Very little is known in the horse world about 5 known diseases that Quarter Horses, Paints and Appaloosas can have. Many other breeds including heavy horses can also have these diseases and any horse with these breeds in their lines should be tested before breeding.
There is a 5 panel test that is available to test for PSSM1, HYPP, HERDA, MH, GBED and a muscle biopsy to test for PSSM2.
In New Zealand the AQHA-NZ require all registered and classified stallions to be 5 panel tested BEFORE any foals can be registered.
Brief look at genetic diseases in QH related breeds
Known Genes Contributing to Tying-Up in Horses:
Polysaccharide storage myopathy (PSSM). Two forms appear to exist, type 1 and type 2. The mutation for PSSM1 has now been identified but not all forms of PSSM are due to the GYS1 mutation.
Type 1 PSSM
Breeds affected: At least 20 breeds. Quarter Horse-related bloodlines, Gypsy Cob, Belgians, Percherons, Morgans, Mustangs, Tennessee Walkers, and some Warm-blood breeds.
Bloodlines: Widespread in many breeds; in the Quarter Horse it is most common in halter horses.
Prevalence: Up to 80% in some draugh breeds, 36-50% of Belgians and Percheron, 8% of the Quarter Horse-related breeds. Rare in Clydesdale's and Shires. N.B. This mutation has not been found in the Thoroughbred.
Age affected: Signs usually begin by 2 to 3 years of age but may occur in weanlings. May be sub-clinical.
Clinical signs: Firm painful muscles, stiffness, skin twitching, sweating, weakness, and reluctance to move with light exercise. Sometimes gait abnormalities, mild colic, and muscle wasting. Serum CK and AST activity elevated except in drafts.
Inheritance: Autosomal dominant. (Autosomal means that the gene affected is not a sex determining one, the condition is not sex linked). Only one copy of the disease gene are required for symptoms to show.
Mutation: Point mutation that results in an arginine to histidine substitution in the GYS1 gene that codes for the skeletal muscle form of the glycogen synthase enzyme.
Testing: Genetic testing on mane or tail hair roots, or unclotted blood samples at the Neuromuscular Laboratory at the University of Minnesota and Animal Genetics. Muscle biopsy samples evaluated for presence of amylase-resistant crystalline polysaccharide.
The following table describes the number of randomly samples horses that tested positive for the PSSM1 mutation out of the total number of horses tested as well as the % of horses that were positive for the mutation (prevalence).
Number with PSSM1 Prevalence
QH 22/335 6.6%
Paint 15/195 7.7%
Appaloosa 9/152 5.9%
Morgan 2/214 0.9%
Percheron 93/149 62%
Belgian 58/149 39%
Shire 1/195 0.5%
Clydesdale 0/48 0%
Belgian draught 34/37 92%
Trekpaard 17/23 74%
Comtois 70/88 80%
Breton 32/51 63%
Type 2 PSSM (McCue et al., 2009a)
Breeds affected: Quarter Horse-related breeds, a few Arabians and possibly other light breeds.
Age affected: Signs usually begin by 2 to 3 years of age but may occur in weanlings. Some horses are subclinical.
Clinical signs: Rhabdomyolysis with or without exercise.
Inheritance: Suspected genetic link. Search for the mutation is underway.
Mutation: Unknown. We are currently mapping PSSM2 with funding from the AQHA.
Testing: Muscle biopsy samples evaluated for presence of abnormal polysaccharide at the Neuromuscular Laboratory at the University of Minnesota.
Malignant hyperthermia (MH)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Present at a very high frequency in two Quarter Horse bloodlines. Often coexists with PSSM.
Prevalence: <1% of the Quarter Horse breed is affected.
Age affected: Adults.
Clinical signs: High temperature, metabolic failure and death under anesthesia. Exertional rhabdomyolysis especially if present with the GYS1 PSSM1 mutation. (When combined with the PSSM 1 mutation in the same animal, this gene will worsen PSSM 1 symptoms which may then cause an MH attack).
Inheritance: Autosomal dominant. Only one copy of the disease gene are required for symptoms to show.
Mutation: Point mutation that results in an arginine to glycine substitution in the RYR1 gene. (This gene is positioned very close to PSSM type 1 and once found together in an individual any offspring have a far higher likelyhood of inheriting both disease genes).
Testing: Genetic testing at Neuromuscular Diagnostic Laboratory at the University of Minnesota.
Glycogen branching enzyme deficiency (GBED)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Horses descended from Zantanon and King.
Prevalence: 8% of the Quarter Horse breed are carriers.
Age affected: Signs usually present in utero or at birth.
Clinical signs: Abortion or stillbirth, may be born alive and are weak at birth. With supportive care may live to up to 18 weeks of age. Death may be sudden when exercised on pasture, associated with weak respiratory muscles or the result of euthanasia due to persistent recumbency. Treatable flexural deformities of all limbs and recurrent hypoglycemia (low blood sugar) and seizures occur insome affected foals.
Inheritance: Autosomal recessive Two copies of the disease gene are required for symptoms to show.
Mutation: A point mutation in exon 1 changes a tyrosine to a premature stop codon in the glycogen branching enzyme gene (GBE1) that is expressed in numerous tissues.
Testing: Histopathological tissue samples (muscle and heart) stained for periodic acid Schiff’s (PAS) show a variable amount of abnormal PAS positive globular and crystalline intracellular inclusions. Genetic testing is done by Veterinary Genetics Laboratory at the University of California Davis or Vetgen in Michigan on mane or tail hair roots.
Hyperkalemic Periodic Paralysis (HYPP)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Horses descended from the Stallion Impressive.
Prevalence: From 27,000 blood samples presented to the UC-Davis Veterinary Hospital for testing in 1992-1996 taken from horses tracing to ‘Impressive 36% were heterozygous (H/n) 1% were homozygous (H/H).
Age affected: From birth onwards, some have shown no symptoms until in their teens.
Clinical signs: Sporadic attacks of muscle tremors (shaking or trembling), weakness and/or collapse. Attacks can also be accompanied by loud breathing noises resulting from paralysis of the muscles of the upper airway. Occasionally, sudden death can occur following a severe paralytic attack, presumably from heart failure or respiratory muscle paralysis.
Inheritance: Autosomal Dominant. Only one copy of the disease gene are required for symptoms to show.
Mutation: A point mutation in the sodium channel gene.
Testing: Genetic testing is done by Veterinary Genetics Laboratory at the University of California, Davis on mane or tail hair roots.
Hereditary Equine Regional Dermal Asthenia (HERDA)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Horses descended from the Stallion Poco Bueno.
Prevalence: 3.5% carrier frequency.
Age affected: From 2 years of age typically when started under saddle
Clinical signs: Hyperextensible skin, scarring, and severe lesions along the back of affected horses.
Inheritance: Autosomal Recessive. Two copies of the disease gene are required for symptoms to show.
Mutation: A missense mutation in PPIB alters a glycine residue that has been conserved across vertebrates.
Testing: Genetic testing is done by Veterinary Genetics Laboratory at the University of California, Davis on mane or tail hair roots.
Much more is known about genetic disorders in Quarter Horses because the AQHA supports research into genetic diseases and because there are millions more horses of this breed than other equine breeds. Table 1 is modified from Tryon et al. (2009). This table provides estimates of the percentage of horses within performance Quarter Horse types that are affected by known genetic muscle diseases. Note, for example, that 56% of halter horses have genetic susceptibility to HYPP, 28% of halter horses have genetic susceptibility to PSSM, and 26% of western pleasure horses carry GBED. This study also showed that a significant percentage of horses have two or more of these genetic mutations.
Table 1. Observed percentages of horses carrying a disease-causing allele for Quarter Horses (QH) and Paints (APH) as well as for elite competitive subgroups.
Population HYPP (affected) PSSM1 (affected) GBED (carriers)
QH 1.5% 11.3% 11.0%
APH 4.5% 4.5% 3.9%
Halter 56.4% 28.2% 5.1%
Western pleasure 1.1% 8.6% 26.3%
Cutting NO 6.7% 13.6%
Reining NO 4.3% 3.1%
Working cow horse NO 5.7% 9.5%
Barrel racing 1.2% 1.4% 1.2%
Racing NO 2.0% NO
NO = Not observed in the data set.
Conclusions
Genetic disorders are common in horses in part as a result of popular breeding practices. The development of genetic tests provides owners with the ability to make informed choices in their breeding programs. In addition, such tests can be used by veterinarians to diagnose genetic diseases and also to use when performing pre-purchase examinations. The development of new equine genetic tools may well produce a rapid expansion in the number of genetic tests available in the near future.
If looking at breeding or buying in the future please think about asking the right questions, it may save you some heartache.
http://www.cvm.umn.edu/umec/lab/PSSM/
Here is a fantastic website where you can add your tested horse to the data base http://www.bridgequine.com
There is a 5 panel test that is available to test for PSSM1, HYPP, HERDA, MH, GBED and a muscle biopsy to test for PSSM2.
In New Zealand the AQHA-NZ require all registered and classified stallions to be 5 panel tested BEFORE any foals can be registered.
Brief look at genetic diseases in QH related breeds
Known Genes Contributing to Tying-Up in Horses:
Polysaccharide storage myopathy (PSSM). Two forms appear to exist, type 1 and type 2. The mutation for PSSM1 has now been identified but not all forms of PSSM are due to the GYS1 mutation.
Type 1 PSSM
Breeds affected: At least 20 breeds. Quarter Horse-related bloodlines, Gypsy Cob, Belgians, Percherons, Morgans, Mustangs, Tennessee Walkers, and some Warm-blood breeds.
Bloodlines: Widespread in many breeds; in the Quarter Horse it is most common in halter horses.
Prevalence: Up to 80% in some draugh breeds, 36-50% of Belgians and Percheron, 8% of the Quarter Horse-related breeds. Rare in Clydesdale's and Shires. N.B. This mutation has not been found in the Thoroughbred.
Age affected: Signs usually begin by 2 to 3 years of age but may occur in weanlings. May be sub-clinical.
Clinical signs: Firm painful muscles, stiffness, skin twitching, sweating, weakness, and reluctance to move with light exercise. Sometimes gait abnormalities, mild colic, and muscle wasting. Serum CK and AST activity elevated except in drafts.
Inheritance: Autosomal dominant. (Autosomal means that the gene affected is not a sex determining one, the condition is not sex linked). Only one copy of the disease gene are required for symptoms to show.
Mutation: Point mutation that results in an arginine to histidine substitution in the GYS1 gene that codes for the skeletal muscle form of the glycogen synthase enzyme.
Testing: Genetic testing on mane or tail hair roots, or unclotted blood samples at the Neuromuscular Laboratory at the University of Minnesota and Animal Genetics. Muscle biopsy samples evaluated for presence of amylase-resistant crystalline polysaccharide.
The following table describes the number of randomly samples horses that tested positive for the PSSM1 mutation out of the total number of horses tested as well as the % of horses that were positive for the mutation (prevalence).
Number with PSSM1 Prevalence
QH 22/335 6.6%
Paint 15/195 7.7%
Appaloosa 9/152 5.9%
Morgan 2/214 0.9%
Percheron 93/149 62%
Belgian 58/149 39%
Shire 1/195 0.5%
Clydesdale 0/48 0%
Belgian draught 34/37 92%
Trekpaard 17/23 74%
Comtois 70/88 80%
Breton 32/51 63%
Type 2 PSSM (McCue et al., 2009a)
Breeds affected: Quarter Horse-related breeds, a few Arabians and possibly other light breeds.
Age affected: Signs usually begin by 2 to 3 years of age but may occur in weanlings. Some horses are subclinical.
Clinical signs: Rhabdomyolysis with or without exercise.
Inheritance: Suspected genetic link. Search for the mutation is underway.
Mutation: Unknown. We are currently mapping PSSM2 with funding from the AQHA.
Testing: Muscle biopsy samples evaluated for presence of abnormal polysaccharide at the Neuromuscular Laboratory at the University of Minnesota.
Malignant hyperthermia (MH)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Present at a very high frequency in two Quarter Horse bloodlines. Often coexists with PSSM.
Prevalence: <1% of the Quarter Horse breed is affected.
Age affected: Adults.
Clinical signs: High temperature, metabolic failure and death under anesthesia. Exertional rhabdomyolysis especially if present with the GYS1 PSSM1 mutation. (When combined with the PSSM 1 mutation in the same animal, this gene will worsen PSSM 1 symptoms which may then cause an MH attack).
Inheritance: Autosomal dominant. Only one copy of the disease gene are required for symptoms to show.
Mutation: Point mutation that results in an arginine to glycine substitution in the RYR1 gene. (This gene is positioned very close to PSSM type 1 and once found together in an individual any offspring have a far higher likelyhood of inheriting both disease genes).
Testing: Genetic testing at Neuromuscular Diagnostic Laboratory at the University of Minnesota.
Glycogen branching enzyme deficiency (GBED)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Horses descended from Zantanon and King.
Prevalence: 8% of the Quarter Horse breed are carriers.
Age affected: Signs usually present in utero or at birth.
Clinical signs: Abortion or stillbirth, may be born alive and are weak at birth. With supportive care may live to up to 18 weeks of age. Death may be sudden when exercised on pasture, associated with weak respiratory muscles or the result of euthanasia due to persistent recumbency. Treatable flexural deformities of all limbs and recurrent hypoglycemia (low blood sugar) and seizures occur insome affected foals.
Inheritance: Autosomal recessive Two copies of the disease gene are required for symptoms to show.
Mutation: A point mutation in exon 1 changes a tyrosine to a premature stop codon in the glycogen branching enzyme gene (GBE1) that is expressed in numerous tissues.
Testing: Histopathological tissue samples (muscle and heart) stained for periodic acid Schiff’s (PAS) show a variable amount of abnormal PAS positive globular and crystalline intracellular inclusions. Genetic testing is done by Veterinary Genetics Laboratory at the University of California Davis or Vetgen in Michigan on mane or tail hair roots.
Hyperkalemic Periodic Paralysis (HYPP)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Horses descended from the Stallion Impressive.
Prevalence: From 27,000 blood samples presented to the UC-Davis Veterinary Hospital for testing in 1992-1996 taken from horses tracing to ‘Impressive 36% were heterozygous (H/n) 1% were homozygous (H/H).
Age affected: From birth onwards, some have shown no symptoms until in their teens.
Clinical signs: Sporadic attacks of muscle tremors (shaking or trembling), weakness and/or collapse. Attacks can also be accompanied by loud breathing noises resulting from paralysis of the muscles of the upper airway. Occasionally, sudden death can occur following a severe paralytic attack, presumably from heart failure or respiratory muscle paralysis.
Inheritance: Autosomal Dominant. Only one copy of the disease gene are required for symptoms to show.
Mutation: A point mutation in the sodium channel gene.
Testing: Genetic testing is done by Veterinary Genetics Laboratory at the University of California, Davis on mane or tail hair roots.
Hereditary Equine Regional Dermal Asthenia (HERDA)
Breeds affected: Quarter Horse-related bloodlines.
Bloodlines: Horses descended from the Stallion Poco Bueno.
Prevalence: 3.5% carrier frequency.
Age affected: From 2 years of age typically when started under saddle
Clinical signs: Hyperextensible skin, scarring, and severe lesions along the back of affected horses.
Inheritance: Autosomal Recessive. Two copies of the disease gene are required for symptoms to show.
Mutation: A missense mutation in PPIB alters a glycine residue that has been conserved across vertebrates.
Testing: Genetic testing is done by Veterinary Genetics Laboratory at the University of California, Davis on mane or tail hair roots.
Much more is known about genetic disorders in Quarter Horses because the AQHA supports research into genetic diseases and because there are millions more horses of this breed than other equine breeds. Table 1 is modified from Tryon et al. (2009). This table provides estimates of the percentage of horses within performance Quarter Horse types that are affected by known genetic muscle diseases. Note, for example, that 56% of halter horses have genetic susceptibility to HYPP, 28% of halter horses have genetic susceptibility to PSSM, and 26% of western pleasure horses carry GBED. This study also showed that a significant percentage of horses have two or more of these genetic mutations.
Table 1. Observed percentages of horses carrying a disease-causing allele for Quarter Horses (QH) and Paints (APH) as well as for elite competitive subgroups.
Population HYPP (affected) PSSM1 (affected) GBED (carriers)
QH 1.5% 11.3% 11.0%
APH 4.5% 4.5% 3.9%
Halter 56.4% 28.2% 5.1%
Western pleasure 1.1% 8.6% 26.3%
Cutting NO 6.7% 13.6%
Reining NO 4.3% 3.1%
Working cow horse NO 5.7% 9.5%
Barrel racing 1.2% 1.4% 1.2%
Racing NO 2.0% NO
NO = Not observed in the data set.
Conclusions
Genetic disorders are common in horses in part as a result of popular breeding practices. The development of genetic tests provides owners with the ability to make informed choices in their breeding programs. In addition, such tests can be used by veterinarians to diagnose genetic diseases and also to use when performing pre-purchase examinations. The development of new equine genetic tools may well produce a rapid expansion in the number of genetic tests available in the near future.
If looking at breeding or buying in the future please think about asking the right questions, it may save you some heartache.
http://www.cvm.umn.edu/umec/lab/PSSM/
Here is a fantastic website where you can add your tested horse to the data base http://www.bridgequine.com