Breeding Responsibly

New research examines the prevalence of genetic diseases in American Quarter Horses by discipline.

New research examines the prevalence of genetic diseases in American Quarter Horses by discipline.

Breeding Responsibly picBy Christine Hamilton of America’s Horse

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); and 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 AQHA.)

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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 They 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 It 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.

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What Next?

“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 the Disorders

From America’s Horse and the American Association of Equine Practitioners

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 – Foals with lethal white foal syndrome are 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.

17 thoughts on “Breeding Responsibly”

  1. There is a simple solution to this – return to breeding for the all around versatility the breed was once famous for. Breeders should take into consideration all traits of the horse, not just the one that wins.

  2. I do agree with Susan, We need to keep with the all round QH….but I do think that with the racing line and barrel racing subgroups the crossing of Thoroughbred(Appendix), this adds more to the genetic breeding, in some case this is good. We have over breed the QH. I love the breed very much! I grew up with them…they are my love.
    Thank you for your time.

  3. My suggestion would have been to end the article with an action. Such as Thinking of breeding, here is where to get your horses tested. Affordable testing would be even nicer.


  4. You can talk about responsible breeding all you want, but why don’t you practice what you preach. When HYPP was linked to Impressive, it was made mandatory to test all offspring going back to him. Now that Herda has been linked to Poco Bueno, you should have mandatory testing for all offspring going back to him. If you want responsible breeding, and eleminating defects, practice what you preach.

  5. Stallion and mare owneres need to apply morals and ethics into selecting individuals for breeding. To eliminate defects, don’t breed animals with defects. Get real breeders, don’t breed your defective genetic animals, don’t breed for the almighty dollar.
    The association needs to use the research information and get proactive to eliminate the genetic defects. Now – not later.

  6. Let’s take it a little farther. The association should consider ways in entice these breeders to be more selective and/or careful. Over breeding, using embrio transfers so that more than 1 or 5 or 10 foals are produced from one mare each year, breeding to anything around; are just a few of what I consider careless breeding techniques. Limit the number of transfers that can be registered. Payoff breeders with horses that carry these genes. Mark the papers the same way you do the HYPP. Give an incentive for everyone to have their breeding stock tested!

    The breeders on the other hand need to STOP!!! They are bringing down not only the quaility of our beloved horses but the price also. I realize the expense involved with these horses, and the incentive to put off spring on the ground, but, what is the acceptable ratio for Good minded, healthy to the sick, unhandlable ones?.

    I have two, beautiful well bred mares. I have been careful in their selection. I still plan on having them tested, and on asking that any stallion I look at to breed to is also tested. I also look at every aspect of the stallion. Including his health, mind, looks, and disposition.

  7. Let AQHA have some of the blame here. Breeders have to go with what type of horse is wining if they want to stay in the business. Go back to the all around athlete wining the classes and I guarantee a lot of this disease problem well diminish. Also I find it unacceptable that Impressive bred horses are required to get tested but not Poco Bueno bred horse. Both line are great line keep it fair.

  8. October 28, 2009, my 4 year old grandson of Invitation Only on the top side and great grandson of Impressive on the bottom was euthanized for multiple pain issues. I didn’t have him tested for PSSM, but he had classic symptoms which would cause him to tie up after five minutes of light riding. For 6 weeks prior to euthanasia the only think he ate was grass hay, it was the only thing he would eat. The poor horse also suffered from gastric ulcers, tendon damage, etc. He was HYPP N/N, and I didn’t realize when I purchased him that he should have additional genetic testing. Diet and exercise is the key to managing PSSM, but when your horse stops eating everything but grass hay it’s tough to get them to ingest corn or soy oil. It is impossible to exercise a horse with tendon injuries that doesn’t tolerate NSAID’s because of ulcers, etc, etc. AQHA needs to require all breeding animals be tested for these debilitating diseases. Also, there are 2 types of PSSM, one of which is extremely difficult to manage and AQHA needs to acknowledge that fact and publish the info.

  9. Im still a little confused on why the association isn’t limiting the number of offspring a stallion can produce a year and to what QUALITY mare with genetic testing done this should all be part of the process to REGISTER your foal. 1) Who is daddy and did he get all these tests done before cover of the mare? 2) who is the mare did she have all these tests done before cover? 3) what points do dam and sire have BEFORE breeding or what can they do as far as work?

    When the AQHA first started you HAD TO PROVE that your hose could PREFORM like a QH or they wouldn’t let you in WHY AREN’T WE STILL DOING THIS lets limit the number of horses and INCREASE the number of QUALITY horses

  10. As a responsible breeder, I would like to QH stallions graded similar to other breed associations and a requirement to geld those that do not meet standards. There are way too many mediocre studs out there producing more of the same.

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