6 Panel Genetic Testing: What to Know

6 Panel Genetic Testing: What to Know

AQHA offers a panel test for six genetic tests. Learn more about the six equine diseases it covers – HYPP, PSSM, MH, GBED, HERDA and MYHM.

sorrel foal and palomino mare stand together in a field with mountains in the background (Credit: Briana Malmquist)

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Genetic diseases cause emotional and financial pain for horse owners and breeders. To help breeders make informed decisions, AQHA offers a genetic health panel test for six genetic diseases:

  1. GBED: glycogen branching enzyme deficiency

  2. HERDA: hereditary equine regional dermal asthenia

  3. HYPP: hyperkalemic periodic paralysis

  4. MH: malignant hyperthermia

  5. PSSM1: polysaccharide storage myopathy Type 1

  6. MYHM: myosin-heavy chain myopathy

When the test is ordered, AQHA will send a test kit, and the owner will mail the hair sample directly to the Veterinary Genetics Laboratory at the University of California-Davis for testing. Once the tests are complete, AQHA will notify the owners and put the results on the horse’s record and certificate of registration.

  • Health panel test: $100 for members and $165 for nonmembers. 

  •  Healthpanel and DNA test:  $120 for members and $185 for nonmembers.

Note: Breeding stallions must have DNA and a health panel on file, while breeding mares are only required to have DNA for foals to be eligible for registration.

The effects of these genetic 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.

GBED: Glycogen Branching Enzyme Deficiency Disorder

An autosomal recessive disease caused by mutation in the GBE1 gene.

Affects: Approximately 8-10 percent of American Quarter Horses, most commonly found in western pleasure and cutting horse industries. Paints can be carriers, as can Appaloosas and breeds that descend from the American Quarter Horse. An estimated 3 or more percent of second- and third-term abortions are caused by GBED. Affected animals may be aborted or stillborn, and foals that survive to term typically die or cannot stand or nurse on their own. Foals may appear healthy for a time but eventually may develop seizures, become too weak to stand, or in some cases, die suddenly.

Description: The mutation of the GBE1 gene reduces the function of the glycogen branching enzyme so that cardiac and skeletal muscle, the liver and the brain cannot store and mobilize glycogen. Because glycogen provides energy to the muscles, the inability to properly store and mobilize it leads to muscle weakness and eventually death. GBED results in second- and third-term abortions and stillborn foals, and researchers think many aborted and stillborn foals whose cause of death was not previously identified might have had GBED. Foals that survive to birth generally die or are euthanized within 8 weeks of age. Although a few foals have survived to the age of 4 months, GBED is always fatal. GBED is a recessive mutation. This means that a horse must inherit TWO copies to be affected. A horse that inherits one copy, known as a carrier, will not experience symptoms of the disorder but can pass the mutation on to offspring.

Continue learning about GBED. 

HERDA: Hereditary Equine Regional Dermal Asthenia Disorder

An autosomal recessive disease caused by mutation in the peptidyl-prolyl isomerase B (PPIB) gene.

Affects: Approximately 3.5 percent of American Quarter Horses are carriers, with most cases found in cutting and cow horse disciplines. It is estimated that 28% of horses in these disciplines carry one copy of HERDA. Breeding horses should be tested to ensure risk of producing HERDA affected horses is minimized.

Description: HERDA is a skin condition that is characterized by hyperextensible (more elastic than normal) skin, severe scarring, and open wounds along the back of affected horses. HERDA causes collagen between skin layers to not form properly, resulting in a horse that is easily injured. Collagen is important in connective tissues throughout the body including components in the eyes, heart, tendons, cartilage, and skin.  The mutation in the PPIB gene results in defective collagen that causes the outer layer of the skin to split from the layer underneath. In many cases, the outer layer of the skin sloughs off entirely, leaving raw wounds. Young horses with HERDA might appear to have an unusual number of nicks and cuts on their skin, but the disease is most often noticed when the horse starts training under saddle. The pressure of the saddle on the back causes the skin to tear and separate, leaving raw areas. Sunlight is suspected to increase damage to collagen thus making the top of the horse more effected (i.e. top of the neck, back, and rump), which is more exposed to UV rays. These areas are slow to heal, and many horses with HERDA are euthanized due to slow-healing injuries. HERDA is a recessive mutation. This means that your horse must inherit TWO copies to be affected. A horse that only inherits one copy, is known as a carrier, and will appear normal and will not experience any symptoms caused by this mutation.

Continue learning about HERDA.

HYPP: Hyperkalemic Periodic Paralysis Disorder 

An autosomal dominant disease caused by point mutation in the SCN4A gene.

Affects: Approximately 4.4% of quarter horses are carriers of HYPP, most commonly found in halter horses.  HYPP is characterized by sporadic attacks of muscle tremors (shaking and trembling), weakness, and/or collapse. Attacks can also be accompanied by loud breathing noises resulting from paralysis of the muscles of the upper airway. Under the proper care, many horses have minimal symptoms and can go on to have successful performance careers.

Description: The mutation in the sodium channel gene causes dysfunction in a specific type of sodium ion channel. These channels are involved in generating electrical impulses associated with muscle contraction. The mutation disrupts the proper conduction of these impulses, causing muscle tremors and even temporarily paralysis in affected horses. In severe cases, HYPP can cause collapse or sudden death. HYPP was first identified in AQHA stallion, Impressive. Any horse that has Impressive bloodlines is required to be tested for HYPP. HYPP was seen in Impressive’s sons and daughters because to be expressed, the disease does not require two copies of the defective gene. However, successive generations of offspring that received two defective genes often show more severe versions of the disease. A horse with two copies of the gene mutation (H/H) is ineligible for registration.

Continue learning about HYPP.

MH: Malignant Hyperthermia Disorder

An autosomal dominant disease caused by mutation in the ryanodine receptor 1 (RYR1).

Affects: American Quarter Horses and several other breeds; the percentage of affected horses is unknown, though it is most commonly found in halter horses. MH is a genetic mutation that causes a life-threatening condition triggered by certain anesthesia drugs such a halothane or isoflurane. MH can also be triggered by stress or excitement.

Description: The mutation results in a malfunctioning calcium-release channel of the sarcoplasmic reticulum in skeletal muscle. During an episode a horse with MH will release uncontrolled amounts of calcium into the bloodstream, which results in painful muscle cramps, extremely high temperature up to 113 degrees Fahrenheit, irregular heart rhythm, excessive sweating and shallow breathing. This can cause a hypermetabolic state (increased metabolism) and may result in death. A horse that has PSSM or MYHM in conjunction with MH will likely have much more severe episodes.  MH is a dominant mutation. This means that your horse only needs one copy of MH to be affected.

Continue learning about MH.

PSSM: Polysaccharide Storage Myopathy Disorder

An autosomal dominant disease caused by mutation in the glycogen synthase 1 (GYS1) gene.

Affects: Approximately 11% of quarter horses are affected by PSSM, as well as many other breeds.  PSSM is a disease that causes an abnormal accumulation of glycogen, the form of sugar stored in the muscle. This excess sugar causes mild to severe muscle cramps, sore muscles and/or muscle weakness. Horses that are managed properly can generally go on to have successful performance careers.

Description: PSSM is a common form of tying up. The mutation in the GYS1 gene causes unregulated synthesis of glycogen, which results in excessive sugar in muscle cells. This leads to muscle pain and stiffness, sweating, exercise intolerance and weakness. Because of the pain and stiffness, horses are reluctant to move.. Two types of PSSM have been classified: PSSM1 and PSSM2. The genetic test used by AQHA identifies PSSM1 mutation. Type 2 PSSM refers to PSSM symptoms that occur in horses without the known PSSM1 variant. At this time, aside from PSSM1, there is not a genetic test for other forms of PSSM. PSSM2 can, however, be diagnosed with a muscle biopsy. PSSM2 is most commonly found in warmbloods and Arabians. PSSM Type 1 and 2 can present the same symptoms but are caused by different issues. The cause of PSSM2 remains unknown, though multiple causes are suspected. PSSM is a dominant mutation, which means horses with just one copy will experience effects, though likely less severe than those that have two copies.

Continue learning about PSSM.

MYHM: Myosin-Heavy Chain Myopathy

An autosomal dominant disease caused by the mutation of the MYH1 gene.

Affects: Approximately 7% of quarter horses have the MYHM variant. It is most commonly found in reining horses, cowhorses, and halter horses. MYHM is a genetic muscle disease that can result in two distinct clinical disease presentations that both involve muscle loss or damage and are linked to the same genetic variant. A horse with MYHM is prone to presenting with one or both during their lifetime, while some horses with the mutation may never experience symptoms.

Description: MYHM is a relatively newly discovered genetic disorder. This mutation makes horses susceptible to disease. Horses with the mutation exposed to environmental triggers will develop symptoms of the disease. Not all environmental risk factors are currently known. Therefore, it is impossible to say if or how a horse with the MYHM mutation will be affected. This makes it important to have your horse tested, as management is key to preventing an episode.

Immune-Mediated Myositis (IMM) is one form of clinical disease caused by MYHM, this results in muscle atrophy that is suspected to be the result of a response to a vaccine or infectious agent such as strangles. The immune system misinterprets the muscle cells as foreign and rapidly attacks them. Horses initially experience stiffness, weakness, and a decreased appetite followed by the rapid loss of 40% of muscle mass within 72 hours.

The second presentation of MYHM is Nonexertional Rhabdomyolysis and often presents as stiffness, like "tying up", and possible swelling of muscles along the back and haunches without exercise.

Nonexertional rhabdomyolysis causes pain, muscle cramping, muscle damage and may or may not result in muscle loss. Horses affected by IMM or nonexertional rhabdomyolysis can recover but may have more frequent episodes.

MYHM is a dominant mutation, which means your horse only needs one copy to be affected, though not all horses with the mutation will become affected. They must be exposed to a trigger to experience symptoms. Horses that are homozygous (My/My) are likely to experience more severe symptoms.

Continue learning about MYHM.