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Kansas State University

Overo lethal white syndrome


Although it is often desirable to breed Paint horses and other breeds for color, the trait of “loud color” can have its drawbacks. It is well-recognized that breeding for color results in the occasional production of a foal suffering from overo lethal white syndrome (OLWS). This is a condition most common among overo paint crosses. However, this condition does not exclusively occur from overo crosses and in many overo to overo crosses this condition does not result.

What is OLWS?
This is a condition where a foal has inherited a recessive gene from both parents that encodes for a mutated protein located on some cells. The protein directly influences cells developing into color (pigment) and nerve cells. What this means is that OLWS foals are white (non-pigmented) and do not have normal nerve cells signaling to the intestines, because of the altered protein receptor. Ultimately, the result is a foal with little to no coat coloration (mostly or completely white) and cannot process milk, resulting in colic.

Who is at risk for developing OLWS?
This is an interesting question that has been answered by some outstanding genetic investigators. The frame, calico, and breeding-stock white overo coat patterns are most commonly associated with the presence of the mutated gene that encodes for the protein receptor of concern (endothelin receptor B). This problem is one that paint breeders should be aware of since it can occur with a variety of Paints and even solid color crosses. What is important to remember is that both parents need to be carriers of the defective gene (heterozygotes: one normal gene, one defective gene) and both must contribute the defective gene to result in a homozygote (two defective genes) foal with OLWS. If a foal is a homozygote for the mutation, it is always fatal. This is a complex problem, but currently available DNA testing makes the determination of carriers (heterozygotes) possible. This is important since we can not determine which horses are carriers based on their appearance (phenotype) alone.

Are all white Paints going to develop OLWS?
No, but most do. Occasionally white colored foals are breeding-stock white and have normal function of the gastrointestinal tract, although this is not very common. Most foals affected with OLWS develop clinical signs of colic by 24-36 hours of age. These foals will not survive; there is no treatment or surgery that will correct the functional abnormality of their gastrointestinal tract.

How can I tell if my horse is a heterozygote?
Genetic testing should be performed prior to entering a breeding program. As previously stated, some horses are heterozygotes and do not express the classic phenotype of heavy white pattern (calico, splash white or frame) or, on the other hand, some horses have a lot of white such as a frame overo, yet are not heterozygotes. Therefore, allele specific genotyping of DNA must be performed to accurately determine an individual horse’s potential to produce a foal affected by OLWS. Testing can be performed by scientists at the University of California at Davis.

Below are some of the myths that have been recently described by investigators at the University of Minnesota associated with OLWS:

Myth #1: All overo horses are carriers of the lethal allele.
Fact: There are many overos that do not carry the lethal allele.

Myth #2: Twenty-five percent of foals from two overo parents will be lethal whites.
Fact: Because there are overos that do not carry the allele, the incidence of lethal white syndrome is less than 25 percent in overo-to-overo matings.

Myth #3: Registered tobianos, Breeding Stock, or Paint crosses cannot carry the lethal allele.
Fact: There are tobianos that have overo bloodlines, and these horses can be carriers of the lethal allele. Breeding Stock and Paint crosses can carry the lethal allele.

Myth #4: Totally white Paints are not carriers of the lethal allele.
Fact: These white horses are often carriers of the lethal allele.

Myth #5: All totally white foals born to two overo parents are lethal whites.
Fact: There are totally white Paints that are not affected by the lethal white syndrome.

Myth #6: Mares cannot produce lethal foals in consecutive years.
Fact: The genetic make-up of one foal does not affect subsequent births.

Myth #7: Only one parent determines if a foal will be a lethal white.
Fact: Both sire and dam contributes a copy of the lethal allele.

Myth #8: Crop-out Quarter Horses cannot carry the lethal allele.
Fact: A small number of crop-outs have been tested and found to be carriers of the lethal allele.

Myth #9: You can reliably tell the carrier status of a Paint by their color pattern.
Fact: This is false.