The following information should be used as a GUIDE ONLY and you should consult with your Veterinarian.
The Genetic orders listed below are disorders that can be found in Appaloosas. Not all horses have these disorders and genetic testing is available through the association for our members and their registered horses.
Hyperkalemic periodic paralysis (HYPP) is an inherited disease of the muscle which is caused by a genetic defect. In the muscle of affected horses, a point mutation exists in the sodium channel gene and is passed on to offspring.
Sodium channels are “pores” in the muscle cell membrane which control contraction of the muscle fibers. When the defective sodium channel gene is present, the channel becomes “leaky” and makes the muscle overly excitable and contract involuntarily. The channel becomes “leaky” when potassium levels fluctuate in the blood. This may occur with fasting followed by consumption of a high potassium feed. Hyperkalaemia, which is an excessive amount of potassium in the blood, causes the muscles in the horse to contract more readily than normal. This makes the horse susceptible to sporadic episodes of muscle tremors or paralysis.
This genetic defect has been identified in descendants of the American Quarter Horse sire, Impressive. The original genetic defect causing HYPP was a natural mutation that occurred as part of the evolutionary process. The majority of such mutations, which are constantly occurring, are not compatible with survival. However, the genetic mutation causing HYPP produced a functional, yet altered, sodium ion channel. This gene mutation is not a product of inbreeding. The gene mutation causing HYPP inadvertently became widespread when breeders sought to produce horses with heavy musculature.
To date, confirmed cases of HYPP have been restricted to descendants of this horse.
Homozygous horses are affected more severely than heterozygous horses. Under ideal management practices, the defective gene does not appear to have adverse effects, but stress and/or increased potassium in the serum can trigger clinical signs of muscle dysfunction. Why some horses manifest severe signs of the disease and other exhibit little or no signs is unknown and currently under investigation. Unfortunately, a horse carrying the defective gene but showing minimal signs has the same chance of passing the gene to future generations as does the affected horse with severe signs.
HYPP is characterised by sporadic attacks of muscle tremors (shaking or trembling), weakness and/or collapse. Attacks can also be accompanied by loud breathing noises resulting from paralysis of the muscles of the upper airway. Occasionally, sudden death can occur following a severe paralytic attack, presumably from heart failure or respiratory muscle paralysis.
Attacks of HYPP can take various forms and commonly have been confused with other conditions. Because of the muscle tremors and weakness, HYPP often resembles exertional rhabdomyolysis (“tying-up” syndrome). “Tying-up” syndrome can be caused by many different circumstances, including exercising a horse beyond the capacity to which it has been trained, as well as nutritional deficiencies and metabolic diseases. A distinguishing feature of HYPP from “tying-up” syndrome is that horses usually appear normal following an attack of HYPP. Horses with “tying-up” syndrome, on the other hand, tend to have a stiff gait and painful, firm muscles of the hind limbs, rump and/or back. “Tying-up” syndrome is also generally associated with some type of exercise. HYPP, by contrast, is not usually associated with exercise, but occurs when horses are at rest, at feeding time, or following a stressful event such as transport, feed changes, or concurrent illness.
Because a horse may be down and reluctant or unable to stand during an HYPP attack, many owners have thought their horses were experiencing colic. HYPP has also been confused with seizures due to the pronounced muscle trembling and collapse. Unlike seizures and other conditions that cause fainting, horses with HYPP are conscious and aware of their surroundings during an attack and do not appear to be in pain. Respiratory conditions and choking have also been confused with HYPP because some horses make loud breathing noises during an attack.
Environmental factors can actually cause an attack of muscle weakness. Owners of HYPP-positive horses should be aware that external stimulus and events could increase the chance of paralysis onset. These factors include dietary changes, fasting, general anesthesia, and concurrent illness and exercise restriction.
HYPP is inherited as an autosomal dominant trait, which means it can occur in both males and females and only one copy of the gene is required to produce the disease. The trait is inherited from generation to generation with equal frequency; it does not get “diluted” out or skip generations.
Breeding an affected heterozygous horse (N/H) to an affected heterozygous horse (N/H) will result in approximately 50% carrying the defective gene (N/H), approximately 25% will be normal (N/N) and approximately 25% will be homozygous carriers (H/H).
Breeding an affected heterozygous horse (N/H) to a normal horse (N/N) will result in approximately 50% normal offspring and approximately 50% carrying the defective gene (N/H).
Breeding an affected homozygote (H/H) will result in approximately 50% carrying the defective gene (N/H) and approximately 50% will be homozygous carriers (H/H) regardless of the status of the other parent.
That the disease can be diluted out and not carried to distant generations.
This is false because an affected horse has just as much chance to pass on the trait as the affected parent which passed the gene to him.
The horse will “grow out of it.”
This is not true. For unknown reasons, attacks of HYPP tend to occur most often at the beginning of intense training and fitting for shows (age three to seven years old). It is important to realise that horses with HYPP are affected for life. It is possible that older horses do not experience the same conditioning stresses as young horses or owners have discovered the best management strategies for the older horses with HYPP.
If a horse does not show any signs up to a certain age, it does not carry the trait.
Unfortunately, this is not the case. Once again, horses with HYPP are affected for life. There was a stallion and a broodmare with HYPP who did not show signs of the disease until age eight and 15, and both horses only experienced one isolated attack.
Helpful Website: www.vgl.ucdavis.edu/services/hypp.php
The Australian Appaloosa Association is committed to removing HYPP from our genetics and we have the following rules in place:
Rule 3.6, 1 Hyperkalemic Periodic Paralysis Disease (HYPP)
In addition to the above rules, any Horse, Semen or Embryo imported must be HYPP N/N to be eligible for registration with the Association.
Hereditary Equine Regional Dermal Asthenia (HERDA), also known as Hyperelastosis Cutis (HC), is a rare but painful genetic skin disease. While it is found in Quarter Horses, it can also be found in other breeds, such as Paints and Appaloosas, that have a genetic link to the QH Stallion Poco Bueno.
HERDA is characterised by hyperextensible skin, scarring, and severe lesions along the back of affected horses. Affected foals rarely show symptoms at birth. The condition typically occurs by the age of two, most notably when the horse is first being broke to saddle.
When a horse is affected (Hr/Hr) by HERDA, there is a lack of adhesion within the dermis, the deep layer of the skin, due to a collagen defect. Collagen serves a form of glue that holds the skin layers together. In horses with HERDA, the “glue” is inferior and the skin layers separate.
There is no cure, and the majority of diagnosed horses are euthanized because they are unable to be ridden and are inappropriate for future breeding. HERDA has an autosomal recessive mode of inheritance and affects stallions and mares in equal proportions.
The DNA test for HERDA that has been developed allows identification of horses that are affected or that carry the specific mutation. Other skin conditions can mimic the symptoms of HERDA.
| N/N | Normal – horse does not have the HERDA gene. |
| N/Hr | Carrier – horse carries one copy of the HERDA gene, but is not affected by the disease. |
| Hr/Hr | Affected – horse has two copies of the HERDA gene and will be affected by the disease. |
For breeders, identification of carriers is critical for the selection of mating pairs.
Breeding of Normal horses (N/N) to Carrier horses (N/HR) will result in approximately 50% Normal horses (N/N) and approximately 50% Carrier horses (N/Hr).
Breeding of Carrier horses (N/Hr) to Carrier horses (N/Hr) will result in approximately 25% Normal horses (N/N), approximately 50% Carrier horses r/Hr)H
(N/Hr) and approximately 25% Affected horses (Hr/Hr).
Breeding of Carrier horses (N/Hr) to Affected horses (Hr/Hr) will result in approximately 50% Carrier horses (N/Hr) and approximately 50% Affected horses (Hr/Hr).
Breeding Affected horses (Hr/Hr) to Affected horses (Hr/Hr) will result in 100% Affected horses (Hr/Hr)
Helpful Website: www.vgl.ucdavis.edu/services/herda.php
The Australian Appaloosa Association is committed to the Management of HERDA and we have the following rules in place:
Rule 3.6, 2 Hereditary Equine Regional Dermal Asthenia (HERDA)
As of 1st August 2012 all AAA Ltd Appaloosa Stallions whose pedigree traces to Poco Bueno, or whose pedigree contains unknown breeding are to provide evidence of their HERDA status by means of genetic testing results from facilities deemed appropriate by the AAA Ltd. Prior to the 1st September 2012, the owner is to supply an original or certified copy of the genetic report and declaration confirming that the hair sample was taken from the relevant horse to satisfy the section. As from 1st September 2012, all scientific testing to determine parentage and/or the presence of genetic disorders must be carried out through the AAA Ltd Office. Should the parents of such stallion already haven been tested N/N for HERDA, it is the Stallions Owners responsibility to provide evidence of such results.
Any Stallion that does not have their HERDA Status on file with the AAA Ltd by 1st August 2012 the following suspensions will apply:
Once the requirements as set out in 1 have been met, all rights and privileges for that stallion will be reinstated.
The AAA Ltd, while it records carriers (N/Hr), it condemns the intentional breeding of two (2) carriers, by doing so risk the production of an afflicted Hr/Hr horse.
Any existing horses registered with the AAA Ltd that have tested or do test HERDA N/Hr or Hr/Hr must state this fact in ALL advertising pertaining to the horse.
In addition to the above rules, any Horse, Semen or Embryo imported must be HERDA N/N to be eligible for registration with the Association.
Overo Lethal White Syndrome (OLWS), also called Lethal White Overo (LWO) syndrome, occurs when a horse is homozygous (OO) for the frame overo gene. This genetic disorder causes the intestinal system not to develop properly (involving aganglionosis of the bowel). The foal will die within the first 72 hours after birth when its first meals cannot be digested properly. The lethal white foal will be born almost pure white.
This genetic abnormality is caused by a dinucleotide TC–>AG mutation, which changes isoleucine to lysine of the EDNRB protein.
Horses that do not have OLWS can still be carriers of the OLWS gene. When they are carriers of this gene, they are said to be heterozygous (nO) for the OLWS gene and may pass it on to offspring. The heterozygous OLWS gene in a horse occurs when the diploid (one copy from mother and one from father) of the OLWS gene contains one frame overo copy and one non-frame overo copy and is often referred to as positive for frame overo.
The OLWS gene has been associated with Paints, Thoroughbreds, Quarter Horses as well as Miniature Horses. Identification of frame overo can be definitively done by genetic testing since the frame overo pattern is not always expresses and can be masked by other genetic traits.
A universal method of describing the genetic testing results has been adopted. The letter “O” symbolises the DNA mutation of the lethal white foal syndrome (LWFS) overo gene and “n” for the normal, non-overo allele. Therefore, the lethal white foals or Homozygous positive for the mutation are characterised as OO. Heterozygous horses or overos are identified as nO and Homozygous negative or non-overos as nn.
This chart shows the possibilities of offspring when mating two horses.
| – | nn | nO |
| nO | 50% nn 50% nO | 25% nn 50% nO 25% OO |
nO – one overo/one non-overo – probable overo pattern
nn – both non-overo – no overo pattern
OO – lethal white overo – horse will not survive
Breeders breeding two overo horses (heterozygous nO) can expect a 50% chance of producing an overo foal, a 25% chance of producing a lethal white foal OO and a 25% chance of producing a non-overo foal nn.
Breeders breeding a frame overo horse (heterozygous nO) with a non-overo horse (homozygous negative nn) can expect a 50% chance of producing an overo foal nO 50% chance of producing a non-overo foal nn and no chance of producing a lethal white foal OO.
Helpful website: www.horsetesting.com/lwo.htm
The Australian Appaloosa Association is committed to keeping OLWS out of our genetics and we have the following rules in place:
Rule 3.6, 3 Overo Lethal White Syndrome (OLWS)
In addition to the above rules, any Horse, Semen or Embryo imported must be OLWS N/N to be eligible for registration with the Association.
PSSM is a muscle disease in horses with Quarter Horse bloodlines such as Quarter Horses, American Paint Horses and Appaloosas. PSSM also occurs in many other breeds including Drafts, Draft crossbreeds, and warmbloods. Many of the clinical signs in these breeds differ from those found in Quarter Horses and related breeds.
There are two different types of PSSM found in horses, Type 1 and Type 2. Type 1 is found in over 20 breeds and commonly affects Quarter Horses, Quarter Horse-related breeds, Morgans, some Draft breeds and some warmbloods. Type 2 PSSM is found in Quarter Horses, Arabians, Thoroughbreds and potentially other light breeds. The Draft breeds affected by Type 1 PSSM are Belgians, Percherons and many Continental European Draft breeds. A high percentage of Continental European Draft breeds (62%) were found to carry the mutation responsible for Type 1 PSSM. The mutation that causes Type 1 PSSM is found in very low prevalence in Shires and Clydesdales, which are of British and Scottish origin, possibly indicating a greater genetic difference between these breeds and mainland European breeds (and their descendants). However, Type 1 PSSM is not neatly geographically distributed in the United States or Europe.
Horses with both forms of PSSM have signs typically associated with tying-up. These signs are most commonly muscle stiffness, sweating, and reluctance to move. The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. Horses with PSSM can exhibit symptoms without exercise.
During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year.
Rarely, episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down. The urine in such horses is often coffee coloured, due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse’s kidneys if they become dehydrated.
Very young foals with PSSM occasionally show signs of severe muscle pain and weakness. This occurs more often if they have a concurrent infection such as pneumonia or diarrhoea. Some weanlings and yearlings, particularly those with type 2 PSSM can develop muscle stiffness with daily activities and difficulty rising.
Polysaccharide storage myopathy (PSSM) is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (polysaccharide) in muscle tissue. Thousands of horses have been identified with tying-up associated with polysaccharide accumulation in muscles. There are two forms Type 1 and Type 2 PSSM. We know that both are the result of the accumulation of muscle glycogen which is the storage form of glucose in muscles.
Type 1 PSSM is caused by a mutation in the GYS1 gene. The mutation causing PSSM is a point mutation on the GYS1 gene which codes for the skeletal muscle form of the glycogen synthase enzyme. The cause of Type 2 PSSM has yet to be identified. Both types have an abnormal type of glycogen staining in muscle biopsies, and the types can be distinguished by genetic testing. Horses with Type 2 PSSM lack the mutation that is specific for Type 1 PSSM. At present there is not a specific genetic test for type 2 PSSM and we do not have conclusive evidence that it is inherited.
Carbohydrates that are high in starch, such as sweet feed, corn, wheat, oats, barley, and molasses, appear to exacerbate type 1 and type 2 PSSM. That is why they should be avoided and extra calories can be provided in the form of fat. An important part of the management of PSSM horses is daily exercise. This enhances glucose utilization, and improves energy metabolism in skeletal muscle. If only the diet is changed, we found that approximately 50% of horses improve. If both diet and exercise are altered, then 90% of horses have had no or few episodes of tying-up.
An old theory about tying-up is that it is due to too much lactic acid in the muscle. Many exercise studies have proven that this is absolutely not the case with PSSM. PSSM is actually a glycogen storage disease and there are several diseases in other species and in human beings that also result in the storage of too much glycogen in skeletal muscle. In these other diseases, glycogen accumulates because the muscle lacks an enzyme (protein) necessary to burn glycogen as an energy source. These similarities led us to test PSSM horses for the disorders in glycogen metabolism identified in human beings. We found that PSSM is a unique glycogen storage disease because the PSSM horses have all the necessary enzymes to burn glycogen as a fuel in their muscles. With exercise, PSSM horses show the expected decrease in muscle glycogen as it is burned as fuel.
The unique feature of PSSM is that the muscle cells in PSSM horses remove sugar from the blood stream and transported into their muscle at a faster rate, and make more glycogen than normal horses. Our recent research shows that the reason for this is that PSSM muscles are very sensitive to insulin beginning as early as 6 months of age. Insulin is a hormone released by the pancreas into the bloodstream in response to a carbohydrate meal. It stimulates the muscle to take up sugar from the bloodstream. Once inside the cell the muscles of PSSM horses make much more glycogen than a normal horse due to an overactive enzyme called glycogen synthase in the case of type 1 PSSM.
Currently, PSSM type 1 can be diagnosed with a genetic test, however, at present PSSM type 2 must be diagnosed with a muscle biopsy. Consult with your Veterinarian first.
| P1/P1 | Affected | Positive for dominant PSSM1 gene, indicates the animal carries two inherited copies. Homozygous PSSM horses are genetically bound to pass the gene to 100% of their progeny when bred, and all foals will be PSSM horses. |
| n/P1 | Affected | Both the normal and PSSM1 alleles were detected. Horse tested heterozygous for PSSM1. The horse is affected with the PSSM genetic disorder and there is a 50% chance this horse will pass a PSSM1 allele to its offspring. |
| n/n | Clear | Horse tested negative for PSSM1 and does not carry thePSSM1 gene mutation. The horse will not pass on the defective gene to its offspring. |
Helpful website: www.cvm.umn.edu/umec/lab/pssm/
At this time the Association has no rules pertaining to compulsory testing for PSSM1, however testing for PSSM1 is encouraged for responsible breeding.
Glycogen Branching Enzyme Deficiency (GBED) is a fatal condition caused by the bodies inability to properly store sugar.
In a normal horse, the body stores sugar as energy by converting glucose to glycogen. This inherited disorder prevents the body from producing the enzyme needed to branch the glycogen structure, preventing the horse from being able to adequately store the sugars. This means that the horse will not be able to store enough energy to fuel important organs, such as the muscles and brain.
Foals born affected by GBED suffer from a range of symptoms associated with this lack of fuel, such as low energy, weakness, and difficulty rising. Other symptoms include low body temperature, contracted muscles, seizures, and sudden death. Unfortunately, GBED is always fatal; most affected foals will die before the age of 8 weeks. GBED often causes fetuses to be aborted in utero.
GBED is an autosomal recessive trait, meaning a foal can only be affected if the foal inherits the disease from both parents. Horses that are carriers of the GBED have 1 copy of the mutation, but do not have any symptoms associated with the disorder. This makes DNA testing important to screen for carriers and prevent this fatal condition.
Horses with both forms of PSSM have signs typically associated with tying-up. These signs are most commonly muscle stiffness, sweating, and reluctance to move. The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. Horses with PSSM can exhibit symptoms without exercise.
During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year.
Rarely, episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down. The urine in such horses is often coffee coloured, due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse’s kidneys if they become dehydrated.
Very young foals with PSSM occasionally show signs of severe muscle pain and weakness. This occurs more often if they have a concurrent infection such as pneumonia or diarrhoea. Some weanlings and yearlings, particularly those with type 2 PSSM can develop muscle stiffness with daily activities and difficulty rising.
Until recently, GBED was not recognised in horses because the wide variety of clinical signs resembles many other foal diseases. The signs can be:
GBED is inherited in horses, just as in human beings. GBED is an autosomal (non-sex cell) recessive disease. This means that horses can be carriers and not show signs of the disease, but have affected offspring. Foals with disease receive an abnormal allele (copy) from both the dam and the sire.
For breeders, identification of carriers is critical for the selection of mating pairs.
| Gb/Gb | The horse carries two copies of the GBED mutation and is homozygous for GBED. The horse is affected with the GBED genetic disorder. |
| N/Gb | Both the normal and GBED alleles were detected. Horse tested heterozygous for GBED and is a carrier of GBED. There is a 50% chance this horse will pass a GBED allele to its offspring. |
| N/N | Horse tested negative for GBED. |
Breeding of Normal horses (N/N) to Carrier horses (N/Gb) will result in approximately 50% Normal horses (N/N) and approximately 50% Carrier horses (N/Gb).
Breeding of Carrier horses (N/Gb) to Carrier horses (N/Gb) will result in approximately 25% Normal horses (N/N), approximately 50% Carrier horses r/Hr)H
(N/Gb) and approximately 25% Affected horses (Gb/Gb).
GBED is always fatal in horses that are Gb/Gb.
Helpful Website: www.cvm.umn.edu/umec/lab/gbed/
At this time the Association has no rules pertaining to compulsory testing for GBED, however testing for GBED is encouraged for responsible breeding.
Malignant Hyperthermia (MH) was initially recognised as a fatal syndrome in humans, and the term describing its occurrence in swine (pigs) is known as porcine stress syndrome. MH is most prevalent in swine but this syndrome has also been reported in dogs (especially Greyhounds), cats and horses.
In horses MH is thought to be confined to Quarter Horses and members of related breeds such as Appaloosas and Paints, however less than 1% are affected.
Horses with the MH mutation may not show any physical signs of the disorder until triggered by exposure to anesthesia or extreme exercise or stress. Symptoms can include high temperature, increased heart rate, high blood pressure, sweating, acidosis, and muscle rigidity. Symptoms develop rapidly, and if not treated quickly, this condition can be fatal.
Although this condition is rare, testing for MH is recommended in case a horse must undergo anesthesia. Horses that are known to have the MH mutation can be given medication prior to administering anesthesia to help reduce the severity of the symptoms.
MH is inherited as an autosomal dominant trait, so the disorder can be passed on even if only one parent has the defective gene. The mutation can be present along with PSSM, and if a horse also has PSSM, the symptoms associated with MH can be more severe. Therefore, testing for both PSSM and MH is recommended for Quarter Horse and related breeds.
| MH/MH | Affected: The horse carries two copies of MH and is homozygous for the genetic mutation associated with MH in Quarter horses. |
| n/MH | Affected: Both the normal and MH alleles were detected. Horse tested heterozygous for the genetic mutation associated with MH in Quarter horses. |
| n/n | Horse tested negative for the genetic mutation associated with MH in Quarter horses. |
Breeding of Normal Horses (n/n) to Affected horses (n/MH) will result in approximately 50% normal (n/n) and approximately 50% Affected (n/MH).
Breeding of Affected horses (n/MH) to Affected horses (n/MH) will result in approximately 25% normal (n/n), approximately 50% Affected horses (n/MH) and approximately 25% Affected (MH/MH).
Helpful Website: www.ker.com/library/equinews/
At this time the Association has no rules pertaining to compulsory testing for MH, however testing for MH is encouraged for responsible breeding.