Overo lethal white syndrome (OLWS)

What is an overo?

Overo describes a coat pattern, where white markings don’t cross over the horse’s back between the withers and tail. An overo can be mostly dark or white.

What is OLWS?

OLWS is a genetic mutation that appears in horses with white markings. A foal must obtain two copies of the mutated gene to get the disease. The foals are born alive with blue eyes and a white coat. These foals may have small black markings on the head, mane or tail.

At birth, the foals appear normal except for their unique coloring. After some time, signs of colic occur because the foals can’t pass feces. This results because the intestines don’t fully develop.

Not every completely white foal has OLWS. Completely white foals may be a carrier of the gene or simply have another white coat color gene.

Treatment

There isn’t a treatment for OLWS. Surgery to bypass the intestinal damage remains unsuccessful. Euthanasia is advised for all OLWS foals with colic.

Who carries the gene?

Horses with only one copies of the gene are known as carriers.

The OLWS gene appears in the following breeds:

• American Paint Horses

• American Miniature Horses

• Half-Arabians

• Thoroughbreds

• Quarter Horses with a lot of white markings

Overos are at greatest risk of carrying the gene, particularly of American Paint Horse and American Miniature Horse breeding. Over 94 percent of framed overos are carriers. Framed overos have white patches centered on the body and neck.

Horses from overo lineage but they themselves don’t have white markings, can still carry the gene. Up to 20 percent of broodstock without excessive white carry the gene.

A small number of tobiano and breeding stock horses also carry the gene. Some other breeds have a very small number of carrier horses. Pinto horses can be carriers. They receive their white markings from other breeds, which may also carry the OLWS gene.

Gene testing

The Veterinary Genetics Laboratory at the University of California, Davis performs genetic tests for the OLWS gene. We recommend this test for frame overos and their descendants. Mares or stallions likely carry the gene if one of their foals has OLWS. Don’t breed carriers to each other.