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xc overo/lethal white


Nicola M.A. Parry, BSc, MSc, BVSc, DACVP
Cummings School of Veterinary Medicine at Tufts University
Excerpted from: Compendium, Vol 27(12) December 2005 p. 945 -950

Abstract
Overo lethal white syndrome is an autosomally inherited disease associated with horse breeds that register white coat patterns. The syndrome is associated with a single amino acid substitution at residue 118 on the endothelin-B receptor gene and occurs in white foals born to American paint horses of overo lineage, specifically the frame overo subtype. Affected foals appear normal at birth but fail to pass meconium and develop severe colic as a result of ileus caused by a functional intestinal obstruction. In the absence of veterinary intervention, death ensues, usually within 24 to 48 hours postpartum. Because there is no treatment for this condition, euthanasia is warranted to minimize unnecessary pain and suffering.

Of the different subtypes of overo horses, lethal white foals occur most often in the frame overo subtype, although there is a report of an affected foal being produced from an overo-buckskin cross. Overo lethal white syndrome also occurs in miniature horses, half-Arabian horses, Thoroughbreds, and so-called cropout quarter horse foals that are born with too much white to be accepted into the breed’s registry.

Foals with overo lethal white syndrome are totally, or almost totally, white and, if not euthanized, die within days of birth from complications of intestinal aganglionosis. Although this condition is fatal, it must be remembered that not all white foals born of paint horses are affected. Consequently, such foals may not have aganglionosis and should not be euthanized at birth unless they develop signs of severe colic or are clinically diagnosed as lethal white foals. (emphasis added) Foals with signs of colic must be examined carefully to differentiate between this fatal syndrome and conditions such as meconium impaction or atresia of the distal intestinal tract. Because this syndrome cannot be treated, affected foals must be euthanized, and genetic testing is therefore essential to prevent its occurrence. Paint horses can be tested for the allele associated with this condition, allowing positive identification of breeding stock animals that are carriers of the lethal gene. Breeders can then avoid mating such carriers and locate new pedigree sources, subsequently breeding overo animals to only genetically proven non-overos. This genetic information can significantly help horse breeders prevent the emotional and economic effects associated with overo lethal white syndrome.