All horses that are to be considered for registration with the AQHA must have DNA markers on file and be parent verifIED
19. GENETIC DISORDERS
19.1 The Association may require, at its sole discretion, scientific testing to determine the incidence of genetic disorders at any time.
19.2 Results
a) The results of any genetic testing, generated in compliance with the Rules & Regulations of the Association, is and remains the property of the Association for its use in its absolute discretion;
b) Test results for all genetic testings will only be accepted from testing facilities deemed appropriate by the Association;
c) Results for all genetic testing will be marked on the horses papers.
19.3 HYPP/MYHM/MH/PSSM1 - Autosomal Dominant Genetic Disorders
Horses carrying one copy (heterozygous) or two copies (homozygous) of the mutation are usually affected by the disease. The severity of the disease can vary. Heterozygous horses carrying one copy of the mutation will pass the disease on, causing mutation to approximately half of their offspring. Therefore, mating to non-carriers will result in approximately half the foals being affected and half being noncarriers. Homozygous horses carrying two copies of the mutation will result in all the foals being affected, even if the mate is not a carrier.
19.3.1 HYPP - Hyperkalemic Periodic Paralysis HYPP is a muscular disease caused by a hereditary genetic disorder that leads to uncontrolled muscle twitching or profound muscle weakness and in severe cases may lead to collapse and/or death. According to research this condition exists in certain descendants of the stallion “Impressive” AmQHA # 076724.
19.3.2 MYHM - Myosin-Heavy Chain Myopathy MYHM can cause two different types of disease. One is muscle weakness and stiffness, followed by a rapid, significant loss of muscle, particularly from the topline of the horse. This is often associated with an infection or vaccination, particularly with Strangles or other respiratory viruses. This syndrome was originally named “immune mediated myositis” or IMM. A mutation associated with IMM was identified by researchers at UC Davis and Michigan State University in the Myosin Heavy Chain 1 (MYH1) gene. MYHM is co-dominant, meaning that the action of the variant is independent of the second variant. If a horse has one copy of the MYHM mutation it can be affected with MYHM. MYHM has ‘incomplete penetrance’, so not every horse carrying this mutation will show the same severity of symptoms. It is thought that if a horse has two copies it is most likely to be more severely affected.
19.3.3 MH - Malignant Hyperthermia MH is a muscle disorder that may only become apparent if the horse is subjected to an extreme stress or exposed to a halogenated anaesthetic. When exposed, the mutation triggers the release of excess calcium in skeletal muscle cells causing a high temperature (hence the name), increased heart rate and blood pressure, sweating and muscle rigidity. MH is frequently fatal. MH sometimes occurs in horses which are also positive for PSSM1, causing them to have more severe tying up symptoms. MH is rare and only found in some Quarter Horses, however because it is potentially fatal it is recommended all possible carriers be tested before undergoing anaesthesia. MH is associated with a mutation in the RyR1 gene and is a dominant trait, meaning a horse only needs 1 copy of the mutation (MH/n) to be affected.
19.3.4 PSSM1 - Polysaccharide Storage Myopanthy 1 PSSM1 causes a build-up of abnormal sugars in muscle. This is one of the causes of tying up with clinical signs that include muscle twitches, stiffness, sweating, reluctance to move and painful cramps. Symptoms can vary widely in severity and age of onset. PSSM1 appears to be quite an old mutation so it is found in many breeds including Quarter Horses and draft breeds. It is a complex disorder that can often be controlled with changes to diet and exercise management. PSSM1 is associated with a mutation in the GYS1 gene and is inherited in a dominant fashion, so a horse only needs to carry one copy (PSSM1/n) to show symptoms. There is some evidence that homozygous horses (PSSM1/PSSM1) are more severely affected than heterozygotes. Note that this test only detects this one specific type of tying up, and horses may still exhibit signs of tying up even if they are not positive for PSSM1.
19.3.5 Autosomal Dominant Traits
Any horse applying for registration:
a) Whose pedigree does not contain “Impressive” lineage will not be required to be tested for HYPP;
b) Where both parents have been tested for HYPP, PSSM1, MH, MYHM and both parents have a negative result on file for that genetic disorder, the horse is not required to be tested for that genetic disorder;
c) That returns a negative result to HYPP, PSSM1, MH, MYHM will be considered for registration;
d) Whose pedigree does have a parent with a positive (heterozygous/carrier) result to PSSM1, MH, MYHM will require testing for that genetic disorder and if the horse returns a positive (heterozygous/carrier) result to PSSM1, MH, MYHM will be considered for registration;
e) That returns a positive (heterozygous/carrier) result to HYPP will be considered for registration with the Association if gelded or spayed. Chemical alternatives to surgical spaying will not be accepted by the Association;
f) That returns a positive (homozygous/afflicted) result to HYPP will not be accepted for registration or recording.
g) That returns a positive (homozygous/afflicted) result to PSSM1,MYHM, MH will be considered for registration with the Association if gelded or spayed. Chemical alternatives to surgical spaying will not be accepted by the Association;
h) Geldings whose parents are both carriers of the same genetic disorder (HYPP/MYHM/MH/PSSM1) will be required to be tested, and if they return a HYPP/HYPP, PSSM1/PSSM1, MH/MH or MYHM/MYHM result will not be eligible for registration.
19.4 HERDA/OLWS/GBED - Autosomal Recessive Genetic Disorders
Horses carrying one copy of the mutation (heterozygous) are usually unaffected by the disease. However, carriers will pass the disease-causing mutation onto approximately half their offspring. Thus, mating to non-carriers will result in approximately half the foals being carriers and half being non-carriers. Mating to other carriers should be avoided as there is a 25% chance the resultant foal will be homozygous affected (afflicted) by the disease. Horses carrying two copies of the mutation (homozygous) are usually affected with the disease.
19.4.1 HERDA - Hereditary Equine Regional Dermal Asthenia. HERDA is a devastating disease that causes the skin to lift and peel away. The condition, which renders a horse unable to wear a saddle or harness, is known by two names: Hyperelastosis Cutis and Hereditary Equine Regional Dermal Asthenia. The reported age of onset ranges from birth to four (4) years old. According to research this condition exists in certain descendants of the mare Miss Taylor, AmQHA #0002636 and stallion, Poco Bueno AmQHA #0003044.
19.4.2 OLWS - Overo Lethal White Syndrome. A genetic disorder in which the caecum, colon and sometimes the rectum undergo a large dilation and fill with faecal mass. Associated with homozygosis of the Overo Lethal White gene it results in incomplete migration of nerve cells to the large intestine during embryonic development. Affected foals may die within seventy-two (72) hours of birth;
19.4.3 GBED - Glycogen branching enzyme deficiency GBED is a lethal storage myopathy caused by a mutation in the GBED gene that prevents the animal from properly storing glucose. The horse will eventually run out of stored energy, which will damage its organs. The symptoms observed are associated with the lack of energy preventing the organs from working correctly, and may include general weakness, failure to thrive, low body temperature, seizures and difficulty rising. GBED is always fatal, with most affected foals dying before the age of 8 weeks. GBED can also cause foetuses to be aborted or born prematurely. It is inherited as a recessive trait, so only homozygous (GBED/GBED) horses are affected. If a horse is a carrier (n/GBED), it will not show any clinical signs of GBED. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.
19.4.4 Autosomal Recessive Traits
Horses applying for registration:
a) That do not carry lineage to Poco Bueno/Miss Taylor will not be required to be tested for that disease;
b) Where both parents have been tested for HERDA/OLWS/GBED and both parents have a negative result on file for that genetic disorder, the horse applying for registration is not required to be tested for that genetic disorder;
c) That are geldings, who are by or out of a HERDA carrier, are not required to be HERDA tested;
d) That are geldings displaying excessive white marking, are not required to be OLWS tested if parent validated to two (2) registered Quarter Horses;
e) That have lineage to Poco Bueno/Miss Taylor must be tested for HERDA;
f) That return a negative result to HERDA/OLWS/GBED may be considered for registration;
g) That return a positive (Heterozygous/Carrier) result to HERDA/OLWS/GBED may be considered for registration;
h) That return a positive (Homozygous/Afflicted) result to HERDA/OLWS/GBED will not be accepted for registration;
i) That are geldings, whose parents are both HERDA carriers will be required to be tested, and if they return a Hr/Hr result will not be eligible for registration.
19.5 Foundation Recorded Horses Genetic Testing
a) All ASB registered horses to be considered for Foundation Recording must return a negative result to HERDA, OLWS, PSSM1, MH, GBED, MYHM;
b) All ASHS registered horses to be considered for Foundation Recording must return a negative result to HYPP, HERDA OLWS, PSSM1, MH, GBED, MYHM;
c) Horses applying for Foundation Recording that display excessive white markings beyond the limits described in Rule 11.4 will not be accepted for Foundation Recording
19.6 Stallions and Mares
a) All breeding stallions and mares with lineage to Poco Bueno/Miss Taylor, and no HERDA results are on file with the Association, will be suspended and no paperwork will be processed for the stallion or mare;
b) All breeding mares who cannot prove that they do not have lineage to Poco Bueno/Miss Taylor and do not have their HERDA results on file will be suspended and no paperwork will be processed for that mare;
c) Once a negative or carrier HERDA result has been received, all rights and privileges for the stallion/mare will be reinstated;
d) All breeding stallions and mares with no PSSM1, MH, GBED & MYHM results on file with the Association, will be suspended and no paperwork will be processed for the stallion or mare. Once results are received for PSSM1, MH, GBED, MYHM, all rights and privileges for the stallion/mare will be reinstated.
19.8 Other Genetic Disorders
a) At the direction of the Association any horse may be required to be tested for any genetic disorder at any time;
b) On notification from the Association, a registered veterinarian will, using an official testing kit, be required to take the specified sample and forward it to the place of testing directed by the Association;
c) The horse’s registration papers must be forwarded to the Association where they will be marked with the official result.
