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OLWFS is a fatal disorder of the digestive system that is associated with a particular white pattern called “frame overo”

Full name

Overo lethal white foal syndrome

Breeds known to be affected

Many breeds, although it is found at its highest frequency in Paint breeds

Inheritance pattern

OLWFS is autosomal recessive. The white spotting pattern expressed by a heterozygous OLWFS horse is incomplete dominant.


A single O allele causes the “frame” or “frame overo” spotting pattern. Expression of white in frame horses is variable, ranging from lots of white “framed” by the horse’s base colour, to minimal or just a few white hairs. It is possible, although unusual, for a horse with no white to carry Frame.

Overo Lethal White Foal Syndrome occurs when a horse is homozygous for the O mutation. These O/O foals are born almost or completely white, but do not have properly formed intestinal nerves and cannot pass faeces. They only survive a few days if not euthanised for compassionate reasons. Carrier horses (O/n) have no documented health issues. OLWS is associated with a two base pair change in the EDNRB gene. Because O can be minimally expressed, it is important to test any horse that might be a carrier even if it has little to no white on it, to prevent the birth of an affected foal.

Interpretation of results

n/n: Horse does not carry the mutation associated with OLWFS.

O/n: Horse has one copy of the mutation associated with OLWFS.

O/O: Horse has two copies of the mutation associated with OLWFS.


Santschi et al. Endothelin receptor B polymorphism associated with lethal white foal syndrome in horses. Mammalian Genome 1998;9:306-309. doi: 10.1007/s003359900754

Metallinos et al. A missense mutation in the endothelin-B receptor gene is associated with Lethal White Foal Syndrome: an equine version of Hirschsprung Disease. Mammalian Genome 1998;9:426-431 doi:10.1007/s003359900790

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