DNA test can predict overo lethal white syndrome

By Marty Metzger
Correspondent

Overo lethal white syndrome is an unexpected and cruel malady in the world of horses.

Eleven months of gestation, and previous time spent planning bloodlines and a foal's future, are lost. These pure white or nearly white babies initially appear normal, perfect in every way, but within 12 hours, they exhibit signs of colic and then die within three days.

The syndrome, which is also known as congenital intestinal aganglionosis, causes a nonfunctioning colon, for which no cure exists. Death occurs in all cases. Some owners choose to have an OLWS foal euthanized. Regardless, the dead foal's dam becomes depressed and the owner emotionally and financially distressed.

OLWS was so named because it was initially discovered in white foals born to parents of overo lineage. Scientific explanations are relatively recent, the most extensive research having been conducted during and since the 1990s.

Every gene is comprised of two units called alleles. Foals receive one allele from each parent. In horses of overo lineage, genes can be either normal (N/N) or normal/lethal (N/L). Normal/lethal horses are carriers of OLWS. If two N/L horses are bred, the resulting foal stands a 25 percent chance of inheriting the lethal allele from each, thus being L/L and doomed at birth.

Not all overos carry the lethal allele, and some solid-colored horses do.

To produce color in some breeds, overo Paints were crossed into them. Generations later, the lethal allele can still be present in solid-colored animals. Therefore, an OLWS foal could be born even if neither parent has overo coloring. Carriers have been discovered in all patterns of pintos and Paints, in solid-colored horses of overo lineage and in crop-outs of other breeds. These individuals, if bred to another with a N/L gene, can produce white foals with OLWS.

Not all solid-white Paint or pinto foals have the lethal syndrome. Not every pure white horse of Paint or pinto lineage carries the defective gene. To know for certain, a DNA test can identify the lethal gene.

Experts at the University of California at Davis, where the test was developed, recommend all breeding stock be tested. Horses that carry the lethal white gene should not be summarily rejected as breeding animals, however, since careful selection of mates free of the gene will safeguard resulting foals from the syndrome.

Gauging numbers affected by OLWS is difficult. Several area veterinarians specializing in equines report they have no personal experience with the syndrome. Not all dead foals are reported. Some L/L foals are re-absorbed or miscarried, the dam's body rejecting the fatally flawed fetus.

Because there is no cure, veterinarians have no intervention option other than to perform euthanasia. The best defense is a good offense, which combines the U.C. Davis test with proper selection of stud/mare crosses.

For more scientific data, access www.ucdavis.edu or www.apha.com and other pertinent sites.