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| Hyperkalemic Periodic Paralysis (HYPP) |
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Hyperkalemic periodic paralysis, or HYPP, is a condition characterized by muscle tremors and weakness. The problem results from a genetic defect that causes abnormal function of the sodium channel in muscle cells. Sodium and potassium concentrations become unbalanced, leading to abnormalities in the electrical signals that control muscle contractions.
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How does HYPP affect horses?
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Episodes of weakness and trembling may last from a few minutes to several hours, usually resolving spontaneously. The problem may be noticed in muscles of the face, larynx, trunk, or limbs. Severely affected horses may collapse with little warning. Others may show difficulty in chewing or swallowing. In cases where the condition affects the muscles involved in breathing or heart function, horses have died from HYPP.
Foals with HYPP often show general weakness, drooling, difficulty in nursing and swallowing, and noisy breathing as well as muscle tremors and stiffness. Stallions with the condition may have trouble mounting a mare because of muscle tremors and weakness. Large numbers of deformed sperm cells have been noted in the semen of affected stallions.
In a few cases, anesthetized horses with HYPP have developed malignant hyperthermia, a condition involving a dangerous rise in body temperature accompanied by an increased serum potassium level. Not all HYPP horses have exhibited this problem, and although a correlation is suspected by some researchers, attempts to find a definite link between the conditions have been inconclusive.
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Which horses are susceptible?
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HYPP occurs only in horses inheriting a dominant gene from one or both parents. While the condition is most often associated with Quarter Horses, any equine that has the Quarter Horse stallion 'Impressive' in its pedigree has a chance of being affected. The best way to determine whether a particular animal has the HYPP gene is by DNA testing.
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Do all HYPP-positive horses show signs of the disease?
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Heterozygous horses (those with one defective gene and one normal gene) may never exhibit disease signs. Horses that are homozygous for HYPP (two defective genes) almost always develop signs, although there is a wide range of severity, and it is common for these horses to die by the age of seven or eight. Episodes in some horses seem to be triggered by excitement, transport, stress, feeding, or exercise, but no clear pattern applies to every animal.
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Can HYPP be eliminated from the equine population?
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The only way to wipe out HYPP is to refrain from breeding horses that test positive for the disease. Before horses with 'Impressive' bloodlines can be registered with the American Quarter Horse Association they must be tested for HYPP, with results indicated on the registration certificate. This measure allows Quarter Horse breeders to select HYPP-negative stallions and broodmares. Unregistered animals and horses of other breeds are not screened by this rule and owners may be unaware that their horses carry the defect.
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Knowing the risk, why would an owner choose to breed a horse that is HYPP-positive?
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Along with a defective gene, Quarter Horses from the 'Impressive' line inherit heavy muscling that has taken them to numerous championships in halter classes. In 1995 the top ten AQHA halter sires all carried Impressive breeding.
While a homozygous mare or stallion carries two HYPP genes and therefore passes the trait to all offspring, a heterozygous parent has both a normal and an HYPP gene. Therefore only about half the offspring from a heterozygous animal will inherit the problem. If two heterozygous horses are mated, about one in four of the offspring will be normal. An owner might be tempted to breed an affected horse, gambling that the success of unaffected offspring will compensate for any loss from HYPP-positive foals.
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How can affected horses be managed to minimize disease signs?
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Horses with HYPP benefit from management steps aimed at reducing the amount of potassium in the horse’s blood.
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- Daily administration of acetazolamide helps to lower potassium levels through increased urination. The horse should have access to fresh water at all times. A veterinarian should be consulted about dosage. If the horse requires medical treatment or anesthesia for any condition, the veterinarian must be informed as to the horse’s HYPP status.
- Regular light exercise (hand walking or lunging) and pasture turnout are preferable to keeping an affected horse in a stall, as potassium balance is improved by exercise. Anyone working around an HYPP horse needs to pay attention and be ready to move away quickly if the horse develops muscle tremors or weakness. Depending on the severity of the condition, riding affected horses may not be advised because of the danger of collapse.
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How important are dietary changes?
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Adjustment of the feeding program to limit potassium intake is one of the most effective management steps. Although forage is rich in potassium, steps can be taken to minimize potassium intake. Grazing is ideal because the high water content of fresh grass keeps ingested potassium at a low level. Grass or oat hay is preferable to alfalfa hay, although potassium level in any hay can be influenced by forage type, maturity, application of fertilizer, and rainfall. Nutrient analysis can help an owner choose hay with a low potassium content.
The feeding schedule for affected horses should provide two or three grain meals per day to avoid spikes in serum potassium. Soybean meal and molasses contain high levels of potassium and should be avoided. The carbohydrate portion of the diet should be built around some combination of corn, oats, barley, and beet pulp. Keeping dietary potassium no higher than 1% of total intake offers the best chance of minimizing problems for HYPP horses.
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Where can I find out more about genetic testing for HYPP?
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DNA testing is available to test if the horse is carrying the HYPP genes. Contact your vet for further information.
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For further information about your horses nutrition and feeding contact Kentucky Equine Research for free diet analysis and advice on 1800 772 198, or email at equivit@ker.com
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