Extension Logo
Extension Logo
University of Minnesota Extension
extension.umn.edu

Overo lethal white syndrome (OLWS)

Quick facts

  • OLWS is a genetic mutation that affects horses with white markings and can lead to death in foals.

  • Foals with two copies of this gene are born white with blue eyes and have intestines that don’t fully develop.

  • There is no treatment for OLWS.

  • Don’t breed carriers of the gene with each other.

Foal with OLWS genetic mutation nursing. Foal is all white with blue eyes.
OLWS foals are born white with blue eyes and have the gene mutation.

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.

Frame overo refers to a specific color pattern that “frames the horse.” White markings of frame overo horses often occur on the barrel, center of the neck, or face. But coloring of the frame overo can vary greatly. The only way to confirm whether a horse is a frame overo is through genetic testing.

What is OLWS?

OLWS is a genetic mutation that appears in horses with white markings, often frame overos. A foal must obtain two copies of the mutated gene to get the disease. The foals are born alive with blue eyes, pink skin 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.  Often within 12 hours the foals develop signs of colic such as rolling or thrashing because they can not pass feces. This results because their intestines do not fully develop. Foals with OLWS usually die within a few days because of damage to the intestines or a buildup of toxic waste materials in the blood.

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 is no 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 copy of the gene are known as carriers and do not develop OLWS.

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. Horses from an overo lineage that do not have white markings, can still carry the gene. Up to 20 percent of broodstock without excessive white carry the gene.

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. Horses with color registries such as Pinto 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. Do not breed carriers to each other.

 Author: Nichol Schultz, DVM, formerly with the University of Minnesota and Molly McCue, DVM

Reviewed in 2023

Page survey

© 2024 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.