Selective equine breeding has led to a number of equine medical issues. These side effects are the consequence of breeding for conformation, movement or specific performance skills such as jumping, reining, cutting and roping without understanding the potential for associated health conditions.
It's only within the last few decades that researchers have been able to accurately define genetic disorders specific to certain horse breeds. Quarter horses are known for problems with tying up. Appaloosas, for eye problems. Arabians, for throwing foals with immune deficiencies. While it's common knowledge that certain horse breeds suffer more from certain conditions than others, understanding how to control and alleviate these conditions is very new territory.
Many of the descendants of "Impressive," a renowned Quarter Horse sire, have been found to suffer from HYPP or hyperkalemic periodic paralysis, more commonly known as tying up. In order to discourage breeders from passing on the trait, the AQHA (American Quarter Horse Association,) established mandatory testing in 1996 for all of "Impressive's," offspring. Any foals testing homozygous (a genetic condition where an individual inherits the same alleles for a particular gene from both parents.) for HYPP are not allowed to be registered with the AQHA.
Given that anyone can breed their mare or stallion, many owners don't find out there's a problem until it's too late. You don't have to be a student of equine genetics to buy sperm or offer your stallion's services. Which is why it's critical that you do the due diligence necessary prior to breeding any horse. Understanding your horse's potential for genetic conditions is critical to the health of the horses you breed as well as the overall health of the breed.
Compared to humans, horses suffer far fewer genetic issues. In this article, we will focus on some of the more commonly seen disorders as experienced by specific horse breeds. There are 3 types of genetic disorders common to horses:
1. Simple Genetic Disorders Also known as the, "classic Mendelian genetic disorder," this is where one or both parents pass their issues onto their offspring. The majority of better known equine disorders fall into this category. A good example is SCID or "severe combined immunodeficiency, an issue that is common to Arabian foals. In simple language, this is where the foal inherits the same mutated gene from both parents. The result: a foal with no functioning white cells, whose body cannot react proactively to diseases.
2. Abnormal Chromosome Numbers One of the more common conditions reflecting this kind of genetic issue is “63,X karyotype,” an issue that impacts female horses. A normal female horse has two X chromosomes, however, with "63,X karyotype" mares, a spontaneous mutation causes the filly to have only one X chromosome. While she will appear outwardly normal, her reproductive tract will fail to develop normally.
3. Multiple Gene Abnormalities Having now sequenced the entire equine genome (Figuring out the order of DNA nucleotides, or bases, in a genome, i.e., the order of As, Cs, Gs, and Ts that make up an organism's DNA,) we now know that the equine genome is made up of 2.7 billion DNA base pairs. Horse health issues such as "roaring," (laryngeal neuropathy) and osteochondrosis are thought to be the result of more than one genetic abnormality.
Roaring, also known as laryngeal neuropathy or RLN is common to both Thoroughbred racehorses and Draft horses and is known to impact performance. Earlier research indicates that the taller the horse breed, the higher the likelihood of reproducing foals with RLN. Studies of normal and affected Thoroughbreds have shown that a gene found on ECA3 appears to be a associated with the disease. This gene also is associated with horse height, suggesting a link. (Nature designed horses to be approximately 14 hands or smaller. Going taller has invited a variety of genetic complications.) This means that horses can be selectively bred to discourage the incidence of RLN, but that this strategy could result in slightly shorter horses.
Friesians are prone to a number of breed specific issues including "megaesophagus," (chronic esophagus dilation) dwarfism, aortic rupture and fluid on the brain or "hydrocelphalus." Researchers have also recently discovered that corneal dystrophy is another genetically related issue with Friesians. Breed specific blood counts are currently underway at both the University of Wisconsin and the Fenway Foundation.
German Warmbloods are known to be prone to "guttural pouch tympany." The affliction typically develops subsequent to a bacterial (primarily Streptococcus spp) infection of the horse's upper respiratory tract. Clinical signs may include intermittent nasal discharge, painful swelling in the salivary glands behind the ears, area, stiff head carriage and noisy or labored breathing. Fever, depression, and anorexia may be seen. Diagnosis is determined by endoscopic examination of the guttural pouch. Interestingly, a study involving 373 German Warmbloods appeared to pinpoint a region on ECA3 as the culprit. Additional studies are being carried out to better determine the precise nature of this genetic mutation.
Two of the more significant health problems afflicting Draft Horses are PSSM 1 (Polysaccharide Storage Myopathy and Junctional Epidermolysis Bullosa or JEB. Researchers at the University of Minnesota Equine Center found the incidence of PSSM 1 amongst randomly tested draft horses to be rather high. Type 1 PSSM is an autosomal dominant trait, meaning only one copy of the gene is needed for horses to be affected. Horses with PSSM are unable to properly store glucose, causing muscle related issues such as tying up and in some severe cases, the inability to move.
Junctional Epidermolysis Bullosa is caused by a recessive mutation of ECA5 or ECA8. Foals afflicted with JEB can experience deep skin ulcerations following the slightest trauma, along with optical, dental and hoof abnormalities. Euthanasia may be necessary when foals fail to thrive.
While PSSM 1 affects at least 20 horse breeds, Quarter Horses are among those most commonly impacted with the problem. Muscle soreness, reluctance to engage the hind end, muscle weakness and atrophy are all clinical signs of PSSM 1. It is caused by a dominant gene mutation on GYS1 which is located on ECA10.
The skeletal muscle abnormality, "malignant hyperthermia," is caused by a mutation on ECA10. With malignant hyperthermia, death, a decrease in body pH known as acidosis and significant increases in body temperature can all be triggered by inhalant anesthesia.
A mutation of ECA26 is responsible for glycogen-branching enzyme deficiency, which can preclude overall muscle malfunctioning, include the heart. Miscarriages and stillbirths may be clinical signs of the condition. Foals with the condition may survive birth, only to succumb to weakness, seizures, cardiac and respiratory failure and potentially, death.
A recessive mutation on ECA1, is responsible for HERDA (hereditary equine regional dermal asthenia.) Easily tented or hyper-elastic skin is a sign that the horse may be affected with the condition. Other symptoms include: easily developed seromas (the development of fluid filled pockets just under the skin,) open wounds that are slow to heal, white hair re-growth in healed areas, sloughed skin and scarring.
An underlying recessive mutation found on ECA17 is thought to be responsible for OLWS or Overo lethal white syndrome. It is commonly found to impact the newborn foals of a variety of paint horse types including: frame breeding stock, sabino, tobiano, white calico and sometimes even solid-frame breeding stock. Sadly, afflicted foals are primarily white and tend to ultimately colic and then pass away.
The best way to curtail the risk of genetic diseases with the horses you bring into this world is by testing the horses you plan to breed. An extensive number of tests are available on the market and by consulting with your veterinarian, you'll be advised on the best tests to run.
As an ethical horse person, your goal should be to eliminate potential carriers of genetically related equine diseases from the breeding pool. By doing so, you are doing the individual horses you breed, as well as the entire breed your horse belongs to, a tremendous good deed. Artificial selection by animal breeders, done to enhance desirable traits, has exacerbated unattractive trait issues in animals.
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