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PAINT HORSE

History | Characteristics | The APHA | Lethal Whites

Lethal Whites:

A Light at the End of the Tunnel
by Dawn Walker

Several experts clarify what we now know, and what we hope to know soon about this devastating syndrome Lethal White: A foal of overo lineage, born all white or mostly white. The foal may seem normal at birth, but usually begins showing signs of colic within 12 hours because of a non-functioning colon. Because the syndrome is always fatal, lethal white foals are often euthanized.

In Hardy Oelke's book, The Paint Horse; An American Treasure, lethal white syndrome is referred to as the "greatest nightmare" of Paint breeders. All breeders--large and small--agree that producing a lethal white foal is one of the biggest disappointments and frustrations of the trade. Is it simply the price to pay for breeding overos, or is tre hope for a day when breeders can produce overo color without the risk of lethal whites?

Since the origin of the American Paint Stock Horse Association in 1962, lethal white syndrome has been a plaguing presence, lurking in the shadows of the breed's success. For lack of a scientific explanation, or perhaps to downplay the one negative aspect of the breed, Paint breeders and the association itself adopted a "sweep it under the rug" policy for many years.

But now, as the APHA watches its registration and membership numbers climb ever skyward, and breeders see their black and white overos bringing premium prices across the country, each is uncomfortably aware of a matching increase in the incidence of lethal whites.

After 30 years of questions, everyone seems to agree it is time to get some answers.

The closest thing to an explanation of the lethal white foal syndrome thus far has been an article written by Dr. Ann Bowling of the University of California at Davis, which was published in the August 1993 issue of the Paint Horse Journal. In her article, Bowling proposed that the three classes of horses that resulted from overo-to-overo matings (overo, solid and lethal white) fit a model of overo as a dominant gene with lethal effects in the homozygous state. This model predicted that 25 percent of foals from two overo parents would be lethal whites.

Bowling went on to say that the studbook definition of overo may include more than one genetic entity, which complicates the application of her model for understanding overo inheritance and predicting lethalwhites.

In a recent interview, Bowling stood by her proposition, pointing out that there has been no successful homozygous overo stallion to date, leading her to believe that whatever causes homozygosity in overos is linked to lethal white syndrome.

At the same time Bowling was formulating her theory, the APHA and Portland State University in Portland, Oregon, were doing pedigree studies of their own. For the past 10 years, the APHA has collected surveys on lethal white foals from its members and submitted them to a team at the university for analysis.

In the first half of this article, Dr. Debbie Duffield, a researcher at Portland State University who has been working on the project since its inception, shares her team's conclusions on how to predict the occurrence of lethal white foals.

Recently, lethal white syndrome has also been examined from a genetic standpoint. In July of 1996, a team of researchers at the University of Minnesota sent the APHA a comprehensive proposal for a grant to describe the inheritance of lethal white syndrome and to locate the gene responsible. In November, the APHA Executive Committee approved the $7,650 grant, and the project is now under way.

The project is headed by three researchers at the university--Dr. James Mickelson, Dr. Stephanie Valberg, and Dr. Elizabeth Santschi. In the second part of this article, Dr. Santschi tells us what they know now, what theories they plan to test, and what they hope to accomplish during the next six months.

Find out more:
*Myths about Lethal Whites
*Question and Answer Discussion on Lethal Whites