Veterinarian Amy Grice, VMD, says it often starts with a vague phone call from a horse owner:

“My horse hasn’t been himself the last couple of days, and it seems like he’s lost weight.”

“He was off just a little on the right hind yesterday, but today it seems like the left hind.”

“My mare normally loves to be groomed, but was flinching, pinning her ears, and kicking out at me today.”

As ambiguous and varied as the reported clinical signs are, they often spark at least the possibility of Lyme disease (also called equine borreliosis) for Grice, especially if there are no other presenting signs.

“The challenge with identifying Lyme disease in horses is that the clinical signs can range from changes in behavior to lameness in different limbs to weight loss,” says Grice, who works at Rhinebeck Equine in Rhinebeck, New York.

Joe Bertone, DVM, MS, Dipl. ACVIM, professor of equine medicine at the Western University of Health Sciences College of Veterinary Medicine, in Pomona, California, concurs: “It becomes a ‘rule out’ diagnosis. Lyme disease can present as so many things: lameness in more than one leg, muscle pain, muscle wasting, depression, changes in behavior, etc. It’s very difficult to identify from just clinical signs alone.”

So how do horses get this mysterious disease that is so elusive to diagnose? The same way as humans do, through the spirochete (spirally twisted bacterium) Borrelia burgdorferi. The ticks that transmit the bacteria are the deer tick on the East Coast and black-legged tick in the Western half of the United States.

The ticks go through various life cycles on diffe