Frame Overo Factor Horse (OLWS)

Frame Overo and Overo Lethal White Syndrome (OLWS)

The Overo White Factor is an allele responsible for a splashed white pattern in horses, but breeding two carriers together can be risky as it leads to the death of homozygous offspring. This allele is caused by a semi-dominant mutation in the endothelin receptor type B (EDNRB) gene. Horses that carry one copy of the mutation exhibit a coat pattern known as Frame Overo or Overo, characterized by broad white patches primarily on the belly and muzzle, and sometimes blue eyes.

Overo Lethal White Syndrome

Foals with two copies of the mutation develop Overo Lethal White Syndrome (OLWS or LWS), distinguished by a completely white coat at birth. These foals are born without a functional colon, which prevents them from passing waste properly, leading to severe, painful constipation and colic within a few days of life. Unfortunately, the condition is fatal, and affected foals are usually euthanized to prevent suffering.

Relevant tests

• P902