Horse Disease Focus - Overo Lethal White Syndrome
What is overo lethal white syndrome?
Overo lethal white syndrome (OWLS) is a genetic defect associated with a specific coat pattern in horses. It is most commonly seen in American Paint horses, but can occur in any breed in which pinto markings are allowed or in grades.
A foal with overo lethal white syndrome is born with no visible pigmentation, making it pure white with blue eyes.
Not all white foals have lethal white syndrome. Foals that are homozygous for the sabino gene are also sometimes born white. Foals with multiple dilution genes, especially ones that carry both cream and champagne, can be extremely pale. True dominant white horses have dark eyes.
The foal will generally appear normal and will suckle, but it will not pass the meconium (a foal's first feces, usually passed within thirty minutes of suckling). Colic symptoms appear within twenty-four hours.
There is absolutely no treatment for a lethal white foal.
The genetic defect affects the progenitor cells for melanocytes (which produce melanin) and intestinal ganglia. These foals simply do not have ganglia in part of their gut, and thus cannot pass or digest any food. There are some indications that overo horses may be poorer keepers than solid colored relatives.
Generally, euthanasia of the foal is the recommended course of action on identification of the syndrome.
Cause and Prevention
Overo lethal white syndrome is a genetic disease. It is carried by overo paint horses, which are distinguished from other patterns by having white markings develop along the sides of the horse, but not crossing the center of the back. (No white *over* the back = overo).
However, some horses that carry the overo gene may have little or no white on them. Judging by the number of apparently solid horses that produce overos, this minimal expression is common or the mutation itself occurs spontaneously.
Overo shows up most often in American Paint and American Quarter Horses. It is also seen in some Thoroughbred lines and is fairly common in miniature horses. It is far less commonly seen in Europe and amongst the traveling people, an overo foal is considered unlucky, is not bred and is often got rid of at the next auction. This probably reflects a working knowledge of the dangers of crossing overo to overo.
Preventing OWLS is 'easy'...never breed two horses with the overo gene together. In the past, however, this was hard. Horses with multiple Paint genes or little or no expression can easily hide overo, as can horses that also carry the leopard complex (Generally Appaloosas). Fortunately, there is now a cheap and reliable genetic test commercially available for the overo gene.
It is recommended that all pinto horses, especially Paints, and all solid horses that have pinto in their lines be tested for the overo gene before being used for breeding. Most reputable breeders test their stallions and advertise their overo status publicly. If one parent is tested clean, it is not technically necessary to test the other, but it is good practice to be aware of the overo status of all stock horses that are being used for breeding.