Audrey Kelleman, DVM, Dipl. ACT, clinical assistant professor and large animal reproduction service chief in the University of Florida’s Department of Large Animal Clinical Sciences, described the importance of this test during the university’s 2020 Healthy Horses Conference.
First, a Primer on Genetics
Kelleman defined genetics as the way physical traits and characteristics get passed from one generation to the next—also called heredity. Genetics includes the study of genes, which have a special code called DNA that determines physical traits and susceptibility to certain illnesses.
“Our bodies are made up of cells, and within those cells are a center portion called the nucleus,” she explained. “The nucleus has chromosomes that contain genetic material for our bodies. Horses have 32 pairs of these chromosomes, making 64 total. A foal gets one copy from the mare and one from the stallion. If we unfurl these chromosomes, they have genes contained on them.”
At a basic level, genes can be categorized as recessive or dominant. If an animal has one copy of a recessive gene, that trait is hidden, and that animal is considered a carrier. If the animal has two copies of the gene, that trait or disease will appear in the animal. With dominant genes, an animal with one copy will show the trait, and animals with two copies might show the trait twofold. If that gene codes for a disease, for instance, it can manifest more severely.
What This Means for Quarter Horse Breeders
The AQHA recommends owners screen their Quarter Horses for five diseases: glycogen branching enzyme deficiency (GBED), heredity equine regional dermal asthenia (HERDA), hyperkalemic periodic paralysis (HYPP), malignant hyperthermia (MH), and polysaccharide storage myopathy (PSSM).
The organization says it provides the panel test because “the effects of these diseases are wide-ranging, from mild and manageable to severe and terminal. Passing these diseases on to successive generations often causes unnecessary suffering and also leads to financial losses for breeders.”
The test is currently required for breeding stallions but not mares, though members can have the test performed on any horse.
Two of the diseases, GBED and HERDA, are recessive, meaning horses can be carriers.
“GBED is a problem where sugars are not used appropriately,” explained Kelleman. “Because of that, muscles, heart, and brain don’t work correctly. In foals, this can cause death due to severe weaknesses and ‘heart attack.’”
HERDA is a disease of the skin that has no cure. “It’s often first seen when fillies and colts start training under saddle where the saddle causes bruising because the skin is so fragile,” she said.
The other three diseases on the five-panel test are dominant. HYPP occurs when a horse’s body is unable to manage potassium properly, causing muscle trembling, falling over, paralysis, and even death, said Kelleman. Horses with two copies of the gene cannot be registered with the AQHA.
Horses with MH can become very hot and have a very high fever, as well as muscle disease, stiffness, and trembling. “They get so hot they can actually die from the high temperature in their body,” she said.
Lastly, PSSM is the horse’s body’s inability to adequately deal with carbohydrates or sugars, causing muscle problems such as stiffness and other disorders.
Kelleman said researchers at the University of California, Davis, where the five-panel test is performed, have discovered another genetic disease of Quarter Horses that might get added to a six-panel test. It’s called immune-mediated myositis, a dominant, autoimmune muscle disease that can result in severe muscle atrophy (shrinking) and loss.
The disease is unique in that “affected animals must have the gene for the disease and also have an environmental or health issue (e.g., respiratory infection, certain vaccinations) that encourages it to be developed and the body to attack the muscles,” Kelleman explained.
Studies have shown that about 7.5% of Quarter Horses have one copy of this gene, with a higher percentage in the reining (13.5%), working cow (8.5%) and halter (8%) categories, but no occurrence in barrel and racing categories.
In summary, said Kelleman, genetic diseases can cause serious problems, and testing for them is important for producing healthy horses.
The five-panel test is easy to do, and the AQHA provides a five-minute instructional video on its website. Simply pluck hair from your horse’s mane or tail in adequate numbers and deeply enough to include the bulb of tissue at the bottom of the follicle, and package and send the sample to the AQHA with the appropriate paperwork and payment.