Horse coats come in a variety of colors and patterns determined by the horse’s genetics. The Overo pattern is a specific coat pattern characterized by white markings that usually spread across the neck and abdomen without crossing over the back. [1]

The Overo pattern is one of three coat patterns in American Paint Horses recognized by the American Paint Horse Association (APHA), along with Tobiano and Tovero patterns. Among Overo variations, the Frame Overo subtype stands out for its distinctive markings. [2]

Unfortunately, breeding two Frame Overo horses together can lead to Overo Lethal White Syndrome (OLWS), a fatal condition in foals characterized by poor intestine function. Foals affected by OLWS typically have white skin and exhibit abdominal pain, usually only surviving a few days after birth. [3][4]

Genetic testing of breeding stock is crucial for identifying carriers of disorders such as OLWS to prevent producing non-viable foals with this condition.

Overo Pattern in Horses

The Overo pattern, commonly observed in American Paint horses (APH), features irregular white markings on the abdomen that do not extend over the back. Across the spectrum of Overo coat variations, the unique pattern of the ‘frame’ subtype stands out. [1][2]

Understanding the distinctions between Overos and their subtypes is essential for both breeders and buyers, particularly in light of the risks associated with genetic disorders such as OLWS.

The American Paint Horse (APH) Breed

The American Paint Horse is a stock breed known for its colorful coat patterns and versatile abilities. It is well-suited to various equestrian sports, including both Western and English disciplines. [2]

The subgroups of APH coat patterns include: [1][2]

  • Tobiano: Characterized by large, rounded patches of white that cross over the dorsal midline (the back) between the withers and tail. Tobiano horses typically have dark-colored heads and legs with limited white markings (i.e. stars, snips, blazes).
  • Overo: Characterized by irregular white markings on the abdomen that don’t cross over the dorsal midline (the back), as well as extensive white markings on the head and face. Overos can be broken down into several categories: Frame, Sabino and Splashed White.
  • Tovero: Characterized by a combination of Tobiano and Overo patterns, with large white patches crossing the back and irregular markings elsewhere.

Frame Overo Pattern

The Frame Overo is an Overo subtype characterized by boxy white patches centered on the side of the body (neck, shoulder, flank, hip), bordered by colored areas. These patches have sharp borders and do not blend into limb markings. [1][2]

Like other Overos, the frame pattern remains horizontally oriented and does not cross over the horse’s topline or back. Overo horses, including Frame Overos, often exhibit extensive white markings on the head and frequently have blue eyes.

While the legs and hooves of a Frame Overo are typically dark, small white markings or white hooves are common.

Frame Overo inheritance follows a dominant pattern, which means the foal only needs to receive the gene from one parent for the coat pattern to develop. However, in some cases, Frame Overos are born from non-spotted parents or horses without any visible white markings or patterns. [2]

These horses may lack body spots but carry the Frame Overo gene, potentially passing it to their offspring. Understanding how genetic traits can be passed from parent to offspring is crucial for breeding and managing risks of disorders like OLWS.

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants | Mad Barn USA

Overo Lethal White Syndrome

Foals born with OLWS can be identified by their predominantly white (or cream) coat and pale blue or grey eyes. Affected foals may exhibit minimal dark pigmentation around the face, tail or ears, and in some cases are also deaf. [4][6][7]

In cases of OLWS, the gestation period and birth are usually normal, and the foal may not show signs of illness immediately after birth. [6][7]

However, affected foals may exhibit very minimal or no passage of meconium, which is the earliest stool produced by a newborn foal. Within 4-12 hours of birth, foals with OLWS typically develop signs of abdominal pain and distension. [3]

Pathophysiology

Foals with OLWS typically do not have borborygmi, or gut sounds, which is usually noticed during a physical exam by a veterinarian. The absence of these sounds indicates a gastrointestinal issue, as fluid and gas are not moving through the digestive tract as expected. [3]

Affected foals have a condition where nerve cells controlling the intestines does not develop properly. These horses lack ganglion nerves, which are involved in peristalsis, the squeezing motion of the digestive tract that enables the passage of feed through the intestines.

A lack of ganglia in the intestines means affected foals are unable to pass feces, leading to severe colic and abdominal pain.

Signs of OLWS that may be observed in newborn foals include: [3][8]

  • Rolling on the back
  • Kicking and looking at the flank and abdomen
  • Restlessness

Genetic Mutation

OLWS in horses is linked to a specific genetic mutation in the Endothelin Receptor Type B (EDNRB) gene. [1][9][10] This gene plays a vital role in regulating the development of the enteric nervous system, which controls intestinal function. [4][6]

The mutation in the EDNRB gene disrupts the normal development of nerve cells in the intestines, causing impaired gut motility. [6] As a result, affected foals cannot properly digest food and die shortly after birth. [3][4]

This genetic mutation is also linked to the distinctive Frame Overo coat pattern observed in affected foals’ parents.

Heritability

The gene responsible for OLWS is characterized as autosomal dominant with variable expression, meaning foals only need to inherit a single copy of the gene from one parent to exhibit traits associated with it, such as the Frame Overo coat pattern. [3]

Horses with just one copy of the genetic mutation show various white coat patterns and may even be solid colored in rare cases. These horses are considered heterozygotes, meaning they carry the OLWS gene without being affected by the condition. However, they can pass the OLWS gene to their offspring.

In order to produce an OLWS-affected foal, both parents must be carriers of the mutation and must pass the mutated copy of the gene to their offspring.

Genetics researchers can calculate the probability of passing genes from parent to offspring. These calculations show crossing two Overo breeding horses results in: [1][11]

  • A 25% chance of a solid-colored foal
  • A 50% chance of a Overo foal
  • A 25% chance of a foal with OLWS

To prevent producing foals affected by OLWS, breeders should avoid mating two horses that are carriers of the OLWS gene.

Can Other Breeds Have OLWS?

While OLWS is most commonly seen in the American Paint horse, there are several breeds that can carry the gene and Overo coat pattern. Although unusual, the Frame Overo coat pattern has been seen in the following breeds: [2][3][5][6]

If you suspect your horse is a carrier of the OLWS gene, consult with a veterinarian or equine geneticist who can provide guidance and testing options prior to breeding your horse.

Do All White Foals Have OLWS?

While OLWS-affected foals are born with predominantly white coats, not all horses with white coat patterns are affected by this condition. Many healthy horses have white coats due to other genetic factors unrelated to OLWS.

In horses, two other genes can result in a white coat: the dominant white genetic trait and mutations in the KIT gene.

  • Dominant White (W) Gene: This genetic trait causes variable depigmentation in a horse’s coat due to the absence of melanocytes (pigment-producing skin cells) in certain areas, resulting in pink skin underneath white markings. [13][14] The extent of depigmentation can vary from partial (50%) to almost complete white coat coverage. Dominant white horses typically have pigmented or brown eyes, rather than the blue or gray eyes seen in OLWS foals. [13]
  • KIT (Proto-oncogene c-KIT) Gene: This gene determines coat color patterns in horses, with mutations resulting in sabino, roan, and white spotting patterns. While full white coat coverage is rare, it can occur due to mutations in the KIT gene. [15]

If your foal is born with a white coat, examination by a veterinarian and genetic testing are necessary to accurately diagnose its health status.

Diagnosis

Diagnosis of OLWS requires a full physical exam by a veterinarian. Usually, diagnosis is suggested based on the parents’ Frame Overo coat coloring.

It is common for foals affected by OLWS to exhibit seemingly normal behaviour at birth, including standing and suckling colostrum at the expected time. [4] Since OLWS progresses rapidly and can cause significant pain for the affected foal, definitive diagnoses are often made post-mortem. [7][11]

Colonic Biopsy

If a definitive diagnosis is pursued, a colonic biopsy is used to evaluate the presence or absence of intestinal ganglia, which are vital for normal intestinal motility. [4]

A small sample of tissue is taken from the foal postmortem and examined under a microscope. In foals with OLWS, the biopsy reveals a lack of ganglion cells in the intestine.

Genetic Testing

A genetic (DNA) test can help confirm the diagnosis of OLWS by identifying the presence of the genetic mutation associated with the condition. This test typically involves collecting a blood or hair sample from the foal and/or the parents. [5]

Prevention

Preventing Overo Lethal White Syndrome in horses primarily involves responsible breeding practices and genetic testing. Breeders should avoid mating horses known to carry the OLWS gene mutation to reduce the risk of producing affected foals. [5][12]

Fortunately, a straightforward genetic test exists to identify horses that carry the OLWS gene. This test looks for mutations in the EDNRB gene to accurately identify carriers.

A OLWS genetic test may have the following results: [5]

  • N/N genotype: The horse does not have the Overo pattern and cannot pass OLWS onto offspring.
  • N/O genotype: The horse does have the Overo pattern and is a carrier, meaning they can pass OLWS onto their offspring. Horses with this genotype should only be bred with N/N horses to prevent OLWS from occurring.
  • O/O genotype: The horse (foal) has OLWS, which is fatal.

Treatment and Prognosis

The prognosis for foals with OLWS is grave as there is no cure or treatment available for this condition. Owners typically choose to humanely euthanize affected foals to minimize suffering. [3][4]

Summary

Overo lethal white syndrome (OLWS) is a fatal genetic condition most commonly seen in American Paint Horses with the Frame Overo coat pattern.

  • Affected foals are typically white with blue eyes, and pass away shortly after birth due to colic caused by poor intestine function
  • Prevention of OLWS through responsible breeding practices and genetic testing of potential carriers is crucial
  • The prognosis for OLWS foals is grave as there is no treatment available; euthanasia is recommended

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Vrotsos, P.D. et al. The Impact of the Mutation Causing Overo Lethal White Syndrome on White Patterning in Horses. 2001.
  2. America Paint Horse Association’s Guide to Coat Color Genetics. America Paint Horse Association’s Guide to Coat Color Genetics.
  3. Lightbody, T. Foal with Overo Lethal White Syndrome Born to a Registered Quarter Horse Mare. Can Vet J. 2002. View Summary
  4. Buechner-Maxwell, V. and Paradis, M.R. Intestine: aganglionosis in Horses (Equis).
  5. Lethal White Overo (LWO) | Veterinary Genetics Laboratory. Accessed Mar. 04, 2024.
  6. Slovis, N. and Irvin, L. Neonates and Periparturient Mares: Tips and Tricks for Diagnosis and Management. Veterinary Clinics of North America: Equine Practice. 2023.View Summary
  7. Roberts, J.N. Congenital and Inherited Anomalies of the Small and Large Intestines in Animals – Digestive System.
  8. Carson, D.M. and Naylor, J. Abdomen: pain – neonate in Horses (Equis). Accessed Feb. 28, 2024.
  9. Ayala-Valdovinos, M.A. et al. New Test for Endothelin Receptor Type B (EDNRB) Mutation Genotyping in Horses. Molecular and Cellular Probes. 2016. View Summary
  10. Metallinos, D.L. et al. A Missense Mutation in the Endothelin-B Receptor Gene Is Associated with Lethal White Foal Syndrome: An Equine Version of Hirschsprung Disease. Mammalian Genome. 1998. View Summary
  11. Young, A. Lavender Foal Syndrome (LFS). 2020.
  12. Marín Navas, C. et al. One Hundred Years of Coat Colour Influences on Genetic Diversity in the Process of Development of a Composite Horse Breed. Vet Sci. 2022. View Summary
  13. Haase, B. et al. Allelic Heterogeneity at the Equine KIT Locus in Dominant White (W) Horses. PLoS Genet. 2007. View Summary
  14. Thiruvenkadan, A.K. et al. Coat Colour Inheritance in Horses. Livestock Science. 2008.
  15. McFadden, A. et al. Spotting the Pattern: A Review on White Coat Color in the Domestic Horse. Animals (Basel). 2024.