Over 20 years ago, a horse called Impressive was winning all the halter classes. Impressive was well-named – he was so strikingly muscled that he never lost a halter class. It is easy to understand why such a grand specimen would be in huge demand as a breeding stallion. And Impressive showed that he had a very important trait as a breeding stallion – he had the ability to pass on his majestically sculptured musculature to his get. It wasn't for quite some years that people started noticing that Impressive's offspring had inherited more than just his looks.
For the gift of Impressive's massive muscles came with a far less welcome inheritance, which came in the form of intermittent episodes of weakness, collapse, or even death. When veterinarians found that these attacks were accompanied by a high level of potassium in the blood, the inherited disease was named hyperkalemic periodic paralysis (HYPP).
Today, HYPP continues to plague the quarter horse industry, although our ability to identify affected individuals, coupled with responsible action on the part of breeders, has helped us to gain control over this genetic, or inherited disease. We have learned much about this disease, yet it persists in the quarter horse and horses with quarter horse blood, such as Appaloosas, paints and palominos.
Although we have named the HYPP for the characteristically high potassium levels that occur during an attack, HYPP is actually due to a mutation in a gene that governs sodium channels in muscle cells.
Genes carry coded messages to the protein-making apparatus in the body. These proteins eventually become things that are as diverse as muscles, hooves, and microscopic channels in muscle cells. Mutations tend to occur spontaneously in certain individuals – indeed, we all likely will have mutations in our genetic code at some point – but, generally, no outward effect is seen. Occasionally, as with the Impressive mutation, the consequences are enormous.
As far as we know, HYPP is seen only in horses that have inherited the gene for the disease of Impressive's bloodline. That is why we generally see the disease in quarter horses and horses that have quarter horse blood, such as Apaloosas, paints and palominos. As fate would have it, not only did Impressive have a genetic mutation for his muscle sodium channel, but he also had an outwardly very desirable trait – his musculature – that resulted in him being a leading sire. Thus, by the early 1990s, over 50,000 quarter horses had Impressive inheritance. So, Impressive's physique was, in a way, a Trojan horse that allowed a huge and consequential flaw to be transmitted to a large number of horses.
HYPP in the horse is a dominant trait. We all receive two copies of most genes – one from our mother and one from our father. With a dominant trait, the mutation only has to be present on one copy of the gene in order for the trait to be seen (heterozygous). With recessive genes, on the other hand, both genes must have the mutation for the disease to be seen (homozygous).
So, if an HYPP positive horse is bred to an HYPP negative horse, at least 50 percent of their offspring will have HYPP, and up to 100 percent of their offspring will have HYPP, depending if the positive horse has inherited the trait from both parents or only from one. It also seems that homozygous horses are more severely affected than heterozygous horses – it may be that with two copies of the abnormal gene, more of the muscle cells are affected.
What to Watch For
Hyperkalemic periodic paralysis was given its name because the veterinarians initially noted that during attacks, horses had a very high level of potassium in their blood (hyperkalemia). It seemed that horses affected with HYPP had an intrinsic, but intermittent inability to regulate potassium (so it got the name periodic), which sometimes left the horse unable to rise (so it got the name paralysis).
Sodium is found in very high levels outside of cells, in the blood, ordinarily, whereas potassium is found in very high levels inside cells. The uneven distribution of the electrolytes, sodium and potassium, allow an electrical voltage to be built up, and without this voltage, muscles would be unable to contract, and nerves would be unable to transmit vital signals throughout the body. In the normal animal, the movement of these electrolytes is highly regulated, and, indeed, the movement of sodium is important in triggering the movement of potassium in and out of cells. If sodium and potassium levels are disturbed, the body suffers severe consequences, ranging from abnormal muscle and nerve function to death.
Small disturbances in blood potassium, that would easily be handled by normal horses, trigger the abnormal sodium channels in horses with HYPP. When the sodium channels open, and start to leak sodium, potassium is triggered to leak into the blood stream. Soon, this vicious cycle leads to the muscle being first hyperexciteable, that is, it contracts far more easily than normal muscle. Then, as time goes on, the muscle becomes flaccid.
As with any genetic disease, HYPP has different degrees of what is termed penetrance, which is the extent to which a genetic trait can be seen in a particular individual. Some horses will be severely affected, and this will be exacerbated if they are homozygous for the trait, and others may look normal for the majority of their lives. Signs that you may see during an attack of HYPP are:
When a horse is having an attack of HYPP, he usually remains very alert – this helps to distinguish HYPP from a seizure, in which the horse is not aware of who or where he is. A horse having an HYPP attack will have normal muscle enzymes. HYPP attacks can also occur at any time – even when a horse is standing quietly in his stall. After an HYPP attack is over, the horse appears normal and does not show signs of stiffness or muscle pain.
This helps to distinguish HYPP from tying up syndrome, which is characterized by painful, stiff muscles that may last for days to weeks after an episode. The horse often sweats profusely, and appears to be painful. Tying up syndrome is also accompanied by high muscle enzymes, such as CPK, because muscle cells are actually breaking down, and is usually precipitated by exercise.
Heart abnormalities severe enough to cause collapse are usually accompanied by other signs of cardiac failure, such as weakness, abnormal jugular pulses, or peripheral edema – even when an attack is not occurring. These horses will also often have murmurs or cardiac arrhythmias – this is not seen with HYPP unless the horse has more than one problem.
When HYPP was first seen in horses, it was often confused with:
In addition to the clinical signs, if your veterinarian is able to obtain a blood sample at the time of an attack, analysis will show a very high potassium level. Blood potassium levels are usually normal in between. In addition, your veterinarian may recommend the following:
It is important to be able to recognize whether your horse is having a mild attack or a severe attack. A moderate to severe attack requires veterinary care, whereas a mild attack may often be treated at home by the owner. If you are not experienced with this disease, it is wise to call your veterinarian in all cases, until both of you feel comfortable with your ability to make the right decision.
A mild attack would consist of mild muscle trembling, but no signs of recumbency, abnormal whinny, or generalized weakness. In the case of a mild attack, there are several things that seem to help.
In the case of a moderate to severe attack, emergency treatment by a veterinarian is needed. The goal of treatment is to decrease the amount of potassium in the blood stream so that the muscles will regain their ability to contract normally.
Many people keep HYPP-positive horses on acetazolamide all the time. It seems to have little adverse effects on the horse, and is relatively inexpensive. Most owners of HYPP horses report that this reduces the number and severity of attacks.
It is sensible to keep your horse on a low potassium diet. Foods that are very high in potassium include:
Foods that are low in potassium include:
Make sure that your horse has a regular exercise schedule – if you are not going to ride your horse one day, make sure that he has plenty of turn-out. In general, turnout with good shelter from the elements is a better housing choice for these horses than being confined to a stall.
Avoid sudden exposure to extremes of heat or cold. Maintain a regular feeding schedule. High potassium hays such as alfalfa and timothy are more likely to precipitate an attack if they are suddenly introduced into a previously low potassium diet. Feed at least three small grain meals (such as oats, barley, or corn) per day.
The problem with eliminating the HYPP gene is that horses with the gene tend to be highly muscled, with the ideal physique for a show quarter horse. Because this desirable look accompanies the defect, people will still want to breed these horses for show. The only way that the disease will every be entirely eliminated is if all owners have their horses genetically tested before breeding AND all owners agree not to breed horses who are positive for the HYPP gene.