Equine Aganglionic Megacolon

Equine aganglionic megacolon, also known as "lethal white foal syndrome," is a rare genetic disorder in horses that occurs when a horse is born with two copies of a recessive gene. This inherited recessive trait prevents the formation of nerves in the portion of the large intestine frequently referred to as the colon, which leads to lethal intestinal obstruction.

  1. Origin

    • The recessive gene that leads to equine aganglionic megacolon controls the encoding of the endothelin-3 receptor, a transmembrine peptide. An endothelin-3 receptor is a 21 amino acid peptide that forms a protein called Endothelin 3. Endothelin 3 helps form neural crest cells and melanocytes in embryos. Neural crests cells form nerves throughout the body, including the gastrointestinal tract, while melanocytes produce melanin, which is a pigment responsible for hair, eye and skin color. When the receptor is mutated, it creates streaks of skin on horses that have no pigmentation. This lack of pigmentation is referred to as an "overo" pattern. When two overo-pattern horses with copies of the inherited mutated endothelin-3 receptor gene mate, they dramatically increase their chances of producing a white foal without sufficient nerve endings in its colon.

    Presentation

    • A white foal possesses no pigmentation on either its skin or coat. White foals afflicted with the disease may appear normal at birth but within a few hours will start to display symptoms of colic. Healthy white foals that may experience trouble producing stool can be aided with an enema. Foals that have equine aganglionic megacolon will not respond to an enema.

    Affected Breeds

    • The American Paint Horse is most frequently afflicted with the disease, although other horse breeds have been affected, including the Quarter Horse and Thoroughbred breeds. Unfortunately, the color of a particular horse's coat is not a reliable method of predicting the incidence of the gene. A single copy of the gene is found in 94 percent of frame overo, white calico overo and frame blend overo horses. Frame overo gets its name from the so-called "framing" of the lack of pigmentation by the surrounding pigmented color of the horse's coat. Breeding stock solid, sabino and minimally blend overo account for a much lower incidence of the gene. Breeding stock solid horses have an almost monochromatic coat color, with occasional spots of white, gray or black. Sabino horses, while displaying white patches or occasionally entirely white coats, owe their coloration to a separate gene known as the "Sabino 1" gene. Minimally blend overo horses display spotty, irregular white coloration on their coats. The only way to determine if a horse possesses a copy of the gene is to perform a genetic test.

    Treatment

    • Unfortunately, there is no known treatment for this disease. Afflicted foals die a painful death within a few days if left untreated. Many veterinarians prefer to euthanize the foals once they have been accurately diagnosed.