Overo lethal white syndrome (OLWS)
Photo credit: Stephanie Valberg, University of Minnesota
The overo coat pattern features white markings that do not cross the back of the horse between its withers and its tail. An overo may be either predominantly dark or white. OLWS is a recessive disorder (two copies of the mutation are necessary to produce disease) that appears to have been selected for in horses where white spotting is a desirable trait. The defective gene has been found in American Paint Horses, American Miniature Horses, Half-Arabians, Thoroughbreds, and crop-out (horses with excessive white markings) Quarter Horses.
Horses heterozygous for the mutation most commonly have an overo-type white coat color pattern with the highest association in frame overos (> 94% heterozygotes, frame overos typically have white patches centered in the body and neck). However, there certainly are many exceptions within horses of overo lineage where horses without white coloration carry the mutation (up to 20% incidence of heterozygotes in broodstock lacking excessive white patterning).
OLWS foals are homozygous for the mutation and are born alive with blue eyes and a white coat. Small black markings on the head, mane and tail can occur in OLWS foals. These foals initially appear normal except for their unusual coloring. After a varying period of time, signs of colic emerge due to the foal's inability to pass feces. The OLWS foal has an underdeveloped, defective 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. Euthanasia is advised for all OLWS foals with colic. There are some completely white foals that do not have OLWS. Although owners call these “living lethals” they actually are heterozygous for the OLWS mutation or have a separate viable white coat color gene.
Testing for the OLWS gene is performed at the Veterinary Genetics Laboratory at the University of California, Davis. This test is recommended for all frame overos and their descendants. Mares or stallions that have previously produced an OLWS foal are in all likelihood heterozygous for the mutation. Carriers should not be bred to each other to avoid the birth of OLWS foals.
The 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 indicate that as other breeds import overo color patterning, they can also import the lethal gene.