Genetic Testing

5 or 6 Panel Genetic Testing – What to Know

To help breeders make informed decisions, AQHA-NZ requires all registered horses to  panel test for five genetic diseases.
Learn more about the five equine diseases it covers – HYPP, PSSM1, MH, GBED, HERDA and MYHM.


Genetic diseases impact on the horse as well as causing emotional and financial pain for horse owners and breeders. To help breeders make informed decisions, AQHA-NZ offers a panel test for six genetic diseases through Massey University:

  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

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.

To get a 5 or 6 Panel test you can either send a Hair Sample to the AQHA-NZ office with your registration form, or you can do this yourself through Massey University.


1. 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; 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.

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.


2. 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.

Description: Collagen makes up connective tissues (skin, bones, tissue, muscles and cartilage). 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 some 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. These areas are slow to heal, and many horses with HERDA are euthanized due to slow-healing injuries.


3. HYPP: Hyperkalemic Periodic Paralysis Disorder 

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

Affects: Approximately 1.5 percent of all Quarter Horses and as many as 56 percent of all halter horses.

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 traces back to the stallion Impressive. 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.


4. 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.

Description: The mutation results in a malfunctioning calcium-release channel of the sarcoplasmic reticulum in skeletal muscle. The malfunction causes excessive calcium to be released into the myoplasm (the contractile part of a muscle cell). This can cause a hypermetabolic state (increased metabolism) and may result in death. MH is triggered by the use of the anesthetic halothane, the muscle relaxant succinylcholine and stress. Affected horses experience increased muscle metabolism, fever often exceeding 109 degrees F, excessive sweating, high heart rate, abnormal heart rhythm, shallow breathing, hypertension, muscle rigidity, breakdown of muscle tissue, muscle protein in the urine and/or death.


5. PSSM: Polysaccharide Storage Myopathy Disorder

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

Affects: Eleven percent of American Quarter Horses. PSSM also affects many other breeds.

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. Research conducted at the University of Minnesota has identified two types of PSSM. Genetic testing determined that some horses with PSSM had a specific mutation in GYS1 (PSSM Type 1), while others did not have the mutation in the GYS1 gene (PSSM Type 2). Horses identified as having a moderate to severe form of the disease according to the muscle biopsy were more likely to have PSSM Type 1 than horses with a milder version of the disease. Researchers have not yet determined what causes PSSM Type 2.


6. MYHM : Myosin-Heavy Chain Myopathy

AQHA and AQHA-NZ are now requiring stallions to be tested for MYHM and have their results recorded.
AQHA-NZ have put together a quick fact sheet regarding MYHM –
please click here to read information regarding this. Testing for MYHM is available in New Zealand at Massey University (MYHM/IMM Test). 

AQHA also have a fantastic resource available with full information regarding MYHM – click here to view.

Affects: approximately 7% of Quarterhorses. Most commonly found in reining horses, cowhorses and halterhorses


  • 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.
  • 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.

Breeding 5 or 6 Panel negative horses

Where AQHA-NZ registers a horse that is parent verified and the 5 or 6 panel are negative then those negative results will be recorded as N/R (not required). Where the parent has positive results and the progeny has been tested for those parents positive tests their negative results will be recorded as N/N. 

In future more inheritable genetic diseases will be discovered and more tests may be available.