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What is Overo Lethal White Syndrome (Foals)?

Even though almost all foals with solid white coats have this condition, there are some that do not. There are other genetic causes of lack of pigmentation in foals. That is why it is important to get your foal checked by an equine veterinary professional before choosing to euthanize. Horses with this syndrome have an undeveloped gastrointestinal system caused by a defect in the embryonic cells that are supposed to form the nerves of the intestines and colon. These are also the same cells that determine the color of their skin so the foal is unable to pass the meconium, causing fatal constipation. There is no treatment and veterinarians always suggest euthanasia rather than letting your horse suffer with the painful symptoms of colic.

Overo lethal white syndrome (OLWS) is a genetic disorder that causes lack of pigment, blue eyes, and a colon that does not work. Although the foal seems outwardly healthy and normal (besides the complete lack of color), within a few hours the signs of colic start to become obvious. This condition has no cure so it is always fatal, and the foal is usually euthanized within hours of birth to prevent suffering due to the painful side effects of the disease. Some of these are colic, inability to pass meconium, and severe abdominal distension. This condition happens when both the mare and stallion have the overo genes which cause the deadly OLW syndrome.

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Symptoms of Overo Lethal White Syndrome (Foals) in Horses

The signs of overo lethal white syndrome are obvious at birth but have to be verified by a veterinarian due to the fact that there are other non-lethal genetic causes of a foal with an all-white coat. The symptoms of OLWS include:

  • Complete lack of color in skin (pink)
  • No color in coat, mane, and tail (some have a bit of black, but it is rare)
  • Inability to pass meconium
  • Cannot get comfortable (laying down, rolling on their back)

 Types

There are four types of horses, which are:

  • Overo is a speckled or spotted horse type which has separate types (shown below)
  • Solid horses have no white spots at all
  • Tobiano has white legs and feet, with some white on the face and body, with colors that are clearly defined
  • Tovero is usually from the mating of a tobiano and overo and has characteristics of each.

There are four types of overo horses, which include:

  • Calico horses have irregular limb markings and irregular white body patches on their body; some have more than one color besides white
  • Frame patterns include those that contain white patches in the body and neck but are framed by color
  • Sabino has a speckled white spotted body with four white feet with white patches on the legs
  • Splashed horses are the least common but are on the rise due to the interest in the pattern; the horse looks like it has been dipped in white paint, with white legs spreading up into the underparts of the horse (belly, lower neck, bottom of the head)

Causes of Overo Lethal White Syndrome (Foals) in Horses

This condition can affect any breed or sex of horse, but is only seen in newborn foals. For a foal to have the overo lethal white syndrome gene, both the mare and the stallion have to have the frame genes. Also, it is most often seen in horses with white in their coats such as:

  • American Paint horses
  • Quarter horses
  • American Miniature horses
  • Half-Arabians
  • Thoroughbreds
  • Crop-out (horses with a large amount of white markings)

Diagnosis of Overo Lethal White Syndrome (Foals) in Horses

Diagnosing OLWS is not a difficult process but the veterinarian will want to do a complete physical and some laboratory tests to verify before suggesting euthanasia. Besides having a coat that lacks any color, an overo lethal white syndrome foal has blue eyes and acts uncomfortable, alternating between lying down and rolling over on its back. The color is caused by breeding a mare and stallion that both have defective genes. 

A DNA test can be done to get a definitive diagnosis, but when the veterinarian does a physical examination, the lack of abdominal noises associated with digestion and the distension of the abdomen are usually proof enough to euthanize.

Post-mortem examination shows absence of ganglion cells in the intestines, intestines full of fecal matter, lesions in the intestinal tract, and a lack of cutaneous melanocytes. If your foal is not euthanized within the first 48 hours of birth, death is due to peritonitis from a rupture of the intestines, which is extremely painful.

Treatment of Overo Lethal White Syndrome (Foals) in Horses

There is no treatment for overo lethal white syndrome at this time so the only humane thing to do is have your foal euthanized by an equine veterinary professional. Attention will also be required for the needs of the mare as she will have to adjust to the loss of the foal (lactation, physical aftercare).

Recovery of Overo Lethal White Syndrome (Foals) in Horses

Since this syndrome is 100% fatal, the prognosis is grave and the only way to prevent it is to check DNA of the mare and stallion before mating.