositive but otherwise clinically healthy horses.