Understanding Genetic Testing
All of our horses have been genetically tested for the original 5 panel that AQHA requires at this time for all breeding stallions. Recently they have added another test, our stallions are tested through Animal Genetics for this but will be tested through AQHA. We will slowly test all of our mares as well; with their results listed on their pages
Some people are unfamiliar with these genetic tests and we thought we would put that information in one place for you to look through. All of the information below has been taken from the Animal Genetics website as we thought this information is best explained there. By clicking here you will be taken to their website or you can read the information below.
Glycogen Branching Enzyme Deficiency
Glycogen Branching Enzyme Deficiency (GBED) is a fatal condition caused by the bodies' inability to properly store sugar. In a normal horse, the body stores sugar as energy by converting glucose (a type of sugar) to glycogen (storable energy). This inherited disorder prevents the body from producing the enzyme needed to create the glycogen structure. This prevents the horse from being able to adequately store the sugars. This means that the horse will not be able to store enough energy to fuel important organs, such as the muscles and brain.
Foals born with GBED suffer from a range of symptoms associated with this lack of energy storage, such as low energy, weakness, and difficulty rising. Other symptoms include low body temperature, contracted muscles, seizures, and sudden death. Unfortunately, GBED is always fatal; most affected foals will die before the age of 8 weeks. GBED often causes the fetus to be aborted in utero. Research suggests that as many as 3% of aborted Quarter Horse foals were homozygous for the GBED mutation.
Studies show that the mutation responsible for GBED is carried by as many as 10% of Quarter Horses, Paint Horse breeds and related breeds. GBED is an autosomal recessive trait, meaning a foal can only be affected if the foal inherits the disease from both parents. Horses that are carriers of the GBED have one copy of the mutation but do not have any symptoms associated with the disorder. This makes DNA testing important to screen for carriers and prevent this fatal condition. If two carriers are bred together, there is a 25% chance per foal that they will develop GBED.
The mutation which causes this disease has been identified by Dr. Stephanie Valberg and Dr. James Mickelson of the University of Minnesota. Animal Genetics Inc. (USA) is licensed for diagnostic testing of GBED.
Hereditary Equine Reginal Dermal Asthenia
Hereditary Equine Regional Dermal Asthenia (HERDA) is also known as Hyperelastosis Cutis (HC). HERDA is a genetic skin disease predominantly found in the American Quarter Horse. Researchers at Mississippi State University and Cornell University believe that the origin of this genetic disorder may be the Poco Bueno's sire line.
The primary symptom of this disorder is a lack of adhesion within the layers of skin. This is due to a genetic defect in the collagen (a fiber with in the body) that holds the skin in place. This defect causes the outer layer of skin to split or separate from the deeper layers. Sometimes these layers tear off completely. Areas under the saddle seem to be most prone to these lesions. There are often permanent scars which prevent the horse from being ridden.
The disorder is recessive, which means that a horse must have two copies of the defective gene to suffer from the disease. A horse with only one copy of the mutation is known as a carrier, and does not present any symptoms. When two carriers are bred with one another, there is a 25% chance per foal born that they will develop symptoms of HERDA. There is a 50% chance per foal born that they will inherit a copy of the mutation and also become carriers of HERDA. This makes testing an important tool while breeding.
Although there is no cure for the disorder, Animal Genetics has developed a simple DNA test in order to detect a single nucleotide polymorphism (SNP) mutation with a high degree of association to this disorder. This makes the test easily available to our customers.
Hyperkalemic Periodic Paralysis Disease
Equine Hyperkalemic Periodic Paralysis Disease (HYPP) is a muscular disease caused by an inherited genetic mutation. HYPP has been traced back to a horse named Impressive. HYPP is also known as Impressive Syndrome, named after this horse. Symptoms of HYPP may include muscle twitching, respiratory noises, and unpredictable paralysis attacks which can lead to sudden death. Severity of attacks varies from unnoticeable to collapses or sudden death. The cause of death is usually respiratory failure and/or cardiac arrest.
HYPP is a dominant disorder meaning both homozygous (H/H) and heterozygous (n/H) horses will be affected. They are both positive for the HYPP mutation. Only homozygous negative (n/n) horses are not affected by HYPP.
Because HYPP is dominant disorder, the effects can also be passed on to other breeds of horses when intermixing breeds occurs. This makes the recognition of this disorder very important in preserving the inherited health of all horses. Horses with HYPP only need one copy of the mutation to present symptoms of HYPP. A horse with HYPP can pass on this mutation to their offspring and can develop symptoms of HYPP. If a horse with HYPP is bred to a non-HYPP horse, there is a 50% chance per horse born that they will develop HYPP. If two horses with HYPP are bred, this increases to a minimum of a 75% chance per horse born of developing HYPP.
Malignant Hyperthermia, or MH, is a genetic muscle disorder that affects Quarter Horses and related breeds. Horses with the MH mutation may not show any physical signs of the disorder until triggered by exposure to anesthesia or extreme exercise or stress. Symptoms include:
high body temperature
increased heart rate
high blood pressure
Symptoms develop rapidly. If not treated quickly, this condition can be fatal.
MH is inherited as an autosomal dominant trait. This means that the disorder can be passed on even if only one parent has the defective gene. The mutation can be present along with another genetic disorder, Polysaccharide Storage Myopathy (PSSM). If a horse also has PSSM, the symptoms associated with MH can become more severe. Therefore, testing for both PSSM and MH is recommended, especially for Quarter Horse breeds.
Although this condition is rare, testing for MH is recommended in case a horse must undergo anesthesia. Horses that are known to have the MH mutation can be given medication prior to administering anesthesia to help reduce the severity of the symptoms. If a horse with one copy (carrier) of MH is bred to a horse with no copies, there is a 50% chance per horse of them developing MH symptoms. If two carriers are bred, there is a 75% chance per horse born that they will develop MH. If a horse has two copies of the MH mutation, there is a 100% chance per every horse born that they will develop MH.
Polysaccharide Storage Myopathy
Polysaccharide Storage Myopathy (PSSM1) is a dominant autosomal hereditary condition that can cause a genetic form of tying-up of muscles, causing muscle damage and the inability to move. There are several forms of PSSM. One of these forms is PSSM1. The PSSM1 mutation causes a disruption in the amino acid sequence. This, in turn, disrupts an energy producing process that allows muscles to operate normally. At least 20 breeds have been identified with Type 1 PSSM. The prevalence of this mutation in Belgian horses is as much as 50%. About 8% of the Quarter Horse-related breeds also have this mutation.
Some horses make and store abnormal muscle glycogen (a form of energy) and cannot tolerate dietary starches and sugars. Horses with PSSM1 can be maintained with low-starch and low-sugar rations and precise exercise protocols. In some horses, symptoms may begin by 2 to 3 years of age. In others, symptoms can remain subclinical, or non-observable. Clinical signs can include skin twitching, stiffness, firm, painful muscles, sweating, weakness, and reluctance to move, even during light exercise. Occasionally, gait abnormalities, mild colic and muscle wasting may also occur. In many cases, horses that have tested positive have had no history of 'tying-up' or other symptoms associated with PSSM1.
Research conducted at Animal Genetics has identified several additional mutations associated with PSSM1. These DNA mutations form a haplotype that allow us to identify horses with PSSM1. A haplotype is a group of inherited genes. Further ongoing research may provide us with a more comprehensive assay for PSSM and enable us to better determine the severity of the disorder in all horses.
An additional genetic mutation in RYR1 gene (a mutation that causes malignant hyperthermia in horses) influences PSSM. This mutation can also increase the severity of the symptoms of PSSM1 in Quarter Horses and related breeds. The combination of PSSM and MH genetic tests are available at Animal Genetics.
Not all cases of tying up are caused by the PSSM1 mutations. These cases are currently being looked at. A horse that tests N/N for PSSM1 but exhibits signs of tying-up or muscle pain may be suffering from an another muscle disorder. Please contact us with any examples of this so we can include these animals in further studies. In many cases, horses that test positive for these mutations will exhibit only minor problems or may never exhibit any noticeable problems at all. Again, please contact us and let us know so we can add these animals to our current research on PSSM1.
Management of Horses That Test Positive for PSSM1:
Horses that test positive for one or two copies of the PSSM mutation should be carefully managed through diet and exercise to help prevent the onset of the disease. For many horses affected by PSSM1, strict control of diet and exercise can reduce, or even prevent the onset of symptoms related to PSSM1. Eliminating high-sugar foods in their diet and consistent exercise are two simple ways to help prevent the disease from developing. Although taking these simple steps may not be effective in every situation, research has shown that often they will provide positive results. It is always important to let your veterinarian know if an animal has tested positive for PSSM1.
Immune Mediated Myositis and MYH1 Myopathy
Immune Mediated Myositis (IMM) is an incomplete dominant autoimmune disorder which causes muscular atrophy and stiffness in Quarter Horses. Horses with two copies of the mutation associated with IMM are more likely to develop symptoms than horses with a single copy, although environmental factors can play a role. IMM typically affects horses younger than eight years old and older than seventeen years old. IMM episodes typically last several days to several weeks and can be fatal if mismanaged.
An affected horse’s immune system attacks the horse’s skeletal muscles. This attack causes the muscular atrophy and stiffness seen in horses with IMM. Horses with IMM have a mutated MYH1 gene, which codes for a protein called 2X myosin. An affected horse’s immune system is unable to tolerate the presence of this protein, leading to an attack on the muscles. Certain infections, such as a Streptococcus infection, and certain vaccines, like the influenza vaccine, are thought to potentially trigger symptoms of IMM. After an immune episode, muscle mass typically returns to the horse within a few months with proper care. Symptoms include:
Muscular atrophy in the back and rear
Loss of appetite
Although IMM itself cannot be cured, this disorder can be managed. Corticosteroids are primarily used to help a horse ease off of an autoimmune episode and can be effective as soon as 72 hours after administration. The sooner a horse is treated, the higher the likelihood that the horse survives the episode. Furthermore, recovering horses should be fed a special protein-heavy diet in order to help them regain muscle mass. If the episode is suspected to have been caused by a vaccine, determining which vaccine caused the episode is important so as to properly manage future incidences. Horses with two copies of the mutation are at the highest risk of recurring episodes.
Horses can inherit this mutation from their parents. Horses with one copy of IMM are susceptible to having autoimmune episodes, while horses with two copies of the IMM mutation are more susceptible to an autoimmune episode and the chance of having recurring autoimmune incidences. As IMM is an inheritable disorder, it is important to test horses prior to breeding in order to best manage potential outcomes.
Lethal White Overo
LWO or Frame as it is also called is listed under pattern tests on the Animal Genetics website.
Frame overo is a highly desirable white pattern gene. All frame overo horses carry a single inherited copy of a gene mutation. This mutation causes pigment loss, producing white markings on certain areas of the horse. While the mutation produces visually desirable horses, it is also linked to a fatal condition known as Lethal White Overo or (LWO). Foals born with LWO are almost pure white in appearance and die within the first few days of life. Correct breeding can avoid this occurrence.
LWO is a recessive genetic disease. This means that a horse requires two copies of the mutated gene from both parents in order to be affected by LWO. Horses with only one copy of the mutated gene present the trademarked spots of frame overo and are not affected by LWO. These horses are known as carriers of the disease, however. A horse with two copies of the mutated gene will suffer intestinal abnormalities caused by undeveloped nerves in the foal's digestive system. These animals die within the first 72 hours of being born and are typically euthanized sooner for humane reasons.
When breeding for a frame overo horse, it is extremely important to know the genomes of the horses being bred. This will prevent two carriers from being bred together. When breeding a frame overo horse to a non-overo horse, there is a 50% chance of the foal inheriting the mutation and developing the frame overo pattern. This is the safest way to breed frame overo horses.
If two frame overo horses (two carriers) are bred with one another, there is a 25% chance per foal born that they will inherit LWO, the fatal disease. There is a 50% chance per foal born that they will develop the frame overo pattern. As the chances of producing a frame overo horse are the same when breeding an overo horse to another overo horse as they are when breeding an overo horse to a non-overo horse, it is highly advisable to breed the latter option. Breeding a carrier to a non-carrier will eliminate the chance for the foal to develop LWO while maintaining the same odds of producing an overo horse.