Overo Lethal White Syndrome (OLWS) is a condition that occurs in newborn foals. The condition is genetic, and both parents carry the defective gene. Horses that carry this gene are most commonly overo white patterned horses (frame overos), but there are exceptions. The defective gene has been found in American Paint Horses, American Miniature Horses, Half-Arabians, Thoroughbreds, and cropout Quarter Horses (foals born to registered Quarter Horse parents that have too much white to qualify for registration with the American Quarter Horse Association).

OLWS foals have blue eyes and are completely or almost completely white at birth. These foals initially appear normal except for their unusual coloring. After a varying period of time, troubling signs of colic emerge due to the foal’s inability to pass feces. The OLWS foal has an underdeveloped, contracted intestine caused by a failure of the embryonic cells that form nerves in the gastrointestinal system. Oddly enough, these cells also play a role in determining skin color. There is no treatment for OLWS, and surgery to bypass the intestinal damage has never been successful due to the extensive nature of this type of lesion. Veterinarians advise euthanasia for all OLWS foals because death will inevitably occur from colic caused by fatal constipation.

The birth of an OLWS foal is emotionally injurious and often financially devastating for small breeders because it is always fatal. Identifying an individual’s propensity for passing on this disease is essential, and research at the University of Minnesota is making prevention a real solution. Horse breeders can now eliminate the possibility of the birth of an OLWS foal by testing their breeding stock. This research found a mutation in OLWS foals using clues from genetic defect studies on lab animals and humans.

Using these clues from other species, researchers at the University of Minnesota investigated the same genes in OLWS foals, and found a mutation. A test for the defective allele (each gene is made of two alleles, one inherited from each parent) was quickly developed. Testing of OLWS foals, their parents, and unrelated horses revealed that all OLWS foals had two copies of the defective gene, their parents had one, and unrelated horses had none. Simply put, if carriers are never again bred to each other, there can never be another OLWS foal born. Horses at greatest risk of carrying the defective allele are overos, particularly of American Paint Horses and American Miniature Horse breeding. A small number of Tobiano and breeding stock horses also carry the defective gene, and a very small number of carrier horses have been detected in other breeds. These other carriers include Pinto horses, which indicates that as other breeds import overo color patterning, they can also import the lethal gene.

We recommend that horse owners concerned about the possibility of their breeding stock carrying the defective gene contact their veterinarian about the possibility and request a professional opinion about how to proceed with further testing.

Contributed By: Elizabeth M Santschi, DVM (AAEP)