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Lethal white syndrome (LWS) is a common genetic disorder primarily associated with American Paint Horses. A foal with this syndrome is born all white or nearly all white and has a non-functioning colon. As of 2005, there is no successful treatment for the disease; such a foal typically dies within two weeks. Because the death is often painful, these horses are generally put down once identified, but care should be taken to not jump to conclusions based on coat color alone.

There is an unrelated fatal genetic defect where two horses carrying the Dominant White gene can produce a stillborn foal. The term "lethal white" normally does not apply to this defect. See white (horse) for details.

Lethal white syndrome first appeared among overos, and so was called overo lethal white syndrome (OLWS), and it is a widespread misunderstanding that all overos carry it. In reality, research by the University of Minnesota has shown that LWS is not directly tied to the overo phenotype: not all overos carry it, and in fact some tobianos and even a small number of crop-out quarter horses and other breeds carry it as well. This misunderstanding is perpetuated by older theories predating the blood test, such as those of UC Davis's Laurie Fio, who claims that LWS results from a homozygous frame overo gene. [1]

In 1997 researchers at the University of Minnesota Genetics Group, sponsored by the American Paint Horse Association, developed a reliable blood test for LWS. Using this test it is now possible to identify heterozygotes so that breedings likely to produce LWS foals can be avoided. Unfortunately, the disease has become so widespread among American Paint Horses that it has become difficult for breeders to avoid horses carrying the disease.

Because LWS foals are born with completely white bodies, there are a variety of misconceptions associating other white horses with the disease. Horses which appear to be entirely or mostly white are usually either white horses, cremellos, perlinos, smoky creams, splash paints, or maximally-expressed sabinos. None of these are any more or less likely to be LWS foals or to carry the LWS gene.

• Paul D. Vrotsos RVT and Elizabeth M. Santschi DVM. University of Minnesota Genetics Group. Stalking the Lethal White Syndrome. Paint Horse Journal. July 1998. Discusses the group's research and several myths about us LWS.

• Elizabeth M Santschi, DVM. Association of Equine Practitioners. Overo Lethal White Syndrome.

• Dilution gene
• Creme gene
• White (horse)
• Pinto horse
• American Paint Horse
• Overo
• Sabino horse
• Equine coat color genetics

• Lethal White Discussion