Position on Genetic Defects

Introduction and Definitions

1. Congenital Defects: Congenital defects include all undesirable traits and pathologic conditions present at birth, whether they are genetic or due to intra-uterine events that result from extra-uterine influences. Congenital defects do not necessarily indicate inheritance but simply indicate that the defect was present at birth.

2. Inherited Tendencies: There are characteristics in horses that are influenced by a wide variety of genes, whose pattern of inheritance is complex and whose expression has strong environmental influences. Horses have been selectively bred for centuries to promote or discourage these characteristics. The selection for or against these inherited tendencies is the basis for our current breed registries. Size, power, color, speed, conformation and many other characteristics that are genetically influenced are selected for or against by certain breed registries. Variations from ideal may be undesirable but they are not deemed to be genetic defects.

3. Genetic Defects: Genetic defects are pathologic conditions of proven genetic origin. These may be the result of a mutation in a gene of major effect or mutations in multiple genes (polygenic) whose effects combine to produce a deleterious or undesirable result. The degree to which some traits are expressed in horses carrying particular mutations can be influenced by environmental factors. This is called incomplete penetrance.

4. Undesirable Traits: An undesirable trait, as designated by certain breed registries, is a condition or behavior which may or may not be present at birth, may develop over time, may or may not be a genetic defect, but precludes registration of that animal. A variation in color is an example of a characteristic that may be considered by a breed to be undesirable. Concealment of such undesirable traits by any means, including surgery, is prohibited by breed registry. It is therefore unethical for a veterinarian to perform such treatments, except when the treatment is intended to improve the health of the horse, and when the veterinarian reports the treatment to the breed registry.

Genetic Tests Available for Horses 

Validated tests for mutations in single genes have been published for these diseases. All tests are commercially available, with exceptions specifically noted below.

Autosomal Dominant

• Hyperkalemic Periodic Paralysis (HYPP) in the Quarter Horse
• Malignant Hyperthermia (MH) in Quarter Horses and related breeds
• Multiple Congenital Ocular Anomalies (MCOA) in multiple breeds (incomplete dominance)
• Myosin heavy-chain myopathy (MYHM) in Quarter Horses and related breeds (autosomal co-dominant with variable penetrance)
• Type 1 Polysaccharide Storage Myopathy (PSSM1) in numerous breeds
• Lethal White Overo (LWO) in the Paint Horse (incomplete dominance: one copy leads to the “frame” overo and two copies leads to lethal white)

Autosomal Recessive

• Cerebellar Abiotrophy (CA) in Arabians horses
• Congenital Myotonia (CM) in New Forest ponies
• Congenital Stationary Night Blindness (CSNB2) in Appaloosa horses
• Equine Familial Isolated Hypoparathyroidism (EFIH) in Thoroughbreds
• Equine Juvenile Spinocerebellar Ataxia (EJSCA) in Quarter Horses and related breeds
• Equine Viral Arteritis Susceptibility Test (CXCL16 haplotype)
• Foal Immunodeficiency Syndrome (FIS) in Dale and Fell ponies
• Fragile Foal Syndrome (FFS) in Warmbloods and Thoroughbreds
• Genetic Disorders of Friesians
  • Distichiasis in Friesians
  • Dwarfism in Friesians
  • Hydrocephalus in Friesians
• Glanzmann’s Thrombasthenia in many breeds (available commercially on a “bleeding panel”)
• Glycogen Branching Enzyme Deficiency (GBED) in Quarter Horses and related breeds
• Hereditary Equine Regional Dermal Asthenia (HERDA) in Quarter Horses and related breeds
• Hoof Wall Separation Disease (HWSD) in Connemara ponies
• Hydrocephalus in Belgians
• Junctional Epidermolysis Bullosa (JEB1) in Belgians
• Junctional Epidermolysis Bullosa (JEB2) in American Saddlebred horses
• Lavender Foal Syndrome (LFS) in Arabian horses
• Naked Foal Syndrome (NFS) in Akhal-Tekes
• Occipitoatlantoaxial Malformation Type 1 (OAAM) in Arabian horses
• Ocular Squamous Cell Carcinoma (SCC) susceptibility in many breeds
• Severe Combined Immunodeficiency (SCID) in Arabian horses
• Skeletal Atavism in Miniature Horses and Shetland ponies

X-linked Recessive

• Androgen Insensitivity Syndrome (AIS) in many breeds

X-linked Dominant

• Incontinentia pigmenti in Quarter Horses and related breeds and Warmbloods (not commercially available)

Y-linked

• XY Sex Reversal in an American Trotter (not commercially available)

New information in equine genetics is being generated very quickly and this list will be updated as it becomes available.

Surgical Correction of Undesirable Traits and Genetic Defects

According to the American Veterinary Medical Association, surgical correction of “genetic defects” for the purposes of concealing the defect is unethical. If surgical correction is undertaken for the purpose of improving the health of the individual, then the animal should not be used for breeding to prevent the perpetuation of the genetic defect. The AAEP recommends genetic testing to make informed breeding decisions. Further, surgical correction of any characteristic specifically named by the breed organization as being prohibited, for the purpose of concealing the characteristic for obtaining registration, would be considered fraudulent and unethical. Such procedures offer no benefit to the horse and are intended only to deceive the breed organization. The AAEP does support surgical correction of conditions that are in the best interest of individual horses.

Identification of Genetic Traits 

AAEP supports the use of genetic testing by veterinarians or breed associations to identify genetic mutations in animals so that owners can make informed decisions about breeding, purchase and specific treatments. Breed associations should be contacted to determine if there are any restrictions on registration of horses with genetic defects. Licensed laboratories should be used for genetic testing.

More information on equine genetic diseases is available at these websites:

• https://www.vgl.ucdavis.edu/dna-tests/horse
• https://www.vdl.umn.edu/
• https://vetsci.ca.uky.edu
• https://equine.ca.uky.edu/news-story/studies-hereditary-traits-horses-using-new-tools 

Revised by AAEP board of directors in 2024.