Lethal White Syndrome LWS (Overo Lethal White Syndrome OLWS)

Equus ferus- Equine Coat Colour Genetic -LETHAL WHITE

One of the mares at the Sand Wash Basin was believed to have given birth to a foal with Lethal White Syndrome.  And although we cannot be absolutely sure the foal was a Lethal White foal, the behaviour strongly suggests it might have been. Lethal White Syndrome has been talked about in the groups and on Facebook, hence this blog post. Without a necropsy (autopsy on an animal) we won't know for certain but here is what we do know...

Similar to Hirschsprung's disease in humans, Lethal White Syndrome affects the colon by making it non-functional and in horses it also affects pigment of the coat. The affected foals are born pure white with blue/grey eyes and occasionally a smudge or darker colour on the body or near the tail however, they die within 72 hours of birth. The colon in these foals is a dead-end and the foals cannot pass feces. They do not act normally and exhibit signs of distress.

Lethal white foal (Picasso x Mingo) photo credit Nancy Roberts 2011

Picasso and Mingo 2012 photo credit Karen McLain

The trait, which is inherited, is carried by the horses who also carry the paint trait frame overo. Frame Overo horses typically have jagged white markings along the center of the body. The back and belly may remain solid colored so the effect is a framed area of white. They may have white faces (apron or bald face) and they may have blue eyes although not always. Some horses may minimally express the trait and the only evidence of the frame overo paint trait is a little spot of white along the neck and an unusually shaped blaze. Some horses may also carry other paint traits such as tobiano and they horses are referred to as toveros. Without genetic testing, nothing is certain so we are basing our assumptions on what we have observed and the reproductive history of the individual mustangs.

Yahtzee (R) & Van Gogh (L)
Photo credit Meredith Hudes-Lowder of Equus ferus Wild Horse Photography

In order to produce a foal with Lethal White Syndrome, both parents must be overo. Not all overo horses carry the trait according to the American Paint Horse Association (http://www.apha.com/breed/geneticsarticles/lethal-whites-1) and not all blue-eyed white foals carry the Lethal White gene. Because  Lethal White Syndrome is autosomal recessive, it means when two horses that are overo and each carry the gene , there is a 25% chance the foal with be born with the syndrome. If a dam with the trait and a sire with the trait have three normal foals, it does not mean the fourth foal will carry the trait; the chances a foal will inherit the syndrome resets each gestation and remains one in four with each subsequent preganncy. 
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In the Punnett Square below (Horse drawings by Karen McLain)
Oo outside the square on top represent the SIRE
Along the left side, the Oo represents the DAM
Both are Overo represented Oo and the carry the trait

The RESULT:
One Solid foal (unaffected) -25%
Two Overo foals- CARRIERS of the Lethal White Trait- 50%
One Lethal White Foal 25%

One Solid Foal (OO) 25%

Two Overo Foals (Oo) 50%

One Lethal White Foal (oo) 25%

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PUNNETT SQUARE REPRESENTING AUTOSOMAL RECESSIVE INHERITANCE

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The photos below are Danielle M. Williams and they show Fleck's foal. Fleck is frame overo and the father is believed to be Eagle, a minimally marked frame overo so it is quite possible the foal carries the Lethal White Syndrome.  Sometimes Lethal White Foals do have darker pigment on the muzzle but this foal is not hunched over in the typical posture of a horse in gastric distress however the witness/photographer Danielle did say the foal did not look well and laid down frequently. She said the foal was unable to stand for any length of time. This is consistent with Lethal White Syndrome and the foals with the disorder often roll from side to side. Another possibility is that the foal may have perished in the fight between the band stallion Eagle and Diego who took over part of the band. Stallion infanticide is unfortunately unavoidable and may be more common than previously thought. Regardless of the manner of death, it is heartbreaking to see a young life extinguished so soon.

Fleck and her foal
Photo by Danielle M. Williams

Fleck and her foal 
Photo by Danielle M. Williams

Fleck and her foal 
Photo by Danielle M. Williams

Please email Meredith with any questions regarding the genetic behind Lethal White Syndrome or horse colour genetic in general.
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*Much thanks to Karen McLain, Heather Robson, Stella Trueblood, Aleta Wolf, Connie Wagner, Danielle M Williams, Nancy Roberts, John Wagner, Joe Tosh, Patrick Brennan, Patti Mosbey, Robin Wadams and all the people who watch observe these horses and report on their behaviour.

Meredith Hudes-Lowder WHNP-BC, MSN, BSN, RNC, BS Biology

Meredith received a Bachelor of Science Degree from Binghamton University with an emphasis in ethology and genetics. She received a Bachelor of Science in Nursing also from Binghamton and a Masters of Nursing in Perinatal/Women's Health from Stony Brook University. She currently practices medicine as a Nurse Practitioner in Manhattan for Advantage Care Physicians. She is also enrolled in the Doctoral program at Stony Brook and anticipates graduation in 2016 as a Doctorate of Nursing Practice. Her doctoral thesis is a research study on cervical cancer screening intervals. She is a member of several professional organizations and was inducted into Sigma Theta Tau- the nursing honor society in 2007.