There are several genetic tests available in Australia that identify carriers of mutated genes that cause disease.
QUARTER HORSES & RELATED BREEDS
HYPP — Hyperkalemic periodic paralysis: This is an inherited disorder that is an autosomal semi-dominant trait. It affects a certain type of sodium pump in muscle cells stopping it from turning off when the levels of potassium in the tissues are high, allowing muscles to continually fire until they become weak and exhausted. This is seen clinically as muscle fasciculations (trembling and shaking), progressing to weakness where the horse may dog-sit or lie down. Sometimes the respiratory muscles are affected, causing difficulties with breathing, respiratory distress and on rare occasions, death. It is seen in Quarter Horses and associated breeds, with horses appearing to be normal between episodes. The disease can be managed by reducing stressful events, dietary changes, and regular exercise. By identifying individuals with this condition, not only can potentially dangerous matings be avoided but owners can be made aware of the status of their horse and so have in place protocols to manage those horses identified as HYPP horses.
HERDA — Hereditary Equine Regional Dermal Asthenia: This is an autosomal recessive trait that affects the skin of horses, predominantly American Quarter Horses and their related breeds, and is thought to be due to a defect with collagen fibres. Clinically the skin becomes hyperextensible, meaning it can be pulled right up away from the body as the skin is not connected properly to the underlying tissues. Horses with this condition become prone to tearing and ulceration of the skin and develop lesions that do not heal well and easily scar. The defect is associated with the structure of collagen fibres and their ability to heal if damaged. It is not usually seen clinically until the horse is around 2 years of age and can correspond in time to when the horse is being broken in and saddled for the first time, as the pressure of the tack on the skin causes large areas of damage. There is no cure, and many horses are euthanised.
MH — Malignant Hyperthermia: This is an autosomal dominant trait that can cause the body to overheat when certain triggers are activated. The common one that is seen with MH-affected horses (and other MH-affected species) is exposure to certain anaesthetics, predominantly some gaseous anaesthetic agents used for horses. The mutated gene enables large amounts of calcium that are stored with the muscle cells to be released into the plasma of the cell, causing muscle fibre contraction. This excessive muscle contraction quickly depletes the cells of energy and generates high amounts of heat, resulting in high body temperatures, lactic acidosis and muscle damage. Clinically, this looks like a horse with a severe episode of exertional rhabdomyolysis (“Tying Up”). It can also be triggered by intense exercise or stress. If an individual is recognised as having this gene, there are precautionary treatments (dantrium) that can be given prior to the horse having to undergo anaesthesia, to reduce the risk of an episode. If the horse does become hyperthermic either due to anaesthesia or intense exercise, it is important to cool them and have the vet treat the horse ASAP.
GBED — Glycogen Branching Enzyme Deficiency: This is an autosomal recessive genetic disease that affects the foal’s ability to store and utilise glycogen in the body and therefore prevents the body maintaining the necessary glucoses levels required in the blood. The body relies on stored glycogen to be converted to glucose so a relatively constant level of glucose can be maintained; without glucose being available to the cells, the body cannot function, and death occurs.
PSSM1 — Type 1 Polysaccharide Storage Myopathy: This is a glycogen storage disease that results in abnormally large amounts of glycogen being stored in the muscle cells. These stores cannot readily be broken down into sugars, which are needed as energy for muscles to function properly. This is seen clinically as a horse that has “tied up” with muscle stiffness, sweating and a reluctance to walk. It is inherited as an autosomal dominant trait, but not all horses will show the full clinical signs, and factors such as diet, exercise and management can decrease the severity of episodes.
LWS — Lethal White Syndrome: This is more common in the paint horse population and is an autosomal recessive condition associated with a lack of neurological development of the intestines. Foals are born as healthy individuals but quickly deteriorate when they begin to suckle. Muscular movements to enable food to pass along through the gut and allow gas and other non-absorbed or undigested material to be passed out of the body do not occur. Foals quickly become bloated and colicky and prognosis for survival is hopeless. As they become so painful quite quickly, foals are usually euthanised within 12 hours of birth. The genetic defect also causes a complete lack of melanocytes in the body, so these foals are born without colour (white) and have blue eyes. Deafness is thought to be associated with this condition; however, this is probably of minor significance, as these foals do not live long enough to make this a clinical issue.
IMM — Immune-Mediated Myositis: This condition differs slightly to many of the other conditions mentioned as it is associated with the presence of a gene but does not follow a recessive/dominant pattern. The horses affected are Quarter Horses and their related breeds and it causes those horses affected to rapidly lose muscle mass (atrophy) symmetrically along their topline and hind-limb muscles after a trigger event such as vaccination for, or infection with Strangles (Streptococcus equi). The muscle atrophy is treatable, although it may take months for the muscle to return to normal and can recur. Some horses may require euthanasia due to the severity of the muscle loss or the prolonged duration of the wastage, causing a poor quality of life. The gene is also seen in Quarter Horses that tend to “tie up” without being exercised (non-exertional rhabdomyolysis).