Horse Muscle Disease


The everyday activity of our equine requires muscle strength. Whether your horse is part of the 50% of the 7.2 million horses in the US that are obligate athletes used for racing and showing (American Horse Council 2017) or are part of the agricultural workforce, muscle strength plays a vital role in our horses’ task and health. So it cannot be denied that when our horse experiences health problems such as muscle diseases, it affects their ability to train, race, and do the work.

When horse muscle disease happens the consequences do not only lie on the horse but it extends to us horse owners as we possibly encounter problems in the harvest of the product or to some extent it will affect the larger athletic community when horses are not up and running at peak performance.

So for this blog, we shall be talking about some causes of horse muscle disease.

Then we are going to tackle the most common which is the genetic muscle diseases and some signs on how we can tell that our horse has a muscle disorder.

Of course, through it all, we will not be forgetting about the management factors that we can do to take care of our horse suffering from a horse muscle disease disorder.



Causes of Horse Muscle Disease

  1. Nutrition

    So, let’s start with nutritional reasons. Certainly, if a horse is undernourished or is not receiving enough food or calories. They’re going to use their body’s muscle stores essentially to keep life supported, (Hiney, 2018). So not enough calories will result in some muscle weakness, disorders, and horse muscle disease.

  2. Intestinal issues

    Some horses may have some malabsorption issues. That means that the intestinal tract is not working correctly anymore. And when horses are not absorbing nutrients out of their intestinal tract, despite their presence in the diet, it could be another reason that we would have a nutritional issue.

  3. Proteins

    When we talk about muscle, obviously we think about protein! And while our horse nutritionist may get down on the details, citing digestion and absorption of forage versus concentrated sources, most horse owners would just look at the amount of crude protein in the horse’s diet. However, we must know that it is not simply about the percentage of protein but also about the types of protein intake.

    Specifically, there are amino acids that are key players in muscle metabolism. We have Lysine, which is always the first limiting amino acid, Threonine, and Methionine, but also some other important ones are branched-chain amino acids like valine, leucine, and isoleucine that are key players when it comes to muscle metabolism, (Hiney, 2018). And buying just about any protein supplements without looking into these key players is not advisable.



4. Vitamin E

Vitamin E is as important in a horse’s muscle health and antioxidant. But unfortunately, some horses may be deficient in Vitamin E which is a cause of horse muscle disease. And to know if your horse is deficient, a simple serum test can determine the vitamin E level, (Hiney, 2018). But you would not want to start a supplemental Vitamin E program without some good rationale behind it and staying within the normal limits.

Because one of the things that we can observe to our animals when we over supplement a nutrient is that their body will adjust to the amount that it is taking in. And so you might change some metabolic processes or some transport and absorption. And that will sort of excite them given the higher levels of nutrients coming into the diet. Then have the level crashing downwards when we seize to administer it, and that may be true for some other nutrients as well (Hiney, 2018). So we recommend never supplementing horses off of a theory, make sure they do need it.

5. Trauma

Trauma can cause muscle disorders in horses, and we, horse owners can cause muscle damage just by overusing the horse, making them muscle sore. And so we must be mindful if our horse is fit enough for the job because any extra glow, tweak, or injury can result in muscle damage. Especially if we horse owners are not aware that our horse might have an undiagnosed lameness, then the compensation of the other limbs can also create some muscle issues and muscle soreness. So whenever we suspect a muscle disorder in our dear equine, it is good to pinpoint the root of the problem rather than just trying some management strategies for it.


Common Horse Muscle Disease Types

Here are the genetic horse muscle disease types;  Recurrent Exertional Rhabdomyolysis (RER), Polysaccharide Storage Myopathy (PSSM) Type 1 and Type 2, Malignant Hyperthermia (MH), Myofibrillar Myopathy(MM).  We will understand this one by one. The symptoms can range quite a bit from one kind of horse to another. Contact us if you have any doubt.

Recurrent Exertional Rhabdomyolysis (RER) :

According to Hiney (2018), this muscle disorder is seen much more frequently in racing breeds; thoroughbreds, standardbreds, and racing quarter horses.

This is because of the excitement level of our horses, or to be more scientific about it, their increased epinephrine level may contribute to bouts of RER. There is also another commonality that may contribute to the bout of RER when the horse had a time off. When they are not receiving the same amount of exercise and then put back into activity.

University of Minnesota research findings concluded that RER is a disorder that has to do with calcium channel regulation. But don’t mistake this for the amount of calcium in their diet. This is literally down within the muscle cell where they, essentially, get too much calcium release for the level of synaptic activity that is needed in general for our racehorse. Typically diagnosed through working up history, as well as doing some blood tests. Because there’s not a genetic test available for this, as there is no gene yet identified that would allow us to test it. So it’d be more of the history, and the blood work rather than a single genetic test.


Malignant Hyperthermia (MH):

It is a genetic disorder with a genetic mutation of the ryanodine receptor 1 gene and this is primarily going to be seen in our stock horses again, quarter horses, and paint (Hiney, 2018). But what do you see with these guys suffering from MH? Essentially, a lot of muscle rigidity, their body temperature is going to be elevated with excessive sweating. They may have high heart rates, even abnormal heart rhythms, and this can be pretty severe for them and death can result. Unfortunately, this one can go with PSSM 1, making them even more difficult to handle. So some horses with unlucky gene pools can have both, they have a disorder in glycogen synthase and a mutation of the ryanodine receptor 1 gene. Luckily though MH does have a genetic test to screen for positive horses. And with the easy determination of the diagnosis comes the accurate management program.

Polysaccharide Storage Myopathy (PSSM) Type 1:

It is most commonly seen in draft horses and stock horse breeds. It may be seen as frequently as 6‐10% of Quarter Horses, Paint, and Appaloosas. Horses of halter‐type breeding are more commonly affected with race-bred horses having lower incidence rates. PSSM1 is caused by a single gene mutation, the glycogen synthase 1. Essentially these horses become “glucose sponges”, storing excess carbohydrates as glycogen in the muscle when readily available glucose is fed to them. Unfortunately, this glycogen is not an advantage as it isn’t easily used for energy by the muscle. Instead, it disrupts normal energy metabolism (University of Minnesota).

When you’re going to see these bouts of tying up essentially, you’re going to see it fairly early into an exercise regimen with your horses. Onset is 20 minutes into light exercise (Hiney, 2018). when you’re just getting started, then the horse may start to tie up again. You’ll see that profuse sweating, the cramping, and it is going to be so far removed from the intensity of the exercise. But the good thing about PSSM 1, compared to RER is that there is a genetic test because again, they know the gene that is affected by PSSM1. Because of it, you can have a suspected horse easily just tested, and then you will know whether or not they actually have this disorder and consequently determine what management program will help with them.

Polysaccharide storage myopathy type 2 (PSSM2):

It is a relative newcomer in identification and unfortunately far less is known in comparison to PSSM1. The causative genes have not been identified, nor has the metabolic pathway causing the disorder been discovered (University of Minnesota).

It is more common in cutters, barrel horses, pleasure horses, and halter horses, well, that’s pretty much everybody. But not the racehorses. These horses have abnormal glycogen in the muscle, and can only truly be diagnosed with a muscle biopsy (Hiney, 2018). But because we do not know what gene is causing this disorder, there is no genetic test available. Therefore poor handling or analysis of the biopsy can result in false positives, and many horses are diagnosed through clinical history and examination.

Myofibril Myopathy or MFM:

It is seen in warmbloods and Arabians, so essentially in the warm ones. This horse muscle disease does not have an early onset though, it’s going to start to creep up on your horse sometime when they are almost in their prime (Hiney, 2018). What happens when a horse is suffering from MFM? Our horses get these little breaks or tears in the muscle fibers. Because the protein called Desmin, which is responsible for the myofibrils lineup in the muscle fiber is no longer present, creating the breaks and tears. When this happens the glycogen deposits in the breaks or gaps between muscle cells. So with glycogen being in the wrong place, our equines may be seen as having poor performance horses, sort of acting out like they are a little bit more unwilling.

According to Dr. Hiney (2018), it seems to happen more when our horses have some time off, similar to RER. This being the case, it looks like you can never go on a long break, we horse owners have to ride them constantly to keep them healthy.


Care of a Horse Suffering From a Horse Muscle Disease

Now let us talk about management of horse muscle disease because that is again probably what everybody wants to know about. So recommendations, what are we going to do? The following are what Dr. Hiney recommends practical tips to manage our dear equines who are suffering these disorders.

  1. First, DIET for ALL horses having these different disorders.

    We want to make sure that forage is maximized. So we’re going to try to keep forage intake high at 1.5 to 2  percent of their body weight. What’s going to be key for these guys is knowing what’s in your forage should be the bulk of what your horse is eating. Because these horses have issues with non-structural carbohydrates and blood glucose and glycogen, we want to keep that value lower than 12 percent. So your hay must be tested. So you know that you’re feeding just the appropriate amount for these kinds of horses.

  2. If your horses are on pasture, you want to lower intake with them, dry lots for one.

    So they don’t get to eat pasture or they use grazing muzzles to restrict how much they’re eating. If you just use partial turnout, horses will just eat twice as fast. And so, the grazing muzzles are probably a better idea for these horses.

  3. When you’re selecting concentrates for them, you have to make sure that it is low in sugars and starches and so making sure that you’re looking for feeds that have that on the label. So that, you know that your feed is going to be safe for those horses. Also, avoid feeds that are going to be high in traditional cereal grains. We want to steer clear from those.
  4. Use fat to provide calories.

    But also when the animal is adapted to a fat diet, we change essentially the enzymes in the metabolic pathways and we get their energy systems geared more. Or towards fatty acid oxidation rather than glucose metabolism. Furthermore, fat is a good choice for those nervous, twitchy horses, where that excitability was a contributing factor. Fat does seem to have a calming effect on horses.

  5. Protein has certainly been recommended for horses that have the Myofibril Myopathies (MFM) that they may respond to. Just be careful with giving them a lot of protein.
  6. And they shouldn’t get rest. But, what does that mean? Do you ride your horse every day? No, that’s not exactly the meaning of it. But stall confinement on their rest days is not going to be good. So we still want these horses to get turned out. Ideally, that turnout is not just a boring dry lot where they stand there and wait to get back in. But having more room to move or having other horses with them would encourage activity. So we don’t want to just let them standstill in a larger area. We want to make sure that the horse is still getting some exercise. Therefore we avoid stall confinement.

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To Prevent & Treat Horse Muscle Disease

At Cave Creek Equine Sports Medicine & Surgery, we offer our equine veterinary services to horse owners in Scottsdale, Cave Creek, Carefree, Desert Hills, New River, the greater Phoenix area, and all the Westside area (including Wickenburg, Wittman, etc). Get to know more about Cave Creek Equine and our services.

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