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Lethal White Foals: What You Need To Know
Your mare's foal is impossibly beautiful—snow white with luminous blue eyes. But this beautiful foal may carry a lethal genetic mutation. Here's what you need to know.
Some horses carry a genetic mutation that impacts development and function of the intestinal tract. If a foal inherits two copies of the mutation (one from the dam and one from the sire), it is born pure white with blue eyes and a malfunctioning intestinal tract. The defective genes cause missing nerve cells in the muscles of part or all of the intestinal tract. Because the foal cannot defecate, it usually dies within a few days. Humane euthanasia is usually recommended to save the foal from suffering.
Some horses carry a genetic mutation that impacts development and function of the intestinal tract. If a foal inherits two copies of the mutation (one from the dam and one from the sire), it is born pure white with blue eyes and a malfunctioning intestinal tract. The defective genes cause missing nerve cells in the muscles of part or all of the intestinal tract. Because the foal cannot defecate, it usually dies within a few days. Humane euthanasia is usually recommended to save the foal from suffering.
If only one copy of the mutated gene is inherited (along with a normal copy of the gene), the foal develops normally.
Because the syndrome most typically occurs in foals born to overo dams or sires, it is is called Overo Lethal White Foal Syndrome. (Overo horses are those who exhibit a distinctive coat pattern featuring white markings that don't cross the back of the horse between its withers and its tail.) But the syndrome also occurs in miniature horses, half-Arabian horses, Thoroughbreds, and cropout quarter horse foals that are born with too much white to be accepted into the breed's registry.
It is NOT always the case that a white foal born to an overo will develop the syndrome. In the past, the foal was watched carefully over the first few days to see whether colic developed—a sign of intestinal dysfunction. Today, genetic testing can be performed to discover whether or not the foal inherited two mutated copies of the gene.
Because the syndrome most typically occurs in foals born to overo dams or sires, it is is called Overo Lethal White Foal Syndrome. (Overo horses are those who exhibit a distinctive coat pattern featuring white markings that don't cross the back of the horse between its withers and its tail.) But the syndrome also occurs in miniature horses, half-Arabian horses, Thoroughbreds, and cropout quarter horse foals that are born with too much white to be accepted into the breed's registry.
It is NOT always the case that a white foal born to an overo will develop the syndrome. In the past, the foal was watched carefully over the first few days to see whether colic developed—a sign of intestinal dysfunction. Today, genetic testing can be performed to discover whether or not the foal inherited two mutated copies of the gene.
Overo Lethal White Foal Syndrome is similar to a human congenital disease called Hirschsprung Disease. The infant suffers obstructive motility disorder of the colon, which can be surgically corrected. In both humans and horses, this disorder is caused by inheriting mutated copies of the endothelin-B receptor (EDNRB) gene.
You can read more about the disorder here:
http://www.vetfolio.com/internal-medicine/overo-lethal-white-foal-syndrome
https://www.extension.umn.edu/agriculture/horse/health/olws/
Copyright Denise Cummins, PhD February 18, 2018
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