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Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are born after the full 11-month gestation and externally appear normal, though they have all-white or nearly all-white coats and blue eyes. However, internally, these foals have a nonfunctioning colon. Within a few hours, signs of colic appear; affected foals die within a few days. Because the death is often painful, such foals often are humanely euthanized once identified. The disease is particularly devastating because foals are born seemingly healthy after being carried to full term.〔 〕 The disease has a similar etiology to Hirschsprung's disease in humans. A mutation in the middle of the endothelin receptor type B (EDNRB) gene causes lethal white syndrome when homozygous. Carriers, who are heterozygous — that is, have one copy of the mutated allele, but themselves are healthy — can now be reliably identified with a DNA test. Both parents must be carriers of one copy of the LWS allele in order for an affected foal to be born. Horses that are heterozygous for the gene that causes lethal white syndrome often exhibit a spotted coat color pattern commonly known as "frame" or "frame overo." Coat color alone does not always indicate the presence of LWS or carrier status, however. The frame pattern may be minimally expressed or masked by other spotting patterns. There also are different genetic mechanisms which produce healthy white foals and have no connection to LWS; another reason for genetic testing of potential breeding stock. Though there is no treatment or cure for LWS foals, a white foal without LWS that appears ill may have a treatable condition. ==Clinical signs== Unlike some coat color dilution lethals, which may result in premature births, stillborn, or weak foals, foals born with lethal white syndrome appear to be fully formed and normal. The coat is entirely or almost entirely pure white with underlying unpigmented pink skin.〔〔 Pigmented regions may be any color, and if present are most common around the muzzle, underside of the barrel, and the hindquarters or tail.〔 The eyes are blue. A few lethal white foals have been shown to be deaf.〔 Healthy foals pass meconium, the first stool, soon after birth, though some healthy foals may require an enema to assist this process. But the meconium of LWS foals is impacted high in the intestine, and never appears, even with the use of enemas.〔 Symptoms of colic begin to appear within the first day,〔 and all LWS-afflicted foals die within the first few days of life.〔 The painful and inevitable death that follows usually prompts veterinarians and owners to euthanize foals suspected of having lethal white syndrome. Death is caused by an underdeveloped part of the digestive system. The large intestine of the horse is a complex system where most digestion takes place, and comprises the cecum, the colon, and the rectum.〔(【引用サイトリンク】title=Horse Nutrition - The Horse's Digestive System. Bulletin 762-00 )〕 Necropsies on lethal white syndrome foals reveal a pale, underdeveloped colon〔 and intestinal obstruction (impaction).〔 Samples of affected tissue show a lack of nerves that allow the intestine to move material through the digestive system, a condition called intestinal agangliosis.〔〔〔 Closer examination of the skin and hair shows both to be unpigmented, and most hair follicles are inactive and many are devoid of hair altogether.〔 All Lethal White Syndrome foals test homozygous for a genetic abnormality. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「lethal white syndrome」の詳細全文を読む スポンサード リンク
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