Rural Heritage Vet Clinic

EPSM—Frequently Asked Questions

by Beth A. Valentine, DVM, PhD

EPSM [Equine Polysaccaride Storage Myopathy] has been around for as long as horses have been recognized to have problems such as shivers, tying up, and Monday morning disease, which is a very long time indeed. We just didn't know what to look for and so we didn't see it.

What is the best diet for an EPSM horse?

The best diet for an EPSM horse is one that includes at least 1 pound of fat (2 cups vegetable based oil or equivalent) per 1000 pounds of horse per day, mixed with a base feed that is as low in starch and sugar as your horse will accept. Forage based products such as alfalfa pellets, water soaked beet pulp, or alfalfa cubes with added oil are ideal for horses that will eat them. Next best are products such as senior feeds or other feeds designed to be complete feeds that can replace hay if necessary. For really fussy horses, try something like Purina Strategy or Nutrena Compete 14% protein, which contain what I consider to be the highest acceptable level of starch and sugar (34%). Strategy or Compete with added oil works well for a lot of horses, but may be slightly too high in starch and sugar for sensitive horses. Mixing these feeds with a forage based feed will help reduce the starch and sugar intake.

If it takes a bit of something "sweet" to get your horse to accept the added fat, that is not a big problem. You can always aim to reduce added starch and sugar later. Many new feeds are coming out that are high enough in fat and low enough in starch and sugar to allow for addition of less fat, but they all so far still need some added 100% fat source. Feeds that are at least 10% fat and also less than 34% starch and sugar allow for less added fat. You can calculate the pounds of fat fed from such feeds by multiplying pounds of fat fed per day by the percentage of fat. Rice bran products can provide some of the fat, but you will still need to add some 100% fat.

For example:
5 lbs per day of a 10% fat feed provides 5 x 0.10 = 0.5 lbs of fat.

Other important parts of an EPSM diet are plenty of good quality hay or pasture. I believe almost all horses benefit from a vitamin E supplement to provide at least 1 international unit (IU) vitamin E per pound of horse per day. In areas that are selenium deficient (ask your veterinarian about this or check Dr. Hintz's selenium map), provide a supplement with at least 1 mg selenium per 1000 pounds of horse per day.

Can you feed an EPSM diet to a foal?

Yes, absolutely. Evidence suggests that part of the risk for bone and joint problems in youngsters is an abnormal response to starch and sugar in the diet. For growing youngsters you want to be sure to add enough vitamin E and, if necessary in your area, selenium. You also want to add a daily "broad spectrum" vitamin and mineral supplement designed to support healthy foal growth. Watch that your foal doesn't get so many daily calories that it grows too fast or gets fat. If that happens, cut down on everything to maintain your foal on the ribby side.

What about dry fat supplements?

Several newer products on the market for horses are granular soy based fat. Milk Specialties Performance Pak 100 and Cool Calories 100 are two such products. They are more acceptable to some horses, and are less messy, than oil, but cost more to feed. Dry fat products generally weight about half of their volume, so you will have to feed twice the volume to get the right amount of fat.

For example:
2 cups (16 ounces) by volume weighs only about 8 ounces, which is 1/2 lb of fat.

How long before I see results after diet change?

Full fat adaptation in EPSM horses takes about four months, and so you may see some ups and downs during this time. Some horses get a bit "over-energized" early on in diet change, I believe because they suddenly feel good again. If your horse acts up, give the horse plenty of turnout time and perhaps work only from the ground until your horse gets used to feeling good.

Many horses have "setbacks" early on in diet change, most often at three to four months after diet change, so don't panic if your horse seems to regress. If you see any positive changes in the first four months, even with setbacks, that is a good indication you are on the right track. Some horses don't show obvious signs of improvement until four to six months of diet change. If your horse is improving after four to six months of diet change, improvement may continue for many months to come.

What is the future for an EPSM horse?

The data to date indicate that more than 90% of EPSM horses respond extremely well to a change to an EPSM diet, along with as much daily turnout and regular exercise as possible. Many EPSM horses never show problems again, although some more severely affected horses may still have episodic problems. If your horse improves, but continues to have some problems after four to six months of diet change, try to increase the fat and reduce the starch and sugar even more. If your horse shows no signs of improvement on diet change, either he is one of the few horses that don't respond to diet change, or the problems are due to something other than EPSM.

Can I breed an EPSM horse?

Breeding an EPSM always poses a bit of a dilemma. If your breeding horse is suspected or confirmed to have EPSM and does not respond well to diet change, I would advise not breeding. If your EPSM horse does well on diet change, and has other good qualities to pass on, I would not have a problem with breeding. At this point it appears that about two-thirds of all draft related horses and about one-third of light horse breeds have the type of metabolism that predisposes to EPSM. They are often the best built, best temperament, and best performing horses, which makes me wonder if we have somehow selected for this type of metabolism. If so, its about time we figured out how to feed them right.


Beth A. Valentine, DVM, PhD, is involved with EPSM research and other veterinary matters at the College of Veterinary Medicine, Oregon State University. She is this site's virtual vet and co-author of Draft Horses, an Owner's Manual.

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26 October 2011 last revision