Overo Lethal White Syndrome Update

Researchers have determined the coat patterns associated with overo lethal white syndrome newborn foals
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The same researchers who identified the gene mutation that causes deadly overo lethal white syndrome (OLWS) have more accurately determined the coat patterns associated with OLWS in newborn foals. The effects of OLWS are wide-reaching, as it has been found in Paints, Miniature Horses, half-Arabians, Thoroughbreds, and cropout Quarter Horses (Quarter Horse foals born with too much white to be accepted into the breed’s registry).

Elizabeth M. Santschi, DVM, Dipl. ACVS, clinical associate professor at the University of Wisconsin’s School of Veterinary Medicine, was a principal researcher in the University of Minnesota studies. Overo lethal white syndrome has been a frustrating problem for breeders, as the condition is always fatal. "Basically you get this all-white foal born apparently healthy, but he never passes feces and he eventually colics and dies," explains Santschi. "There’s nothing you can do. As a surgeon, I always want to do something, and there’s nothing I can do (for OLWS foals). The only way to avoid this condition is to not breed them."

This led to studies examining the genetics behind OLWS. The condition was associated with its namesake overo coloring in carrier parents (characterized by white coloration of the abdomen that does not cross the dorsal midline between the withers and tail). Symptoms are similar to a genetic condition (Hirschprung disease) that appears in humans and rodents, so Santschi’s group targeted the same gene in the horse. In 1998, the group revealed in Mammalian Genome that a mutation of the endothelin receptor B (EDNRB) gene was associated with OLWS. Lethal white foals have two copies of the defective allele (two alleles make up a gene, one from each parent), while their healthy carrier parents have one, and non-carrier horses have none. They deduced that if one-copy horses were not bred to each other, OLWS would never occur. "The beauty of it is, you can test your horses before breeding to see if you have a carrier," says Santschi

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Written by:

Stephanie L. Church, Editorial Director, grew up riding and caring for her family’s horses in Central Virginia and received a B.A. in journalism and equestrian studies from Averett University. She joined The Horse in 1999 and has led the editorial team since 2010. A 4-H and Pony Club graduate, she enjoys dressage, eventing, and trail riding with her former graded-stakes-winning Thoroughbred gelding, It Happened Again (“Happy”). Stephanie and Happy are based in Lexington, Kentucky.

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