Q: I have the opportunity to adopt a mare with HYPP. I would use her as a trail horse. Do you have any suggestions on crafting her diet to keep her healthy and relatively symptom-free?
A: Hyperkalemic periodic paralysis (HYPP) exists in horses descended from the Quarter Horse stallion Impressive. About 4% of Quarter Horses are thought to be affected. The condition is the result of a single point mutation in the sodium channel gene which causes the channel to become “leaky” when blood potassium levels fluctuate. Therefore, management centers around the horse’s diet in the hopes of maintaining a steady blood plasma potassium concentrations.
In affected horses high potassium levels cause sodium to leak in to muscle tissue through the defective channels. This causes muscle spasms and other signs, including muscle stiffness, third eyelid prolapse, involuntary recumbency (inability to rise), and sometimes death. When cellular sodium levels rise, cells push out potassium in an attempt to maintain the correct concentration gradient between the outside and the inside of the cell. Unfortunately, this causes the potassium concentration outside the cell to rise further resulting in a vicious cycle. The kidneys work to excrete the excess potassium, but if they’re unable to the horse’s heart rate slows and death is possible.
Horses with HYPP can be homozygotes for the gene (H/H, meaning they received the faulty sodium gene from both parents) or heterozygotes (H/N, meaning they received the gene from only one parent). Homozygotes tend to display more severe clinical signs than heterozygotes. Horse age, gender, and muscle mass seem be poor predictors for the likelihood of a horse displaying signs of HYPP.
Research conducted at Texas A&M University showed that the dietary potassium level impacts the likelihood of the horse displaying clinical signs, so this is another important consideration. Horses that were N/N (negative for HYPP) had no clinical signs regardless of dietary potassium levels. On the other hand, H/H horses had no signs when fed a diet containing 1.1% potassium, but exhibited clinical signs 52% of the time when fed a diet containing 1.9% potassium and 67% of the time with 1.9% potassium.
Therefore, research results suggest HYPP horses should be fed diets with 1.1% of potassium or less and owners and managers should try to maintain a steady plasma potassium concentration by feeding multiple small meals rather than infrequent large meals. As such, slow feeders for hay can be a very useful management tool for these horses.
Creating a diet with no more than 1.1% potassium is more challenging, however. This is because many hays and forages contain high potassium. It is not unusual for a hay based diet to provide in excess of 10 times the daily potassium requirement even when intakes are restricted to 1.5% of body weight.
Ideally, HYPP horses should consume forages that have been tested to ensure low potassium levels. However, this is not always practical. Some general guidelines are to avoid feeding legumes such as alfalfa, as these tend to be high—often over 2% potassium. Grass hays can still be high but average less than 2%. As such, it is often necessary to substitute some of an HYPP horse’s forage with a hay stretcher that is guaranteed to have lower potassium levels. There are a number of such feeds available on the market. Otherwise unmolassed beet pulp averages about 0.7% potassium, but rarely comes with a guarantee. There are a number of automatic pellet dispensers on the market than can be used to feed pelleted feeds to again insure consumption occurs throughout the day rather than in large meals. Access to pasture and ability to move might also be beneficial, but, again, it is important to know the pasture’s potassium content.
Peak plasma potassium concentrations seem to occur between 2 to 5 hours after a meal. To avoid triggering clinical signs, avoid working horses during this time period, especially if they are fed large meals.
Further, research from Texas A&M suggests that if you are meal feeding, aim to keep total potassium per meal at or below 33 grams. At this level horses with HYPP remained asymptomatic; however, when meal potassium levels increased to 58 grams per meal, clinical signs occurred 52% of the time. If feeding a grass hay with 1.8% potassium this would mean feeding no more than 4 pounds (1.8 kilograms) per meal. For a 1,200-pound horse being fed 2% of body weight as hay per day this would equate to feeding six meals of four pounds each per day.
One interesting observation from the research studies was that over a period of being fed the high-potassium diets (1.9% and 2.9% potassium) for two weeks, horses adapted and both plasma potassium levels and signs of HYPP decreased. This suggests that horses can adapt to diets with higher than ideal potassium levels. It also means that care needs to be taken when diet changes are made. Suddenly switching a horse to a higher potassium diet could trigger clinical signs until adaptation has occurred. Every effort should, therefore, be made to avoid diet changes. However when changes are unavoidable, for example new batches of hay, changes should be made very slowly in order to avoid triggering clinical signs.
In conclusion, HYPP horses can be managed and lead healthy, active lives, but they require extremely detailed management and they will not be a good fit for some barn environments that are unable to meet their needs. Take care to read feed labels, test hay, and create a management plan with your veterinarian. An equine nutritionist can help you find suitable products to stretch forage and can create a suitable feeding plan. Remember: There will always be the risk of your horse developing signs of HYPP, so create a plan with your veterinarian for how to deal with them when they occur.