White spots on a horse's otherwise dark withers are a telltale and unfortunately irreversible sign of an ill-fitting saddle. They appear because intense, localized pressure has so damaged the hair follicles that pigment no longer can be dispersed throughout the hair shaft.
Pressure spots can become apparent in a matter of days on particularly sensitive horses or it may take weeks of repeated trauma for them to emerge. They may become sharper as the hairs that were subjected to less pressure on the perimeter recover, but to some degree they always will remain.
Gray horses may develop black spots as a result of pressure injuries. In a very young horse, they may be white and blend into the coat as he ages and grays out over time. The surest way to avoid these spots is to verify that your horse's tack fits. Pressure from the pommel of a saddle that is too large or too small usually is to blame, but the cantle of a saddle that bridges can produce spots, too.
Pressure spots can also appear in other places on the horse's body. A too-small halter will cause the hairs on the bridge of a horse's muzzle to turn white. And a foal whose dam grabs his crest with her teeth may forever bear white marks in a pattern that corresponds to her bite.
This article originally appeared in EQUUS 249, July 1998.