Lyme disease is caused by a spiral-shaped bacterium called Borrelia burgdorferi that is spread to some mammals via the bite of specific hard-bodied ticks. Also known as borreliosis, it is widely considered the most important tick-borne bacterial infection in North America, and the question remains: Do ticks transmit the bacterium to horses and cause disease, as they do to humans and dogs, or is this a disease presumed also to affect horses simply because they coexist in the same environments as the ticks?

Many adult horses in specific geographic areas are seropositive for Lyme disease (i.e., have antibodies in their blood against the bacterium), indicating they are either currently infected or have been infected with Borrelia, according to clinicians and researchers such as Thomas J. Divers, DVM, Dipl. ACVIM, Dipl. ACVECC, professor of medicine and chief of Large Animal Medicine at Cornell University’s College of Veterinary Medicine.

Here we’ll review the key advances that have been made over the past 10 years and whether equine borreliosis is a real threat to horses or if, as some researchers contest, Lyme disease poses no threat to horses at all