Coloring Gone Wrong?: Lethal White Overo Syndrome (LWO)

The American Paint Horse is known for being a lovely breed, exceptional in flashy colors and patterns, with good disposition and adaptability. This breed portrays three patterns recognized by the American Paint Horse Association (APHA): tobiano, tovero, and over. The tobiano pattern is characterized by patches of white coloring (usually oval or round) that cross the topline between the ears and the tail. Most of the time, the tail is two colors, and all four legs of the horse are white, starting below the knees. The tovero pattern is characterized by dark pigmentation around the mouth, ears, and quite possibly around the forehead and eyes. One or both eyes may be blue. It is a combination of the tobiano and overo patterns. The overo pattern is characterized by no white coloring across the horse’s back and irregular patterns everywhere else. Bald-faced head markings are incredibly prominent, and at least one leg is dark in color, contrary to the other three. Blue eyes are not uncommon, and the tail is usually one color. The overo pattern is unique and subdivided into three subdivisions: frame overo, sabino overo, and splashed white overo.

Tobiano Paint Horse
Tobiano Paint Horse
Tovero Paint Horse
Tovero Paint Horse
Frame Overo Paint Horse

Frame Overo Paint Horse

It is not a surprise that the frame overo is a greatly desired pattern gene. The frame overo has a distinct horizontal orientation of white patches and a colored back, and the pattern is variable in phenotype. The white patches do not exceed the back when the horse is seen from the side view, which is why the word “frame” is used. All frame overo horses carry a single inherited copy of the Ile118Lys EDNRB mutation (Animal Genetics, 2021). It is recognized as the “O” allele in genetic testing. Due to this mutation, it produces white markings on specific areas of the horse. However, it is also linked to Lethal White Overo Syndrome (LWO), which is an autosomal recessive disease in which a foal is born almost absolute white in phenotype and dies within a few days of its lifespan (Young, 2020). This syndrome occurs when the foal receives two identical copies of the gene mutation, one from each parent. Not only are these foals white, but they are born with an underdeveloped digestive tract. This leads to colic because the foal cannot move food through the digestive tract properly and cannot defecate. If the foal has only one copy of the mutation, it will have the desired frame overo pattern.

Genetics for Lethal White

O/O

 

Homozygous

A Lethal White foal, which carries two copies of the frame Overo gene, would test as homozygous for frame Overo. Since no living frame Overo horse over a week old will test as homozygous, it applies only to horses in the Lethal White condition.

n/O

Heterozygous

The horse carries just a single copy of frame Overo and exhibits the desirable frame Overo markings. Since frame Overo is a dominant gene, the coat pattern should be present in all horses with a single copy of the mutated gene.

n/n

 

Negative

Non-Overo or 'solid' horse

Data from Animal Genetics Inc. Test Results

Affected foals of LWO syndrome possess an all-white coat color, pink skin, and blue eyes. There is also great potential in the foal being deaf. Abdominal pain and colic usually occur around 12 hours. They show typical colic symptoms such as rapid breathing, thrashing, sweating, frequently looking at sides, and off-colored mucus membranes (Malone, 2021). Affected foals usually pass away in less than 72 hours. The best ethical decision is to euthanize the foal as a result of excruciating pain. Since this disease occurs rapidly, diagnosis is often made postmortem. There is no medication or cure for this disease, though there is a prevention route. When bred, frame overo horses will carry just a single copy of the gene, which passes one copy to their foals approximately 50% of the time. If the frame overo horse is mated to a solid non-overo horse, then the foal will only get one copy of the mutation, possibly allowing the desired pattern. If two frame overo horses are mated, there is a much higher chance that the foal could develop LWO.

Remember these key points:

  1. If two frame overo horses are mated, there is a much higher chance of the foal acquiring LWO. Try breeding a frame overo horse to a solid non-overo horse.
  2. There is no treatment for this autosomal recessive disease
  3. Affected foals have no chance of survival, and euthanasia is the best humane option

Resources:

  1. Animal Genetics, Inc. (2021). Frame Overo (LWO).
  2. Malone, E. 2021. Colic in Your Horse. University of Minnesota Extension. https://extension.umn.edu/horse-health/colic-your-horse
  3. Overo. 2021. American Paint Horse Association. https://apha.com/breed/overo
  4. Tobiano. 2021. American Paint Horse Association. https://apha.com/breed/tobiano
  5. Tovero. 2021. American Paint Horse Association. https://apha.com/breed/tovero
  6. Young, A. 2020. Lethal White Overo Syndrome (LWO). UC DAVIS Veterinary Medicine, Center for Equine Health. https://ceh.vetmed.ucdavis.edu/health-topics/lethal-white-overo-syndrome...

Photo credits: https://creativecommons.org/licenses/by/4.0/

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