Inherited Diseases in Quarter Horses
Many livestock breeds suffer from genetic diseases and the Quarter horse is no different in that respect. Luckily, with so many horses registered, better diagnostic tools and DNA testing, genetic diseases can often be traced back to one or a few animals and steps taken to halt the progress of such diseases.
Hyperkalemic periodic paralysis (HYPP) is characterised by uncontrollable muscle contractions. This acts much like isometric exercise, causing the muscles to beef up.
An 'impressive' show sire, Impressive, was a muscle-bound, illustrious individual. He was the epitome of a halter champion and in great demand as a sire.
Unfortunately his extraordinary conformation came from HYPP. Impressive was born in 1969 and before long had passed on the dominant HYPP gene to thousands of his offspring. The spread of the disease was hastened, almost promoted, by breeders striving to emulate the muscled bulk of Impressive. Artificial insemination gave a domino effect to HYPP causing its rapid spread throughout the Quarter horse world. HYPP is a dominant gene so needs only one parent to have the gene for it to be transmitted to the offspring.
The gene was identified in 1992. There is now a DNA for HYPP and this is required by the AQHA. Horses which possess the homozygous form of the gene (H/H) are barred from registration. The registration of horses with the heterozygous (H/N) form is under review. Horses born from 2007 onwards and confirmed as having bloodlines going back to Impressive must carry information about the risks of HYPP with their papers.
The muscle mass so popular in halter classes is linked to HYPP. As the condition becomes less common, it is likely that the halter class style of horse will change. There is now a 'Performance Halter Class' with the prerequisite being a Register of Merit in performance.
Also inherited is Hereditary Equine Regional Dermal Asthenia (HERDA) which is caused by an autosomal recessive gene. The disease is also known as hyperelastosis cutis (HC). As the gene is recessive, the condition only passes to foals if both parents carry the gene. The disease manifests in tearing or splitting of the skin usually along the back. Symptoms do not usually appear until there is pressure or injury to the back, neck or hips. A collagen defect prevents the layers of skin from adhering to each other properly. Collagen acts as a 'glue' binding the skin layers together. The outer layer splits from the under layer or may tear off completely. In extreme cases, the skin may split along the back and peel off down the sides. If healing can be promoted, there are usually disfiguring scars. Most horses with HERDA are euthanized two to four years after onset. With such a horrific outcome, euthanasia is really the only option.
Another genetic disease to affect Quarter horses is Glycogen Branching Enzyme Deficiency (GBED). Ten percent of Quarter horses and related bloodlines may carry this mutation. All GBED foals to date have been either Quarter horses or American paints. Most have cutting horse lines. There is considerable controversy over which bloodline is responsible for the condition.
With GBED, the glycogen branching enzyme (GBE) is lacking in the horse's make-up. GBE is a protein which is essential to build glycogen. Glycogen is a complex sugar and a source of fuel for many body tissues. Without glycogen, body tissue cannot be manufactured. Because the body cannot store glycogen (due to the lack of GBE), the heart and skeletal muscles cannot function and death follows rapidly. If both parents carry one copy of the gene, the offspring will be homozygous for the lethal GBED allele. There may be late-term abortion or foals will be born dead or extremely weak. If born alive, they rarely survive for more than a few weeks. The longest period of survival is eighteen weeks.
Foals may have low body temperature at birth. High respiratory rate and weakness of muscles used in respiration, contracted tendons, overall weakness and inability to rise are other symptoms. It is very common for affected foals to have a low white blood cell count and low blood glucose. GBED is not linked to the sex of the foal.
Ten percent of the Quarter horse population is believed to carry the defective gene. King P-234 is one stallion that has been linked to this condition. There is a blood test available to identify the gene in potential breeding horses.
Equine polysaccharide storage myopathy (EPSM or PPSM) causes a horse to 'tie up'. PPSM is a genetic predisposition to properly digest grains, leading to muscle tissue damage during exertion. This metabolic muscular condition is most commonly associated with draught horse breeds. In America, these are the Clydesdale, Shire, Belgian, Suffolk Punch and Percheron. It is related to a glycogen storage disorder. PPSM has been traced to three specific bloodlines with an autosomal recessive inheritance pattern. Genetic testing is recommended before breeding from a Quarter horse as 6% have the condition at a subclinical level. Some control can be achieved by feeding a specialised low-starch diet. Of Quarter horses which show signs of neuromuscular disease, 48% will have PPSM.
Lethal White Syndrome is also called Overo Lethal White Syndrome (OLWS). For a foal to be affected, both parents must carry the defective gene. The condition is most commonly seen in overo white patterned horses. The Quarter horses most likely to carry the gene are 'cropout' Quarter horses. These horses have too much white to be accepted for registration with the American Quarter Horse Association. Affected foals are completely white or nearly so and have blue eyes. Some time after birth, signs of colic manifest as the foal in unable to defecate. This is because the intestine is underdeveloped and contracted because of a failure of the embryonic cells that form nerves in the gastro-intestinal system. These same cells play a role in determining skin colour. There is no treatment and no successful methods of surgery. Euthanasia is recommended before the foal dies of colic caused by constipation. There is a DNA test for this disease. If horses which carry the lethal gene are never bred to each other, the disease will not occur.