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 genetic mechanisms that produce white foals that are not LWS-affected, and such foals may simply be ill and can be treated. The disease has a similar genetic pattern to Hirschsprung's disease in humans.
Lethal white syndrome first was identified in 1982 in horses with the overo color pattern. For this reason, the condition is often called overo lethal white syndrome (OLWS). It is a widespread misunderstanding that all overos carry it; this is not correct. The misunderstanding is based on an incomplete understanding of older theories predating the development of a blood test, such as those of UC Davis's Laurie Fio, who theorized that frame overo was a dominant gene and that LWS resulted from the homozygous form.
However, research by the University of Minnesota indicates that LWS is not directly tied to visible overo color: not all overos carry it, and some tobianos and even a small number of solid or crop-out quarter horses have produced LWS foals. The gene has appeared in other breeds, though usually thought to be horses desended from Paint or Quarter Horse carriers. There has even been a case in a miniature horse. However, many documented cases in non-overos were horses with overo ancestors.
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. 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.
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 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. The term "lethal white syndrome" does not correctly apply to this defect. 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, splash overo paints, 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 not a link between Sabino genes and LWS.
On the other hand, as the LWS gene has no visible effect on a carrier, color pattern is not a reliable indicator; a horse of any color who has ancestors who carried LWS may also carry the gene in addition to other coat color genes. Only genetic testing can reveal if the gene is present in heterozygous form.
