RED DEER – Fearsome sounding genetic diseases haunt the Quarter horse industry, disorders such as hyperkalemic periodic paralysis, glycogen branding enzyme deficiency and hereditary equine regional dermal asthenia.
The breed appears particularly disease ridden because there are millions of Quarter horses in North America and the breed association funds more genetic research.
However, equine veterinarian Stephanie Valberg of the University of Minnesota says other conditions may appear as more breeds are tested.
“If we tested other breeds, we would find a similar number of genetic mutations,” she told the Alberta Horse Owners and Breeders Conference held Jan. 17 in Red Deer.
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The severity of some genetic problems may depend on nutrition, the kind of work the horse does, hormones and environmental factors.
Researchers know one gene is not responsible for all problems, but polygenic tests have not yet been developed.
“The next step is to map the multitude of genes that go together to influence what happens,” she said.
Current tests search only for a single mutation, and she is skeptical of tests that try to predict the prevalence of one trait from the presence of one gene.
Valberg said buyers should ask breeders for tests and equine associations need to fund more genetic research to determine the prevalence of disorders so breeders can make informed choices.
Some of these disorders have occurred because a limited number of popular sires were used. Valberg said tests on affected horses often find they are closely related.
Horses have 20,000 genes on 33 pairs of chromosomes. Matched pairs of genes come from the sire and dam. An allele is one copy of the gene that affects a particular trait.
There are recessive and dominant diseases. Two matching genes from parents create the recessive disease while dominant diseases require only one mutant gene from one parent.
“There are mutations that the body corrects, but sometimes the body can’t correct it,” Valberg said. “That is where we get genetic mutations.”
A horse named Impressive is an example of how one animal can spread a mutation to thousands of animals.
Impressive was a prize winning halter horse with heavy muscling that many producers wanted, but it also carried a gene that caused a condition called hyperkalemic period paralysis (HyPP).
Impressive produced more than 40,000 offspring, and 60 percent of halter horses now have HyPP.
The condition causes shaking and trembling of muscles, and contractions can be so severe foals cannot suckle or breathe properly. Some may die suddenly because their upper airways collapse. Some horses are in so much pain that they go down and cannot stand.
Affected horses have shown higher than normal potassium levels in their muscles, which means it may be managed by restricting that mineral in their diets.
The American Quarter Horse Association requires the testing of all Impressive descendents. Homozygous individuals, which carry the gene on both sides, will not be registered.
A test has been available since 1992.
“The mutation started with Impressive himself or it was in his sperm where the trait started,” Valberg said.
“None of his brothers produced any infected foals.”
Severe combined immunodeficiency is a recessive condition found in Arabians in which foals die by five months of age. Young animals develop recurrent infections once they are off colostrum because they cannot mount their own immunity.
The Arabian Horse Registry recommends testing, but it is not mandatory. Arabian breeders mounted a campaign to help owners make the right breeding choices.
Glycogen branding enzyme deficiency is a common cause of abortion or weak foals and has been found in nine percent of Quarter horses. It was first detected in 2001 and is always fatal. Horses have low blood sugar and seizures and are too weak to stand.
“It doesn’t look like many horses are affected, but you’ll see a concentrated group of performance horses are affected,” Valberg said.
Hereditary equine regional dermal asthenia is also seen in Quarter horses, especially among working cow or cutting horses. It is a skin disease that is often not noticed until horses wear a saddle and develop wounds that do not heal.
