Lethal white syndrome

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. There currently 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. However, care should be taken to not jump to conclusions based on coat color alone; there are many genetic mechanisms that produce white foals that are not LWS-affected, and a white foal that appears ill may be treatable. The disease has a similar genetic pattern to Hirschsprung's disease in humans.^[1]

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Lethal white syndrome first was identified in 1982 in horses with the frame overo color pattern.^[2] For this reason, the condition is often called overo lethal white syndrome (OLWS). In spite of the name and the association of the condition with the "frame overo" coat color pattern,^[3] research by the University of Minnesota demonstrated that LWS is not directly tied to visible overo color; some tobianos and even a small number of solid or crop-out quarter horses have produced LWS foals.^[4][5]

On the other hand, while not all horses with visible overo coloring carry the recessive LWS gene, to date all horses known to have produced an LWS foal do have overo ancestry in their pedigrees.^[6] the recessive nature of the gene means even that having a carrier ancestor cannot be way to know if a horse is a carrier or not, because statistically a recessive is only passed on 50& of the time. Therefore, it is not possible to tell if a horse carries this gene by looking at its color or its pedigree, only a blood test will detect a carrier. Though most commonly linked to American Paint Horses, it may also appear in other breeds that have overo ancestors; it even was found in one case involving a Miniature horse.^[7][8] However, documented cases in non-overos were horses with overo ancestors.^[9][10]

In 1997 researchers at the University of Minnesota Genetics Group, sponsored by the American Paint Horse Association, developed a reliable DNA blood test for LWS.^[4] Using this test, now available through the Veterinary Genetics lab at the University of California, Davis, it is now possible to identify carriers of the gene (heterozygotes) so that breedings likely to produce LWS foals can be avoided.^[8]

Because LWS foals are born with near- or completely white bodies, there are a variety of misconceptions associating other white horses with the disease. There is an unrelated fatal genetic defect where two horses carrying the "Dominant White" gene can produce a reabsorbed or stillborn foal. Thus the term "lethal white syndrome" does not correctly apply to this defect. (See white (horse) for details.) In the Arabian horse, there is a lethal gene known as Lavender Foal Syndrome, also called "coat color dilution lethal," which produces foals who cannot stand at birth, and is also genetically unrelated to LWS.

Healthy horses which appear to be entirely or mostly white are usually either cremellos, perlinos, smoky creams, ivory champagne, maximally-expressed sabinos, or the rarer white horse. None of these color genes are linked to the LWS gene. The maxium white sabino, which occasionally appears in Paint horses, was particularly targeted for a time as a "Living lethal," even though there is no link between Sabino genes and LWS.^[4]

References

• Paul D. Vrotsos RVT and Elizabeth M. Santschi DVM. University of Minnesota Genetics Group. Stalking the Lethal White Syndrome. Paint Horse Journal. July 1998.
• Elizabeth M Santschi, DVM. Association of Equine Practitioners. Overo Lethal White Syndrome.
• OMIA - Online Mendelian Inheritance in Animals. - "Megacolon (Phene ID 1207, Group 000629) in Equus caballus." Summary of condition and extensive bibliography of peer-reviewed research source material: http://omia.angis.org.au/retrieve.shtml?pid=1207

See also

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