We have at our disposal vaccines designed specifically for horses to help prevent a number of common ailments, from West Nile virus and Eastern equine encephalitis to strangles and influenza. But one equine vaccine that remains elusive is that against Lyme disease—an increasingly diagnosed disease transmitted to horses by the Ixodes species of ticks.

There is, however, a canine vaccine that many veterinarians use off-label for horses in Lyme-endemic areas. But to date, there have been no studies evaluating this product’s impact on horses’ humoral (blood) immune responses. So Kathryn Slaughter, a veterinary student at Western University of Health Sciences, in Pomona, California, and colleagues recently evaluated how 42 healthy horses’ bodies reacted after being vaccinated with the canine product.

Slaughter, who completed the study under the supervision of Joe Bertone, DVM, MS, Dipl. ACVIM, a professor of equine medicine at Western University, shared the results of the team’s study at the 14th World Equine Veterinary Association Congress, held Oct. 8-10 in Guadalajara, Mexico.

Lyme disease is caused by the bacteria Borrelia burgdorferi. Horses, like humans and dogs, are incidental dead-end hosts. B. burgdorferi transmission from tick to horse takes 24 to 48 hours, meaning a tick must be engorged in a horse for that time period in order to pass along the bacteria—a fleeting bite won’t do it. Clinical signs generally appear two to five months or more following disease transmission.

Clinical signs of Lyme disease are nonspecific in horses and include chronic weight loss, low-gr