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 Lyme disease is unknown.