Genes causing fatal equine

Ingrid Skjong

For Amanda Purdy, a research graduate student specializing in horse genetics, the biggest discovery of her career involved 10 different genes and a well-timed stroke of luck.
On a whim, Purdy chose three genes from the group and ended up solving a puzzle within six months that has plagued paint horse breeders for years.
“It was a very lucky choice.” Purdy said.
Along with a group of University researchers, Purdy discovered the gene responsible for Overo Lethal White Syndrome — a fatal genetic condition that produces snow-white foals. The horses are born seemingly healthy, but die within 24 hours from complications associated with a nonfunctioning colon.
By pinpointing the culprit gene, paint horse breeders are now able to test their horses for carrier status and avoid combinations that produce lethal whites.
“It’s a little like human genetic counseling,” said Elizabeth Santschi, assistant professor of large animal surgery. “Previously people really didn’t know what the frequency was.”
Paint horses, the second most popular horse breed in the United States, are specifically bred for variations of their splotchy coats. Paints of overo lineage are particularly sought for their distinctive color pattern.
Santschi estimated that of the 20,000 overo paints born each year, about 1,000 to 2,000 are lethal whites. The foals result from breeding two overo paints that carry the deadly gene.
Although the numbers might not appear alarming, frustrations surrounding the doomed births resonate throughout the paint horse community.
Marlys Backes began raising paint horses in 1983 on her Avon, Minn., farm. Though most of her breeding is done between paint and quarter horses, she has experienced several lethal white births.
“It’s so devastating to have spent 11 months nurturing this mare and you end up with a big zero,” Backes said.
Despite the frustration that accompanies lethal whites, large breeding operations tend to stay tight-lipped about the births’ frequency, Backes said. The possibility of discouraging potential clients is a major concern.
To help ease concerns surrounding horse-related genetic problems, equine genetics has become an increasingly growing field. However, funding is low, and genetic studies on production animals receive stronger backing because of their potential impacts on the highly profitable meat markets, Santschi said.
An 11-month equine gestation period also makes genetic research expensive and time-consuming. For the six-month lethal white study, researchers utilized a similar condition found in mice and a human disease called Hirschsprung’s to determine the elusive gene.
The University of California-Davis, a front-runner in equine genetics, was the University’s main competitor during the gene study. Purdy conceded there was a feeling of urgency, but ultimately the University won.
During breeding season, Santschi said she receives four lethal white cases each week and several calls a day from distraught breeders.
“You take a big hit financially as well as emotionally,” said large animal hospital director Paul Vrotsos, who owns and breeds paint horses.
After having several of the stark-white foals two years in a row, Vrotsos implored University researchers to solve the lethal white puzzle. Each of his 12 horses have been tested, and he said he is grateful to be able to make educated breeding decisions based on the results.
The test is a simple blood or hair sample screening costing about $50 per horse — a small price to pay when small breeders like Vrotsos estimate a $2,000 to $3,000 loss for every lethal white birth.
Larger breeding operators might not be as concerned with the situation as smaller breeders. The instances of lethal whites are simply viewed as another business cost.
Despite the proven risks of certain breeding combinations, some breeders will continue to take their chances to achieve the desired blood line.
“Anytime you select for a desired trait, there’s a tendency for undesirable traits as well,” Santschi said.