Medical Index
Genetics Index

When one thinks of genetic disorders an entire host of fear and concern bubbles to the surface. Genetics is a complicated business that is continually being researched in the scientific community. There really is no way to make the understanding of genetics simple--because it isn't!

The following are areas where congenital anomalies and inherited disorders might occur in the horse:

Below is information that will hopefully sort out some of the confusion when it comes to your horse and genetic abnormalities. What constitutes "genetic abnormalities?" How are they defined? Think of genetic abnormalities coming under the umbrella of two separate main categories: Chromosome Abnormalities and Inherited Disease.

Chromosome Abnormalities: "A chromosome anomaly, abnormality, aberration, or mutation is a missing, extra, or irregular portion of chromosomal DNA. It can be from an atypical number of chromosomes or a structural abnormality in one or more chromosomes. A karyotype refers to a full set of chromosomes from an individual that can be compared to a "normal" karyotype for the species via genetic testing. A chromosome anomaly may be detected or confirmed in this manner. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. There are many types of chromosome anomalies. They can be organized into two basic groups, numerical and structural anomalies." W

Disorders in sex development (DSD) include chromosome and DNA mutations. They include variations in chromosome numbers and/or structure. This link provides an excellent overview: Equine Genetic Disorders Reviewed

Inherited Disease

Inherited disorders are passed to the offspring from a parent. Some of these disorders can be breed based. Inherited diseases-- fatal or disabling genetic conditions--exist in many breeds including the Arabian Horse, American Quarter Horse, American Paint Horse, American Saddlebred, Appaloosa, Miniature horse, and Belgian. Below are some examples of "inherited disorders":

American Saddlebred

  1. Junctional epidermolysis bullosa (or JEB), a condition found in American Saddlebred and Belgian foals that causes skin lesions over pressure points of the body in newborn foals and results in large areas of skin loss. The disease is a lethal condition, and affected foals typically are euthanized shortly after birth.
  2. Lordosis (sway back) in the American Saddlebred
  3. Congenital cataracts in American Saddlebred

Arabian Horses

There are six known genetic diseases in Arabian horses, two are inevitably fatal, two are not always fatal but usually result in euthanasia of the affected animal, the remaining conditions can be treated. Three are thought to be autosomal recessive conditions, which means that the flawed gene is not sex-linked and has to come from both parents for an affected foal to be born. The others currently lack sufficient research data to determine the precise mode of inheritance.

  1. Vitiligo aka Leukoderma or Birdcatcher Spots, Arabian Fading Syndrome, Pinky Syndrome and Lavender Foal is a fatal, neurologic disease found in Arabian horses worldwide. Common clinical signs in affected foals include seizures; opisthotonos (severe hyperextension of the head, neck, and spinal cord); stiff, paddling leg movements; nystagmus (involuntary movement of the eyeballs); and, often, a soft lavender color to the foals' coats.
  2. Severe Combined Immunodeficiency (SCID) - Similar to the "bubble boy" condition in humans, an affected foal is born with no immune system, and thus generally dies of an opportunistic infection, usually before the age of five months. There is a DNA test that can detect healthy horses who are carriers of the gene causing SCID, thus testing and careful, planned matings can now eliminate the possibility of an affected foal ever being born.
  3. Cerebellar Abiotrophy (CA or CCA) - An affected foal is usually born without symptoms, but at some point, usually after six weeks of age, develops severe incoordination, a head tremor, wide-legged stance and other symptoms related to the death of the purkinje cells in the cerebellum. Such foals are frequently diagnosed only after they have crashed into a fence or fallen over backwards, and often are misdiagnosed as a head injury caused by an accident. Severity varies, with some foals having fast onset of severe coordination problems, others showing milder symptoms. Mildly affected horses can live a full lifespan, but most are euthanized before adulthood because they are so accident-prone as to be dangerous. Though clinical signs are distinguishable from other neurological conditions, the only way to verify a diagnosis of CA is to examine the brain after euthanasia. An indirect prediction analysis that identifies DNA markers associated with CA is available.
  4. Lavender Foal Syndrome (LFS) - Also called Coat Color Dilution Lethal (CCDL). The condition gets it name because most affected foals are born with a coat color dilution that lightens the tips of the coat hairs, or even the entire hair shaft. Foals with LFS are unable to stand at birth, often have seizures, and are usually euthanized within a few days of birth. There is currently no genetic test for LFS.
  5. Occipital Atlanto-Axial Malformation (OAAM) - This is a condition where the cervical vertebrae fuse together in the neck and at the base of the skull. Symptoms range from mild incoordination to the paralysis of both front and rear legs. Some affected foals cannot stand to nurse, in others the symptoms may not be seen for several weeks. This is the only cervical spinal cord disease seen in horses less than 1 month of age, and a radiograph can diagnose the condition. There is no genetic test for OAAM, and the hereditary component of this condition is not well researched at present.
  6. Equine Juvenile Epilepsy - Sometimes referred to as "benign" epilepsy or "idiopathic" epilepsy, is not usually fatal. Foals are born normal and appear normal between epileptic seizures, usually outgrowing the condition between 12 and 18 months. Affected foals may show signs of epilepsy anywhere from two days to six months from birth. Symptoms of the condition can be treated with traditional anti-seizure medications, which may reduce the severity of symptoms. Though the condition has been studied since 1985 at the University of California, Davis, the genetic mode of inheritance is unclear, though the cases studied were all of one general bloodline group. Some researchers have suggested that epilepsy may be linked in some fashion to Lavender Foal Syndrome due to the fact that it occurs in similar bloodlines and some horses have produced foals with both conditions.
  7. Guttural Pouch Tympany (GPT) - Occurs in horses ranging from birth to 1 year of age and is more common in fillies than in colts. It is thought to be genetic in Arabians, possibly polygenic in inheritance, but more study is needed. Foals are born with a defect that causes the pharyngeal opening of the Eustachian tube to act like a one-way valve. Air can get in, but it cannot get out. The affected guttural pouch is distended with air and forms a characteristic nonpainful swelling. Breathing is noisy in severely affected animals. Diagnosis is based on clinical signs and radiographic examination of the skull. Medical management with NSAID and antimicrobial therapy can treat upper respiratory tract inflammation. Surgical intervention is needed to correct the malformation of the guttural pouch opening to provides a route for air in the abnormal guttural pouch to pass to the normal side and be expelled into the pharynx. Foals that are successfully treated may grow up to have fully useful lives.

Quarter Horses

  1. Hyperkalemic periodic paralysis (HYPP), which is caused by an autosomal dominant gene linked to the stallion, Impressive. It is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis among affected horses. Because it is a dominant gene, 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 bars registration of horses who possess the homozygous form (H/H) of the gene, and though heterozygous (H/N) horses, is still eligible for registration, altering that status is currently being discussed. Additionally all Quarter Horses born 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, the halter classes are undergoing significant changes. Halter classes are dominated by the Impressive bloodline. Impressive, a very prolific halter horse, brought to the stock breeds the muscle mass that is popular in halter competition today. This muscle mass is linked to HYPP, and as the condition is reduced within the breed, the style of horse in halter classes is also likely to change. Already there have been rule changes, including the creation of a "Performance Halter class" in which a horse must possess a Register of Merit in performance before it can compete.
  2. Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC). This is caused by an autosomal recessive gene, and thus, unlike HYPP, HERDA can only be transmitted if both parents carry the gene. When a horse has this disease, there is 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 often splits or separates from the deeper layer, or it 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. The very hotly debated and controversial theory, put forth by researchers at Cornell University and Mississippi State University is that the sire line of the great foundation stallion Poco Bueno is implicated as the origin of the disease. As of May 9, 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.
  3. Glycogen Branching Enzyme Deficiency (GBED) is a genetic disease where the horse is lacking an enzyme necessary for storing glycogen, the horse's heart muscle and skeletal muscles cannot function, leading to rapid death. The disease occurs in foals who are homozygous for the lethal GBED allele, meaning both parents carry one copy of the gene. The stallion King P-234 has been linked to this disease. There is a DNA blood test for this gene.
  4. Equine polysaccharide storage myopathy, also called EPSM or PSSM, is a metabolic muscular condition in horses that causes tying up, and is also related to a glycogen storage disorder. While also seen in some draft horse breeds, PSSM has been traced to three specific but undisclosed bloodlines in Quarter Horses, with an autosomal recessive inheritance pattern. 48% of Quarter Horses with symptoms of neuromuscular disease have PSSM. To some extent it 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.
  5. Lethal White Syndrome. Although "cropout" Quarter Horses with Paint markings were not allowed to be registered for many years, the gene for such markings is a recessive and continued to periodically appear in Quarter Horse foals. Thus, it is believed that some Quarter Horses may carry the gene for Lethal White Syndrome. There is a DNA test for this condition.

Other Genetic Problems

  1. Dwarfism in Miniature Horses
  2. Contracted foal syndrome in Thoroughbreds
  3. Parrot mouth in Thoroughbreds and other breeds
  4. Wobbler SyndromegGenetic influences are still under investigation. Research has shown breeding wobbler horses to other wobbler horses resulted in an increase in limb OCD occurrence, but this effort never reproduced a wobbler. This disease can affect horses of all ages, but if the horse is older than 10 years, he likely has an acquired ostearthritic problem. Breed distribution of wobbler-affected horses is as follows: 37% Thoroughbred, 25% Warmblood, 16% Quarter Horse, 21% other (including a fair number of Tennessee Walkers), and 1% pony."
  5. Shivers - The larger breeds of horse such as Belgian draft horses and other draft horses, warmblood, warmblood crosses and the lighter breed of horse including light harness horses, standardbreds, hunters, hunter-jumpers, hacks, quarter horses, and thoroughbreds have presented with shivers. The pony breeds generally do not develop shivers. There is some suspicion that there is a genetic component to shivers--that horses may be predisposed to it. This, however, has not been scientifically proven, as yet.
  6. Enteroliths - Genetics may play a role in enterolith formation. A recent study showed that around 8% of horses with enteroliths had siblings affected. Also, it is known that the Arabian is the most commonly affected breed with a study showing that Arabians and Arabian crosses. Other affected breeds are quarter horses, thoroughbreds, appaloosas and miniature horses. Warmblood breeds seem to be affected less commonly than other breeds. Researchers at UC, Davis have identified three candidate genes that may be associated with enterolithiasis and this research may provide the foundation for discovering the genetic basis of the disease. If gene mutations can be identified, diagnostic screening methods may be developed to allow early identification of horses at high risk for enterolith formation.
  7. Chimera - This condition is either inherited, or it is acquired through the infusion of allogeneic hematopoietic cells during transplantation or transfusion. In nonidentical twins, chimerism occurs by means of blood-vessel anastomoses. The likelihood of offspring being a chimera is increased if it is created via in vitro fertilization. Chimeras can often breed, but the fertility and type of offspring depends on which cell line gave rise to the ovaries or testes; varying degrees of intersexuality may result if one set of cells is genetically female and another genetically male.
  8. Leg Ailments - Many leg ailments may be the outcome of genetic predisposition.
  9. Tying-up, Azoturia, Monday Morning Disease, Exertional Rhadomyolysis - Valberg and other researchers are unraveling the genetic basis for tying up. As equine genome mapping techniques progress, DNA-based tests are increasingly becoming part of the diagnostic approach to muscle disease in horses. Four DNA-based tests for muscle disorders in horses are available. Individual tests identify hyperkalemic periodic paralysis (HYPP), glycogen branching enzyme deficiency (GBED), malignant hyperthermia (MH), and type 1 polysaccharide storage myopathy (PSSM1). DNA diagnosis uses hair roots or blood samples, and provides a less invasive and more accurate diagnosis than histological interpretation of muscle biopsy.
  10. Equine Metabolic Syndrome (EMS): There can be a genetic predisposition for EMS. Can be any breed; however, pony breeds, domesticated Spanish mustangs, Peruvian Pasos, Paso Finos, Andalusians, European Warmbloods, American Saddlebreds, Morgans, and American Quarter Horses are more prone to developing EMS than others.
  11. "Degenerative suspensory ligament desmitis (DSLD)There is thought to be a genetic component in this debilitating disorder thought to be limited to suspensory ligaments of Peruvian Pasos, Peruvian Paso crosses, Arabians, American Saddlebreds, American Quarter Horses, Thoroughbreds, and some European breeds. It frequently leads to persistent, incurable lameness and need to euthanize affected horses. The pathogenesis remains unclear, though the disease appears to run in families. Treatment and prevention are empirical and supportive, and not effective in halting the progression of the disease. Presently, the presumptive diagnosis of DSLD is obtained from patient signalment and history, clinical examination, and ultrasonographic examination of clinically affected horses, and is confirmed at post mortem examination. Presently, there are no reliable methods of diagnosing DSLD in asymptomatic horses. The goal of this study was to characterize and define the disorder in terms of tissue involvement at the macroscopic and microscopic levels." Veterinary Research on DSLD
  12. This is certainly not a complete list of genetic problems in horses. I have always had my own theories when it comes to diseases and physical problems in horses and any living thing for that matter. We are what is passed down to us through our ancestors. The result might show up immediately or down the road. We have predispositions in our make up. That isn't to say that nutrition, environment and care are not a factor in this equation--they are. To me it is a "no brainer", however, research and science find imperical proof for the "whys?"

    For More Information:

    Horse Congenital and Inherited Problems
    Ontario Ministry of Affairs
First posted: , 2014
Last update: Feb 23, 2014