OVERO LETHAL WHITE SYNDROME
Overo Lethal White Syndrome is the result of an embryo that is homozygous for the Frame Overo gene. A single Frame Overo gene creates the very striking and popular Frame Overo pinto pattern, but when doubled up, the foal's intestinal system does not develop properly, and the foal dies within 72 hours of birth, when it is unable to digest its first meals.

A Lethal White is a pure white (or almost pure - some have color on the tips of the ears and around the mouth) foal born of two parents both carrying the Frame Overo gene. Most Lethal White foals die within 72 hours of birth, as they lack functional intestinal systems. The gene in homozygous form interferes with the development of the proper nerve function in the lower colon, and the food is unable to be 'moved on.' Sometimes the colon is not even connected completely. To date, there is no surgery or other treatment to save these unfortunate foals.

The way to avoid this tragedy is simply to avoid breeding Frame to Frame. The hard part is identifying carriers of this gene. There are many color patterns that resemble Frame Overo (Many loud Sabinos look very much like Frames, for instance), and others that mask it. Frame is sometimes very minimally expressed, so that the horse does not appear to be a Frame at all, but only a "normal" horse with pretty socks or a nice wide blaze, etc. (This is the most likely explanation for the so-called "crop-pouts" that occur in some of the non-color breeds)  Frame Overo is attractive and highly prized by horse owners. The challenge is breeding for it safely. 

A laboratory test using a simply-obtained hair sample is now available at UC Davis, that will help avoid this tragedy by identifying carriers, so that they may be bred safely. Since horses can carry the Frame gene without it being expressed to a recognizable degree, it is a good idea to have all breeding stock tested for this gene.

It is possible for sabinos and splashed overos, as well as apparent Tobianos, to carry lethal white. Most likely, it is because these horses have a frame overo in their pedigree somewhere that passed on the overo gene, but it is not showing up in the coat, as often happens with overo. We recommend testing for lethal white to anyone breeding any overo or a horse that looks solid but has overo in its pedigree.

You can download forms for this and other genetic tests from the UC Davis website-- follow the links from http://www.vgl.ucdavis.edu

A WORD OF CAUTION:


Maximum White Sabino PMU foal

Just because a foal is born pure white, DO NOT ASSUME IT IS A LETHAL WHITE! DO NOT DESTROY IT UNTIL YOU KNOW FOR SURE!
Perfectly healthy, 100% normal and fully-functional Maximum White Sabino and Tovero foals look just like Lethals at birth, and they come from the Overo complex of pinto patterns just as Lethals do, but there is nothing wrong with them. Give a white foal a chance.

Sometimes I wonder why I bother with this website...
...and then something happens that makes it all worthwhile:

This foal, owned by Tammi Vogel, is alive today thanks to information Tammi was able to learn from this website!

When the pure white foal was born, he seemed normal, but local folks all warned that it was only a matter of time until symptoms would start and he would die a painful death - so she should call the vet out and have the foal put down.

Tammi made the vet appointment but luck was with her and the vet couldn't come out until much later. During that time, she researched Lethal White on the Internet and came upon this website, where she learned of Maximum White Overos - in this case, Maximum Sabino, judging from the mother's lacy spotting pattern.

I happened to check my email early that morning and my heart jumped to my throat when I saw the photo and the diagnosis.

DON'T PUT THAT FOAL DOWN! I responded and called her on the phone. We had a nice conversation and she was very relieved to be able to give the foal a chance.

Three days later he was still fine, so the vet never needed to come out to do the dreaded deed.

Many months later, "Go Check Whitie" (named for the frequent text message Tammi sent to her family while she was away at work) is healthy and strong, a completely normal weanling colt!

UC Davis Genetics of Overo Page

For more info about Lethal White Syndrome:

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