Overo lethal white foal syndrome

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Overo Lethal White Foal Syndrome

Heocolonic Aganglionosis

Overo lethal white foal syndrome (OLWS) is a fatal autosomal recessive trait disorder that affects newborn foals bred that receive a copy of the mutated OLW gene from each parent. OLWS affects the foal's nervous system, due to the impaired development of the foal's digestive system; resulting in intestinal abnormalities and leading to colic and death within the first 72 hours of life.

Horses with the white Overo coat pattern are likely to be carriers of the OLW gene. However on rare occasion, solid-colored horses can also inherit the gene. The Overo coat pattern is described as white markings on the lateral and ventral aspects of the neck and torso. It may be easily confused with the tobiano coat pattern, in which the horse's coat has more white on the dorsal cervical and lumbar regions and the legs. Horse breeds that can have the Overo coat pattern include the American Paint horse, American miniature horse, Quarter horse, Thoroughbred, and half-Arabian.

OLWS is the equine equivalent to Hirschsprung disease, a disorder affecting humans. In both horses and humans, the mutated site in the endothelin receptor B gene (EDNRB gene) results in similar gastrointestinal effects from the mutated gene.

DNA Testing

The only way to be certain that a horse is a carrier of the mutated gene is by conducting a DNA test. Samples used for testing include hair or blood. However, it is important that the sample is collected properly in order for accuracy of the DNA analysis.
  • Hair samples: If hair samples are used, they must include between 15-20 hairs, collected from the mane and tail, and include the roots.
  • Blood samples: If blood samples are used, it must be unclotted, kept refrigerated, and delivered to the testing laboratory within 24 hours.
More information on the testing method, cost, speed of results, etc. can be obtained from the testing laboratory. In the United States, Animal Genetics offers OLW testing.


Colic signs soon after birth
Born with distinctive white coat
Slightly elevated heart rate


  • History
  • Clinical signs
  • DNA testing
  • Ultrasound
  • Radiographs



There is no treatment available, foals are usually provided supportive therapy for the pain (flunixin meglumine), and are eventually euthanized.


  • Do not breed horses that may each contain the mutated frame overo trait
  • Submit samples for DNA testing for Frame Overo traits in suspected breeding horses


Invariably grave.

Scientific Research

General Overviews

Risk Factors

  • Breeding two horses that each have the frame overo trait

Commonly Affected Breeds

Quarter Horse iconPaint Horse iconThoroughbred icon