Leading halter breeders Dorn and Debbi Parkinson of Prescott, Arizona, have used genetic testing in their program to manage and breed away from hyperkalemic periodic paralysis since they first became aware of the disease. “The more knowledge you have about what’s happening genetically, the better off you are,” Dorn says. “You can make more informed decisions on how to breed. … I fundamentally believe that testing is the only fair thing a breeder can to do for the consumer, the prospective purchasers of stallion fees or of horses,” Dorn says. “I think it’s important that a buyer knows as much about a horse as the seller.” Because of new research, it is now possible for Quarter Horse breeders to fine tune their genetic testing, honing in on the diseases that are most prevalent in the type of horses they’re breeding.
A team of 11 researchers from the University of California at Davis and the University of Minnesota took DNA from a nationwide random sampling of registered Quarter Horses and tested it for five genetic diseases:
- glycogen branching enzyme deficiency (GBED)
- hereditary equine regional dermal asthenia (HERDA)
- hyperkalemic periodic paralysis (HYPP)
- polysaccharide storage myopathy (PSSM)
- lethal white foal syndrome (LWFS), which is found in overo American Paint Horses and suspected to be carried by some American Quarter Horses.
Researchers then performed the same genetic tests on DNA taken from subgroups of top-level performance Quarter Horses in seven events: barrel racing, cutting, halter, racing, reining, western pleasure and working cow horse. (All DNA samples were anonymous, used with permission from the UC Davis Veterinary Genetics Laboratory and American Quarter Horse Association.)
They compared the results from both groups to see how often and where each genetic disease showed up. The data was published in the January 2009 Journal of the American Veterinary Medical Association.
What Genetic Researchers Found
In the Quarter Horse breed as a whole, researchers found the highest incidence of PSSM and GBED, with about 11 percent of individuals testing positive for those genes, but there was a lower incidence of HERDA (3.5 percent) and HYPP (1.5 percent).
However, disease-causing genes were distinctly distributed within the seven subgroups of performance Quarter Horses competing at the highest level of their discipline.
Of the halter subgroup, 56 percent of the individuals carried the genes for HYPP and 28 percent carried the genes for PSSM.
The cutting, reining and working cow horse subgroups all contained the genes causing HERDA, PSSM and GBED. In the cutting subgroup, 28 percent of the individuals carried the gene for HERDA.
In the western pleasure subgroup, genes for GBED had the highest incidence at 26 percent, although HYPP, PSSM and HERDA were also present.
The barrel racing and racing subgroups had the lowest frequencies of the diseases tested for, with only PSSM appearing in the racing subgroup.
LWFS did not appear in any of the groups, although researchers believe it could appear in a larger sample population due to the number of registered Quarter Horse mares that have produced foals with the recessive disease when bred to Paint stallions.
“All breeds of horses are going to have genetic diseases,” says Dr. Stephanie Valberg of the University of Minnesota, an author on the paper. “AQHA has been very proactive in researching genetic diseases in Quarter Horses. That’s why we know so much about genetic traits in Quarter Horses.”
What This Genetic Research Means
The appearance of these diseases in our elite performance subgroups are due in large part to practices such as linebreeding to concentrate desirable traits, which can also concentrate undesirable traits that might be in a family line. Habits such as “popular sire syndrome” – the tendency for many breeders to breed to a top performing stallion – do the same.
How breeders use assisted reproductive techniques such as frozen semen can also amplify one individual’s influence on the gene pool.
“When a large portion of breeders breed to a single animal, there will be a loss of genetic diversity,” explains Dr. Sharon Spier of UC Davis, who presented this research at the 2009 AQHA Convention. “Too much breeding to one animal will give the gene pool an extraordinary dose of his genes, and this will include whatever detrimental recessives he may carry, to be uncovered in later generations.”
Dr. Spier adds that if we are not careful as breeders, we need to be prepared for more genetic diseases to crop up in the future, and the diseases currently identified could become more prevalent. However, using available genetic tests, we can breed away from those traits and reduce their frequency in the population.
What Next for Quarter Horse Genetics?
“There is no perfect animal,” Dr. Valberg says. “You and I probably have autosomal recessives, deleterious traits; everybody has them. There are always going to be individuals that have unknown genetic traits that when you cross them, you get something you weren’t expecting.
“But we can certainly work with what we do know and avoid the certain problems.”
And these diseases cause problems: although some diseases, such as PSSM, can be successfully managed in affected individuals, others, such as GBED, are categorically lethal.
“You’re not going to eliminate genetic diseases in any breed,” Dr. Spier says. “But you can be knowledgeable about them and do the best you can.
Defining Quarter Horse Genetic Disorders
HYPP – Hyperkalemic periodic paralysis is a muscle condition that leads to uncontrolled muscle twitching or profound muscle weakness. In severe cases, it can lead to collapse and death.
HERDA – Hereditary equine regional dermal asthenia causes the skin on a horse’s back to literally peel away. Early signs include the presence of weeping skin, hematomas, wounds, sloughing skin, scars and loose and easily tented skin that doesn’t return to its initial position.
GBED – Glycogen branching enzyme deficiency doesn’t allow a foal to store enough sugar in its cells for energy, and it dies, usually before 2 months of age. These foals are often still born.
PSSM – Polysaccharide storage myopathy, a form of “tying up,” is characterized by the accumulation of glycogen (storage form of sugar) and an abnormal sugar (polysaccharide) in skeletal muscle. Affected horses develop stiffness, muscle cramping and soreness with light exercise, probably due to a deficit of energy generation in their muscles.
LWFS – Lethal white foal syndrome affects foals born with blue eyes and are completely or almost completely white at birth. They have an underdeveloped, contracted intestine caused by a failure of the embryonic cells that form nerves in the gastrointestinal system. Oddly enough, these cells also play a role in determining skin color. The foals are unable to pass feces and the condition is fatal.