HYPP in Horses and Muscular Health
- What is HYPP in Horses?
- Genetics and Signs of HYPP in Horses
- Supplements That May Lend Support
- Possible Diagnostic Tests for HYPP
- HYPP Treatment Options and Prevention
- Video on HYPP Genetics, Testing & Preventing Episodes
By: Dr. Lydia Gray | Updated March 11, 2025 by SmartPak Equine
What is HYPP in Horses?
Hyperkalemic periodic paralysis (HYPP) is a genetic disease affecting Quarter Horses (and Quarter Horse crossbreds), American Paint Horses, and Appaloosas from the Impressive bloodline. This disease causes a dysfunction in how the electrolytes sodium and potassium move in and out of muscle cells.
Genetics and Signs of HYPP in Horses
Horses can have one or two genes for HYPP, and thus can show a varying degree of signs, from mild to severe. Mild signs generally include sweating and twitching in the neck, shoulders, and flank, while more severe signs may consist of muscle trembling and cramping, which can cause the horse to sway, sit, or even go down.
The entire event lasts anywhere from 15 to 60 minutes and horses can die during episodes due to cardiac or respiratory arrest. Due to the risk of personal injury, only experienced adults should ride and handle horses with HYPP.
Supplements That May Lend Support
Horses that suffer from HYPP may benefit from antioxidants such as Vitamin E as well as muscle supplements specifically designed to help muscle cells recover and rebuild.
However, it is very important to maintain a low level of potassium in the diet, so check with a veterinarian before adding any supplement, especially electrolyte, multi-vitamin/mineral, and kelp products. Plain salt is an excellent alternative to feeding complete electrolyte supplements, which often contain potassium.
Possible Diagnostic Tests for HYPP
Because an imbalance in the blood levels of sodium and potassium can only be seen during an episode, horses related to the Impressive sire that demonstrate muscle tremors, weakness or collapse should be tested for HYPP. This involves having a DNA test of mane or tail hair performed on a horse that either has Impressive in the pedigree or has shown any of the signs described above.
HYPP Treatment Options and Prevention
Prescription Medications Available
The medication most commonly used to prevent HYPP episodes is the human drug acetazolamide, which is a diuretic that helps filter potassium from the blood. If an owner recognizes that an episode is about to occur, they can gently exercise the horse or try feeding something with sugar in order to ward off the attack.
Veterinarians that are present during an episode may give acetazolamide and other medications intravenously.
Management Suggestions & Things to Avoid Feeding Your Horse
- HYPP horses should not be given alfalfa hay, brome hay, molasses, soybean meal, or any other feedstuffs - such as electrolytes - that are high in potassium.
- Later cuts of timothy or Bermuda grass hay, grass pasture, beet pulp, and grains such as corn, oats, wheat, and barley contain low amounts of potassium and are generally safer to feed.
- Feeding multiple small meals throughout the day so that no one meal exceeds 33 grams of potassium is also advised. A veterinarian can further help you design a nutritional program that is complete and balanced yet contains the minimum amount of potassium necessary.
- A consistent exercise program - along with regular turnout on grass pasture - is also important in preventing bouts of HYPP.
Video on HYPP Genetics, Testing & Preventing Episodes
In this Ask the Vet video, Dr. Lydia Gray explains this muscular condition, signs of HYPP, plus what to feed and not to feed your horse that's been diagnosed.
SARAH: "What is HYPP?" And I also want to give Gabby credit. This user has previously asked a question about what fruits and vegetables are OK to feed your horse, which is one of your favorites.
DR LYDIA GRAY: Oh, no. No, it was not. That was a tough one. That was a tough one.
SARAH: But you got to talk about prunes.
DR LYDIA GRAY: Oh, I did.
SARAH: Yeah.
DR LYDIA GRAY: All right. OK.
SARAH: So there was some upside in there for you.
DR LYDIA GRAY: Pros and cons. HYPP.
SARAH: Yeah.
DR LYDIA GRAY: So what it stands for is hyperkalemic periodic paralysis.
SARAH: Obviously.
DR LYDIA GRAY: We can break down each words. So hyperkalemic-- kalemia refers to potassium, obviously.
SARAH: It's the K from the periodic table.
DR LYDIA GRAY: Right. And so hyperkalemia means excessive potassium in the blood. And then periodic just means occasional, sometimes. And paralysis, that's a little bit of a scary word, but that's what horses do when they have this disease, this genetic disease, is they occasionally have muscle tremors and paralysis and fasciculations. Just trying to give hard words to spell.
SARAH: For Nels to put on the screen.
DR LYDIA GRAY: Yeah, right, right. So it's in Quarter Horses and Quarter Horse crosses, like Appaloosa and Paints, kind of the stock horse breeds. And it all comes from a gene from a horse named Impressive that was born in the '60s. And now the AQHA, the American Quarter Horse Association, requires foals that have Impressive in their pedigree to be tested, and then they can only be registered if they're not double positive for this, which would be called homozygous. If you have one copy of the gene and you're heterozygous, those foals can be registered. Two copies of the gene, you can't. And two copies means you're quite likely to develop this, and it's going to be severe.
This can actually be a fatal disease, but they can-- the severe presentation of this is they don't just have the muscle tremors and the shakes and the sweating, and their third eyelid prolapses, and you'll see it. They can actually go down, and they can stop breathing or their heart can stop.
SARAH: Because those are both muscles.
DR LYDIA GRAY: Right. And what that also means is these horses can be dangerous to be around. So you want to be careful if you know you have one. If you don't know if you have one but you could, because you have a quarter horse and there is Impressive in the pedigree, the testing is super easy. It's done at UC Davis, or University of California - Davis. They have the lab. And all you have to do is send in about 20 to 30 mane or tail hairs, pulled out from the root. So you don't cut them. You pull them out from the root.
And the cool thing is-- I just went there. They have videos that show you how to do this. They also have the test kit that has the forms and an envelope that you lay the hairs in, mail it away. Takes about two weeks-- done. So if there's a chance your horse could have this, you need to know, for safety reasons.
And also, there's medications that your horse can be on to sort of-- they're potassium-clearing, so they get rid of extra potassium. Hay has a lot of potassium in it, alfalfa hay in particular. Other things that have potassium-- because you want your horse's diet to be low in potassium.
SARAH: For these horses.
DR LYDIA GRAY: For HYPP horses. That's sort of the main prevention tool. So molasses, things that have molasses should not be given HYPP horses. You have to be careful with electrolytes. Horses need salt. And so with these guys, I would give plain, pure salt, and not a well-rounded or balanced electrolyte.
SARAH: Because potassium--
DR LYDIA GRAY: It's going to have potassium in it, yeah. Kelp is not a good thing. Soybeans have a lot of potassium, so you want to stay away from those. Good things to have-- and this is surprising and sort of contrary to what we've always said-- grains. Oats, corn, barley--
SARAH: Whole grains.
DR LYDIA GRAY: Whole grains are very low in potassium. So these horses can have grain meals. And the thing that experts recommend is feeding small meals frequently. Because the whole diet should be less than about 1%, 1 and 1/2% potassium, and your vet and your nutritionist or FeedXL can help you with that. But each meal should have no more than about 33 grams of potassium. So one of the ways you do that is to feed small meals frequently.
Pasture-- again, not something-- it's something we recommend, but often with a muzzle, you know, because of sugar problems. But grass pasture for these horses is excellent, because the water content of the pasture and how slowly they meander through a pasture and eat keeps the potassium from climbing sky high.
SARAH: As opposed to hay, which is dry, and they can eat a whole bunch at once.
DR LYDIA GRAY: Almost concentrated, yeah. Yeah, so.
SARAH: OK.
DR LYDIA GRAY: Yeah.
SARAH: It was a big question. I think we got a really well-rounded answer.
DR LYDIA GRAY: A huge question.
SARAH: That's awesome.
DR LYDIA GRAY: If you want to know more about it, go to UC Davis and read. And also, AQHA has some wonderful reading materials and documents you can download about it, because they are the group that knows the most about it.
SARAH: We'll try to include links to those that you referenced in the video descriptions that you guys can get right to them from there. And then of course I know you've answered some HYPP questions, and we have some articles on it in our Horse Health Library, too. So we'll link that, as well.
DR LYDIA GRAY: Yep, we've got resources, too. Yep.