Lethal White Overo Syndrome (OLWS)

Genetic Issues in Horses

History of OLWS

OLWS was first discovered in 1982 and the effects were always the same. A foal would be born almost pure white and then slowly die withing three days. Many breeders, the vast majority being American Paint Horse breeders, struggled to understand the genetic component that caused their foals to lose their lives. Without a genetic test many breeders were forced to continue breeding different pairings to hopefully avoid having a foal born with OLWS.

It wasn't until 1997 that a team of three independent research groups came together to find out the root cause of this genetic abnormality (Geneticists at the University of Minnesota, the University of California and a hospital in Australia). By working together they were able to determine the DNA mutation and genes associated with OLWS. Researchers were then able to conclude that the Ile118Lys mutation on the equine endothelin receptor type B gene is responsible for individuals that are carriers for the gene.

By discovering the root cause of Lethal White foals, researchers were able to determine that both parents had to each carry a copy of the mutated gene to end up with a foal that acquired two gene copies. Thus they concluded that OLWS is a recessive genetic defect that only effects homozygous recessive foals and does not adversely effect heterozygous individuals.

Our Gorgeous Lethal White Filly

Why is this genetic mutation an issue?

Lethal White foals are born with an underdeveloped, defective colon caused by a failure of the embryonic cells that form nerves in the gastrointestinal system to fully form.

This essentially inhibits the foal from passing feces and absorbing liquid into their gut. Surprisingly, the cells responsible for intestinal development are also responsible for the foals coat color.

Foals usually show signs of colic and distress shortly after birth and are humanly euthanized to end their suffering. To date there are no treatment options available to breeders other than euthanizing positive foals and preventative genetic testing.

What breeds of horses are carriers?

American Paint Horses, American Miniature Horses, Half-Arabians, Thoroughbreds, and Quarter Horses. Many coat patterns also carry the mutation and can be seen in solid colored horses, tobianos, splash, overos, toveros, but are typically found in frame overos. Furthermore, overos have four different types or subcategories frame, calico, sabino, and splashed white; many coat patterns are inherited together. This is possible due to the placement on the alleles in the Kit Gene.


Lethal white foal 2013

Preventative Measures

There are no options, other than euthanasia, for foals born with two copies of the gene mutation. However, there are steps that can be taken to prevent foals from being born OLWS. Genetic testing is now available to prevent breeding two individuals that are heterozygous for the mutation. Many breeders have chosen to breed mares to non-frame type stallions that are negative to carry OLWS and many breed registries are now requiring genetic testing to be done prior to being registered.

Discussion Question

My question is why would breeders continue to breed heterozygous individuals within a breeding population? It doesn't make sense to continue to promote a color or bloodline with the risk of developing a OLWS foal. Even with careful genetic testing and knowledge, at the end of the day we are dealing with animals and accidents do happen. I personally would never breed a positive individual and would remove them from the breeding population. There are ethical reasons that are directly tied into the issue of OLWS foals and I would want to be a positive influence in the Equine Industry.
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