The 5 Panel Genetic Test for AQHA registration
The AQHA offers its members a genetic test for a panel of 5 different genetic diseases affecting Quarter Horses. The panel tests for Hyperkalemic Periodic Paralysis (HYPP), Polysaccharide Storage Myopathy type 1 (PSSM 1), Malignant Hyperthermia (MH), Hereditary Epidermal Regional Dermal Asthenia (HERDA), and Glycogen Branching Enzyme Deficiency (GBED). All these diseases are genetic, and are caused by single mutations in DNA that are easy to test for. The University of California-Davis performs the testing for the AQHA, using mane or tail hair or blood samples.
The panel has been available for years, but just recently AQHA began requiring that results of the panel be on file for stallions before their foals could be registered. This applied at first only to stallions that were bred to more than 25 mares, but after January 1, 2015, ALL breeding stallions will need to have the 5 panel test results on file. This requirement doesn’t restrict the use of stallions that may test positive for one or more of the diseases, but it does give mare owners valuable information to consider when making breeding decisions. Although the requirements for the panel testing only apply to breeding stallions, owners may request testing of any horse if they are interested or have concerns.
HYPP stands for Hyperkalemic Periodic Paralysis. This disease affects the electrical impulses within the body that control muscle contraction. The defective gene results in clinical signs of muscle tremors and fasciculations. In some severe cases, horses may be unable to stand, or even unable to breathe. Horses can show symptoms with only one copy of the defective gene, but symptoms are often more severe if they have two copies of the mutation. This disease affects mostly halter horses, and can be traced back to the prolific stallion ‘Impressive’. Since Impressive lines were also used in Paint and Appaloosa halter breeding programs, HYPP is found in those breeds as well. AQHA does not allow registration of foals that test positive for two copies of the defective gene (H/H), but will allow registration of foals that are H/N: one defective and one normal gene.
PSSM stands for Polysaccharide Storage Myopathy. This disease causes changes in the way sugars are stored and used by the muscles. It causes frequent episodes of ‘tying up’ if not properly controlled by a special diet and regular low intensity exercise. There are two types of PSSM. Type 1 is caused by a genetically identified mutation, which is testable. Type 2 is suspected to be genetic, but that mutation has not yet been identified by researchers. Most Quarter Horses with PSSM have type 1. Horses will show symptoms of PSSM type 1 with one or two copies of the mutation. Like HYPP, PSSM type 1 is more common in halter QHs than in other lines. Some QHs have been shown to have mutations for both HYPP and PSSM.
HERDA stands for Hereditary Epidermal Regional Dermal Asthenia. Horses with HERDA have defective collagen, an important protein that is part of skin, cartilage, muscles, and tendons. The major clinical sign is skin that is easily injured, torn, or even sloughed off. The skin is also very slow to heal. There is no treatment for the condition, and horses that have it are often euthanized. Horses will only show symptoms if they have two copies of the mutation for HERDA. Horses with only one copy of the mutation are clinically normal. These animals are called ‘carriers’. They can pass copies of the mutation to their foals, and if one carrier is bred to another carrier, the foal might inherit the mutation from both parents and be symptomatic. HERDA is limited mostly to horses with reining and cutting horse bloodlines.
GBED stands for Glycogen Branching Enzyme Deficiency. Like PSSM, this disease also affects how sugars are stored, but in a different and more severe way. It results in abortions, stillborn foals, and foals that are alive but weak at birth and die or are euthanized soon after. Like with HERDA, horses may be carriers for GBED – if a horse has only one copy of the mutation it will be clinically normal. Paints and Appaloosas can also carry the GBED mutation.
MH stands for Malignant Hyperthermia. This disease changes the way muscle cells handle calcium, and thus the metabolism of the cell. Horses with MH will appear normal most of the time, but have specific occasions when they show symptoms. During an attack, horses will have a very high fever, profuse sweating, high and irregular heart rate, high blood pressure, and rigid muscles. Attacks are triggered by certain anesthetic agents or stress, and are sometimes fatal. MH is believed to be less common than either HYPP or PSSM, but the percentage of affected horses is not yet known. Several breeds including Quarter Horses and Paints can be affected. Horses may be positive for both PSSM and MH together, and these animals appear to suffer from more severe episodes of tying up than horses that have PSSM alone.