Genetic Disorders

HYPP – Hyperkalemic Periodic Paralysis.

HYPP causes delayed muscle relaxation. Symptoms range from muscle twitches and weakness to severe muscle spasms, paralysis, respiratory noises, collapse and occasionally death, usually from cardio-respiratory effects. HYPP is a dominant trait, meaning a horse only needs 1 copy of the mutation (HYPP/N) to be affected.  There is some evidence that homozygous horse (HYPP/HYPP) are more severely affected than heterozygotes. The severity and onset of symptoms can be managed with diet, particularly by avoiding high potassium feeds. HYPP is associated with Quarter Horses from the “Impressive” line and is caused by a mutation in the SCN4A gene.

HERDA – Hereditary equine regional dermal asthenia

HERDA is a skin disease found primarily in Quarter Horses. It is characterized by hyper-elastic skin which progresses to severe skin lesions, particularly along the back of the horse. The disorder affects the collagen that holds the skin in place, making it much easier to tear than normal. Any rubbing, such as that caused by saddling, will cause the skin to split so affected horses are unable to be ridden. The lesions are painful and prone to infection. HERDA is associated with horses from the Poco Bueno sire line. HERDA is caused by a mutation in the PPIB gene an id recessive, so the horse must be homozygous (HERDA/HERDA) to be affected. If a horse is a carrier (HERDA/n), it will not show any clinical signs of HERDA. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.

OLWS – Overo lethal white foal syndrome

OLWS is autosomal recessive. The white spotting pattern expressed by a heterozygous OLWS horse is incomplete dominant. Found in many breeds, although highest frequency is in Paint breeds.

A single O allele causes the “frame overo” spotting pattern. Expression of white in frame is variable, ranging from lots of white “framed” by the horses’ base colour, to minimal or just a few white hairs. It is possible, although unusual, for a horse with no white to carry Frame.

OLWS syndrome occurs when a horse is homozygous for the O mutation. These O/O foals are born almost or completely white, but do not have properly formed intestinal nerves and cannot pass faeces. They only survive a few days if not euthanized for compassionate reasons. Carrier horses (O/n) have no documented health issues. OLWS is associated with a two base pair change in the EDNRB gene. Because O can be minimally expressed, it is important to test any horse that might be a carrier even if it has little to no white on it, to prevent the birth of an affected foal.

PSSM1 – Polysaccharide Storage Myopathy

PSSM1 causes a build-up of abnormal sugars in muscle. This is one of the causes of tying-up, with clinical signs including muscle twitches, stiffness, sweating, reluctance to move and painful cramps. PSSM1 is found in many breeds including Quarter Horses and draft breeds. It can be controlled with changes to diet and other environmental factors. It is associated with a mutation in the GYS1 gene and is inherited in a dormant fashion, so a horse only needs to carry one copy (PSSM1/n) to be affected. There is some evidence that homozygous horses PSSM1/PSSM1) are more severely affected than heterozygotes. Please note that this test only detects this one specific type of tying up, and horses may still exhibit signs of tying up even if they are not positive for PSSM1.

GBED – Glycogen branching enzyme deficiency

GBED is a lethal storage myopathy caused by a mutation in the GBE gene that prevents the foal from properly storing glucose. This means the horse will not have enough stored energy, which will eventually damage its organs. The symptoms observed are associated with the lack of energy preventing the organs from working correctly and may include general weakness, low body temperature, seizures and difficulty rising. GBED is aways fatal, with most affected foals dying before the age of 8 weeks. GBED can also cause foetuses to be aborted in utero. It occurs in Quarter horses, paints and related breeds and is inherited as a recessive trait, so only homozygous (GBED/GBED) horses are affected. If a horse is a carrier (GBED/n), it will not show any clinical signs of GBED. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.

MH – Malignant Hyperthermia

MH is a muscle disorder that may only become apparent if the horse is subjected to an extreme stress or exposed to a halogenated anesthetic. When exposed, the mutation triggers the release of excess calcium in skeletal muscle cells causing high temperature (hence the name), increased heart rate and blood pressure, sweating and muscle rigidity. MH sometimes occurs in horses which are also positive to PSSM1, causing them to have more severe tying up symptoms. MH is rare and only found in some Quarter horses and paint families; however, because it is potentially fatal it is recommended all possible carrier be tested before undergoing anesthesia. MH is associated with a mutation in the RyR1 gene and is dominant trait, meaning a horse only needs 1 copy of the mutation (MH/N) to be affected.

MYHM/IMM – Myosin-heavy chain myopathy/previously called Immune-Medicated Myositis

Myosin-heavy chain myopathy can cause muscle atrophy (wastage) and stiffness in Quarter horses and related breeds. MYHM may also cause rhabdomyolysis (tying up) that is not associated with exercise. MYHM is an auto-immune disease that is normally triggered by an infection or vaccination. The immune reaction causes an attack on a protein in the horses’ muscles, leading to the muscle wastage. While MYHM is insurable, affected horsed can often be treated and managed to reduce the impact of symptoms. MYHM is codominant, meaning horses with one copy can be affected.  Horses with two copies of the MYHM mutation tend to be more susceptible to episodes and are more likely to have repeat episodes than those with one copy. MYHM is caused by a mutation in the Myosin heavy chain 1 (MYH1) gene.

Source: This information has been extracted from testing documentation provided by Racing Australia for the AAA Ltd.