Editing American Quarter Horse (section)

==Genetic diseases==
Several genetic diseases are of concern to Quarter Horse breeders. Most can now be identified by [[DNA]] testing so that breeders do not inadvertently produce foals with these conditions:
*[[Hyperkalemic periodic paralysis (equine)|Hyperkalemic periodic paralysis]] (HYPP), which is caused by an [[autosomal]] [[dominant gene]] originally linked to the stallion [[Impressive (horse)|Impressive]]. It is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis. Because it is a dominant gene,<ref>{{cite web |title=Details on AQHA HYP rules for registration |url=http://www.aqha.com/association/registration/hypp.html |access-date=September 9, 2019 |archive-url=https://web.archive.org/web/20090120043813/http://www.aqha.com/association/registration/hypp.html |archive-date=2009-01-20}}</ref> only one parent has to have the gene for it to be transmitted to offspring. There is a DNA test for HYPP, which is required by the [[AQHA]]. Since 2007, the AQHA has barred registration of horses that possess the homozygous form (H/H) of the gene, and though [[heterozygous]] (H/N) horses are still eligible for registration, altering that status is periodically discussed. Additionally, all Quarter Horses born in 2007 or later that are confirmed to be descendants of Impressive must carry a note about the risks of HYPP on their registration papers. Due to HYPP, there have been some rule changes for show competition, including the creation of a "Performance Halter class" in which a horse must possess a Register of Merit in performance or racing before it can compete.<ref>{{cite web|url=http://services2.aqha.com/iphonedev/www/sections/sectionIV/rules/448.html |title=AQHA Handbook, Section 448 Halter Classes, (j) Performance Halter |access-date=30 September 2012 }}{{dead link|date=July 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
*[[Myosin-heavy chain myopathy]] (MYHM) is a genetic muscle disease added to the AQHA genetic testing panel in 2022.<ref name="MYHM added">{{cite web |title=AQHA Adds MYHM Testing to Genetic Health Panel |url=https://www.aqha.com/-/aqha-adds-myhm-testing-to-genetic-health-panel |website=American Quarter Horse Association |access-date=7 March 2024}}</ref> It is a [[dominant gene|genetic dominant]] condition, though not all horses that inherit the gene will show clinical signs of being affected and the environmental triggers are not well understood at present. An estimated 7% of all Quarter Horses carry this gene. There are two forms, each linked to the same genetic variant. Affected horses may exhibit one or both forms of the condition. The first is Immune-Mediated Myositis (IMM). It may occur in response to a vaccination or infection, following which 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 form of MYHM is Nonexertional Rhabdomyolysis (compare to PSSM, below), which often presents as stiffness and possible swelling of muscles along the back and haunches without exercise. Clinical signs include pain, muscle cramping, and muscle damage, which may or may not result in muscle loss. When the condition is triggered, horses can recover but may have more frequent episodes. Horses that are homozygous (My/My) may have more severe symptoms.<ref name="MYHM">{{cite web |title=Myosin-Heavy Chain Myopathy (MYHM) |url=https://www.aqha.com/myhm |website=American Quarter Horse Association |access-date=7 March 2024}}</ref>
*[[Malignant hyperthermia]] (MH) causes a horse's body to release uncontrolled amounts of calcium into the bloodstream when subjected to certain stressors, resulting in painful muscle cramps, extremely high temperature up to 113 degrees Fahrenheit, irregular heart rhythm, excessive sweating, and shallow breathing.<ref name="MH">{{cite web |title=Malignant Hyperthermia (MH) |url=https://www.aqha.com/web/aqha/mh |website=American Quarter Horse Association |access-date=7 March 2024}}</ref> It manifests when horses receive certain anesthesia drugs or in response to stressors such as overwork or excitement.<ref name="pmid10659313">{{cite journal |vauthors=Valberg SJ, Mickelson JR, Gallant EM, MacLeay JM, Lentz L, de la Corte F |title=Exertional rhabdomyolysis in quarter horses and thoroughbreds: one syndrome, multiple aetiologies |journal=Equine Vet J Suppl |volume=30 |issue= 30|pages=533–8 |year=1999 |pmid=10659313|doi=10.1111/j.2042-3306.1999.tb05279.x |doi-access=free }}</ref> Caused by a mutated [[allele]], ryanodine receptor 1 gene (RyR1) at nucleotide C7360G, generating a R2454G amino acid substitution,<ref>{{cite journal|doi=10.1111/j.1939-1676.2009.0274.x | pmid=19220734 | volume=23 | issue=2 | title=Malignant Hyperthermia Associated with Ryanodine Receptor 1 (C7360G) Mutation in Quarter Horses | year=2009 | journal=Journal of Veterinary Internal Medicine | pages=329–334 | author=Aleman M| doi-access=free }}</ref> it is inherited as an [[autosomal dominant]].<ref>{{cite web |last1=Lenz |first1=Tom R. |title=Heritable Diseases of the American Quarter Horse and Their Management |url=http://manc.umd.edu/Abstracts2010/LenzHYPP%20abstract.pdf |website=Tom R. Lenz |access-date=September 9, 2019 |archive-url=https://web.archive.org/web/20140609024924/http://manc.umd.edu/Abstracts2010/LenzHYPP%20abstract.pdf |archive-date=2014-06-09 }}</ref><ref>{{cite web |title=Malignant hyperthermia: a review |url=https://www.researchgate.net/publication/281816894 |website=ResearchGate |access-date=August 24, 2019 |language=en}}</ref> Horses that carry PSSM or MYHM along with MH have more severe episodes.<ref name="MH"/>
*[[Hereditary Equine Regional Dermal Asthenia]] (HERDA), also known as hyperelastosis cutis (HC). This is caused by an [[autosome|autosomal]] [[recessive gene]], and thus produces affected offspring only when both parents transmit the gene, but may produce unaffected carriers if only one copy is transmitted. Horses affected by this disease have a collagen defect that results in the layers of skin not being held firmly together. Thus, when the horse is ridden under saddle or suffers trauma to the skin, the outer layer of the skin often splits or separates from the deeper layer, or can tear off completely. It rarely heals without disfiguring scars. Sunburn can also be a concern. In dramatic cases, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Most horses with HERDA are euthanized for humane reasons between the age of two and four years. Researchers at [[Cornell University]] and [[Mississippi State University]] have theorized that the sire line of the foundation stallion [[Poco Bueno]] is linked to the disease. In 2007, researchers working independently at Cornell University and at the [[University of California, Davis]] announced that a DNA test for HERDA has been developed. Over 1,500 horses were tested during the development phase of the test, which is now available to the general public through both institutions.<ref>{{cite web |title=HERDA: DNA Tests Available for Disfiguring Skin Disease |url=https://thehorse.com/128263/herda-dna-tests-available-for-disfiguring-skin-disease/ |website=The Horse |access-date=August 24, 2019 |date=May 28, 2007}}</ref> Approximately 3.5% of all Quarter Horses are carriers, as are as many as 28% of horses in [[cutting (sport)|cutting]] and related [[working cow horse]] disciplines.<ref name="HERDA">{{cite web |title=Hereditary Equine Regional Dermal Asthenia (HERDA) |url=https://www.aqha.com/web/aqha/herda |website=American Quarter Horse Association |access-date=7 March 2024}}</ref>
*[[Glycogen Branching Enzyme Deficiency]] (GBED) is a recessive genetic disease in which the horse lacks an enzyme necessary for storing [[glycogen]]. In affected horses the heart and skeletal muscles cannot function, leading to rapid death. The disease manifests in foals who are [[homozygous]] for the lethal GBED allele, meaning both parents carry one copy of the gene. The stallion [[King (horse)|King P-234]] has been linked to this disease. A [[DNA]] blood test is available.<ref>{{cite web |url= http://www.cvm.umn.edu/umec/lab/gbed.html|title= Glycogen Branching Enzyme Deficiency (GBED) in Horses|access-date=2008-06-12 |author1= Valberg, Stephanie |author2=James R Mickelson|website= Glycogen Branching Enzyme Deficiency (GBED)|publisher= University of Minnsesota| archive-url= https://web.archive.org/web/20080512055701/http://www.cvm.umn.edu/umec/lab/gbed.html| archive-date= 12 May 2008 <!--DASHBot-->|url-status = live}}</ref> Roughly 10% of all Quarter Horses carry this gene.<ref name="GBED">{{cite web |title=Glycogen Branching Enzyme Deficiency (GBED) |url=https://www.aqha.com/web/aqha/gbed |website=American QuarterHorse Association |access-date=7 March 2024}}</ref>
*[[Equine polysaccharide storage myopathy]], also called EPSM or PSSM, is a metabolic muscular condition in horses that causes [[Equine Exertional Rhabdomyolysis|tying up]], and is also related to a [[glycogen]] storage disorder.<ref>Valberg et al., "[https://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=10659313&dopt=AbstractPlus Exertional rhabdomyolysis in quarter horses and thoroughbreds]", ''Equine Vet Journal Supplement'', pp. 533–38</ref> There are two forms, PSSM-1 and PSSM-2. PSSM-1 is found in Quarter Horses and has a genetic test available. PSSM-2, which is primarily found in other breeds, has no genetic test available but can be diagnosed with a muscle biopsy.<ref name="PSSM">{{cite web |title=Glycogen Branching Enzyme Deficiency (GBED) |url=https://www.aqha.com/web/aqha/pssm |access-date=7 March 2024}}</ref> PSSM-1 has been traced to three specific but undisclosed bloodlines in Quarter Horses, with an [[autosome|autosomal]] [[recessive]] inheritance pattern.<ref>{{cite web |url= http://www.addl.purdue.edu/newsletters/2000/summer/eer.shtml|title= Equine Exertional Rhabdomyolysis|access-date=2008-06-12 |author = Ulman, Katherine|website= Summer 2000 Newsletter|publisher= Purdue University, Animal Disease Diagnostic Lab| archive-url= https://web.archive.org/web/20080513155218/http://www.addl.purdue.edu/newsletters/2000/summer/eer.shtml| archive-date= 13 May 2008 <!--DASHBot-->|url-status = live}}</ref> 11% of the Quarter Horse population carries PSSM, and 48% of Quarter Horses with symptoms of neuromuscular disease have PSSM. To some extent the disease can be diet controlled with specialized low-starch diets, but [[genetic testing]] is advised before breeding as the condition exists at a subclinical level in approximately 6% of the general Quarter Horse population.<ref>{{cite web |title=Prevalence of PSSM in Quarter Horses |url=https://thehorse.com/130289/prevalence-of-pssm-in-quarter-horses/ |website=The Horse |access-date=August 24, 2019 |date=14 September 2006}}</ref>
*[[Lethal White Syndrome]] (LWS) is fatal when homozygous. Affected foals are born pure white in color with an underdeveloped intestinal tract that prevents them from defecating, thus dying within days if not euthanized first.<ref name="LWS">{{cite web |last1=Young |first1=Amy |title=Lethal White Overo Syndrome (LWO) {{!}} School of Veterinary Medicine |url=https://ceh.vetmed.ucdavis.edu/health-topics/lethal-white-overo-syndrome-lwo |website=ceh.vetmed.ucdavis.edu |language=en |date=9 June 2020}}</ref> Although "[[cropout]]" Quarter Horses with [[pinto horse|pinto]] markings were not allowed to be registered for many years because white markings were thought to be a result of undesirable crossbreeding,<ref name="Paint"/> the gene that causes the condition also creates the [[frame overo]] color pattern when [[heterozygous]], and the color pattern was not always visibly expressed. Thus, the condition has continued to appear periodically in Quarter Horse foals. There is a DNA test for this condition, and in part because DNA testing can verify parentage and because the genetic mechanism of LWS is now understood, AQHA has repealed its cropout rules, allowing horses with white patterns to be registered.<ref name="Paint">{{cite web |title=Flashy Paint Coat Color |url=https://www.aqha.com/-/flashy-paint-coat-color |website=American Quarter Horse Association |access-date=7 March 2024}}</ref><ref name=UDCLWS>{{cite web| url= http://www.vgl.ucdavis.edu/services/coatcolorhorse.php| title= Horse Coat Color Tests| author= University of California – Davis| website= Veterinary Genetics Laboratory| publisher= University of California at Davis| access-date= 2008-03-08| archive-url= https://web.archive.org/web/20080219095454/http://www.vgl.ucdavis.edu/services/coatcolorhorse.php| archive-date= 2008-02-19| url-status= dead}}</ref>
*[[cleft palate|Cleft Palate]]: a birth defect linked to multiple causative factors including genetics, hormones, mineral deficiency, tranquilizers, and steroids. Cleft palates are extremely uncommon, but as most of the research done on the condition has utilized Quarter Horses, the defect is linked to the breed. The surgery to repair a cleft palate has about a 20% success rate. Clinical signs include: lifting head high when eating, dropping head low to drink, coughing when beginning of exercise, and taking an extremely long time to fully administer oral medications placed in the side of the jaw.<ref>{{Cite journal|last=Shaw|first=Sarah|date=2015|title=Clinical characteristics of horses and foals diagnosed with cleft palate in a referral population: 28 cases (1988–2011)|journal=Can Vet J|volume=56|issue=7|pages=756–760|pmid=26130841|pmc=4466833}}</ref><ref>{{Cite journal|last=Kirkham|first=LemcN|date=2002|title=Surgical cleft soft palate repair in a foal|journal=Australian Veterinary Journal|volume=80|issue=3|pages=143–146|doi=10.1111/j.1751-0813.2002.tb11375.x|pmid=12019699}}</ref>