Lethal White Overo (LWO) DNA Test

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Breeds: All

Description

Frame overo is a popular white spotting pattern, typically seen as irregular, jagged white patches on the horse’s sides and neck that rarely cross the back, framed by colored areas. It is caused by a mutation in the endothelin receptor B gene (EDNRB). One copy of this mutation produces the frame overo pattern. Two copies cause Lethal White Overo (LWO), also called Lethal White Foal Syndrome, a fatal condition.

Foals with LWO are born almost entirely white. The same mutation that affects pigment cells also prevents the nerves of the intestine from developing, so the foal’s gut cannot move food through. Affected foals develop colic within hours of birth, and the condition cannot be treated. Foals die within a few days, and are usually euthanized sooner for humane reasons.

Carriers can look solid

Most horses with one copy of the mutation (n/O) show the frame overo pattern, but not all of them. Some carriers have so little white that they look solid, and the pattern can also be hidden by other white patterns such as tobiano or sabino. Any horse with frame overo in its pedigree should be tested before breeding, whatever its appearance.

Breeding safely

LWO is recessive, so a foal is only affected if it inherits the mutation from both parents. Carriers themselves are healthy.

Breeding a carrier to a non-carrier (n/O × n/n) gives each foal a 50% chance of being a frame overo carrier and no chance of LWO. Breeding two carriers together (n/O × n/O) gives each foal the same 50% chance of being a carrier, but also a 25% chance of being born with LWO. Since the odds of producing a frame overo foal are the same either way, breeding carriers only to tested non-carriers is the safe choice.

Possible results

Genotype	Description
O/O	Affected: Foal has two copies of the frame overo mutation and has Lethal White Overo. The condition is fatal within the first few days of life.
n/O	Carrier: Horse has one copy of the frame overo mutation. It usually shows the frame overo pattern but may appear solid. Horse is healthy. Each foal has a 50% chance of inheriting the mutation. Breed only to n/n horses to avoid producing an LWO foal.
n/n	Negative: Horse does not carry the frame overo mutation and cannot produce an LWO foal.

Breeds Affected

American Paint Horse, American Quarter Horse, Pinto, Miniature Horse, Thoroughbred, Mustang, Half-Arabian

Reference

A missense mutation in the endothelin-B receptor gene is associated with Lethal White Foal Syndrome: an equine version of Hirschsprung disease.

Metallinos DL, Bowling AT, Rine J.
Mamm Genome. 1998 Jun;9(6):426-31. doi: 10.1007/s003359900790.
PMID: 9585428

Endothelin receptor B polymorphism associated with lethal white foal syndrome in horses.

Santschi EM, Purdy AK, Valberg SJ, Vrotsos PD, Kaese H, Mickelson JR.
Mamm Genome. 1998 Apr;9(4):306-9. doi: 10.1007/s003359900754.
PMID: 9530628

Related Tests

Breeding for white patterns? The Pattern Panel includes LWO plus seven other pattern tests for $75.