Overo Syndrome Research Paper

“The true white horse does not exist”, almost everyone in the Equine world has heard this at one time or another. The reason that a true white horse does not exist, is because of the Overo Lethal White Syndrome or OLWS for short. Although it can be called many names, this is the most common. This syndrome affects horses of the overo gene pool. An overo is a type of white coat pattern that occurs in several breads of horses. The white coat patches can be associated with any type of coat pattern. The white patches can be anywhere on the lateral and ventral areas of the barrel of the horse as well as the lateral sides of the neck and flank. However the patches can never cross over the dorsal portion of the equine as this becomes a different coat pattern called a Tobaino. This coat coloring is acceptable in some breads and registries, but not all. The registries that accept this type of coat patter are the American Paint Horse Association (APHA) and the Pinto Horse Association of America (PtHA). The breads that the overo coloring has been found in are the American Paint horses, American Miniature horses, Half-Arabians, Thoroughbreds, and cropout Quarter Horses (“Overo Lethal White Syndrome” 1999).

However when two overos are bred together they can sometimes produce a pure white foal that seems normal when it is first foaled (Trommershausen-Smith 1977). However very quickly it becomes apparent that it is not healthy, as the foal will soon exhibit symptoms of colic as it fails to pass its meconium, the first feces of its life. Thus as there is intestinal aganglionosis, it cannot survive and it promptly on average 12 hours after its birth. (Vonderfecht et. Al. 1983). The predicted percentage rate for a pure white overo foal to be born from two overo parents is 25%. (Lethal White Overo 2013). This data suggests that it is a recessive disorder that affects these