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Recent advances in PSSM genetic testing

Recent advances in PSSM genetic testing

Polysaccharide Storage Myopathy

Polysaccharide Storage Myopathy (PSSM) in horses

Stephanie Valberg DVM PhD and James Mickelson PhD
College of Veterinary Medicine, University of Minnesota, St. Paul

Frequently Asked Questions

1. What is polysaccharide storage myopathy (PSSM) in horses?
2. What are the signs of PSSM in horses?
3. What causes PSSM in horses?
4. What should I do if a horse is stiff and reluctant to move?
5. Does PSSM differ from HYPP in horses?
6. How do I know if my horse is having an episode of tying-up (not PSSM test)?
7. What if the episode occurred weeks ago and I want to know if my horse is susceptible to tying-up?
8. Can my horse's muscles heal after an episode of tying-up?
9. How is a diagnosis of PSSM in horses established and what is the difference between mild, moderate, and severe forms of PSSM?
10. Is PSSM in horses an inherited disease?
11. Is there a genetic test for PSSM in horses?
12. How do I prevent another episode of tying-up in my horse?
13. Can my horse be cured?
14. Where can I learn more about PSSM in horses?

1. What is polysaccharide storage myopathy (PSSM) in horses?

PSSM is a muscle disease in horses with Quarter Horse bloodlines such as Quarter Horses, American Paint Horses and Appaloosas. The American Quarter Horse Association (AQHA) has funded research into this disease since 1995 and has provided us with the opportunity to learn much about the diagnosis, cause and treatment for this disease.
Another form of polysaccharide storage myopathy also occurs in Draft, Draft crossbreeds, warmbloods. Many of the clinical signs in these breeds differ from those found in Quarter Horses and related breeds.  The signs found in Draft, Draft crossbreeds, and warmbloods include muscle soreness, reluctance to engage the hind quarters muscle atrophy, weakness, difficulty in backing up, and picking up hind feet.
Whether this disease exists in other breeds is controversial. Research from our laboratory suggests that there are several causes for tying-up.  Most racing Quarter Horses, Thoroughbreds, Standardbreds and Arabians with tying-up, suffer from a separate disease from PSSM called recurrent exertional rhabdomyolysis (see RER information). In these breeds of horses, the accumulation of abnormal polysaccharide, the hallmark of PSSM, is rarely if ever present.
Several different acronyms have been used to describe this disorder including PSSM, EPSM and EPSSM. The variety of acronyms used are in part related to preferences of different laboratories, as well as to differences in the criteria used to diagnose polysaccharide storage myopathy.  Many light breeds of horses, various draft breeds, ponies, warmblood breeds, and mules are reported to have PSSM when amylase-sensitive glycogen is used as a diagnostic criterion for PSSM. These criteria result in up to 80% of Draft horses and 33% of all horses outside of draft and Quarter Horse bloodlines being diagnosed with PSSM.

In contrast, Quarter horse-related breeds, draft horses and warmbloods are the breeds most affected with PSSM when amylase-resistant abnormal-polysaccharide is a required diagnostic criterion. Of 1,251 horses with a suspected neuromuscular disease, 40% were diagnosed with PSSM in a recent review of muscle biopsies. Although > 50 equine breeds were included in the study, 63% of horses with PSSM were of Quarter Horse related breeds, 12 % draft breeds and 9% warmblood breeds. We believe this is the most accurate criteria to use to diagnose PSSM and it is the one used by our laboratory.

2. What are the signs of PSSM in horses?

Horses with PSSM have signs typically associated with tying-up. These signs are most commonly muscle stiffness, sweating, and reluctance to move.  The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting.

During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they often stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year.

Rarely, episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down. The urine in such horses is often coffee colored, due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse's kidneys if they become dehydrated.

Very young foals with PSSM occasionally show signs of severe muscle pain and weakness. This usually occurs if they have a concurrent infection such as pneumonia.

3. What causes PSSM in horses?

Polysaccharide storage myopathy (PSSM) is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (polysaccharide) in muscle tissue. About 200 horses of Quarter Horse and warmblood/draft horse breeding have been identified with tying-up associated with polysaccharide accumulation in muscles. This disorder is inherited in Quarter Horses and breeding individuals with PSSM has produced affected offspring.

Horses with PSSM accumulate muscle glycogen due to an unregulated uptake of sugar (glucose) into their muscles and the synthesis of its storage from in the muscle called glycogen. One aspect of the defect involves enhanced sensitivity of the muscles to insulin, resulting in more transport of sugar from the bloodstream to skeletal muscle. The diet can be adjusted to decrease the amount of insulin and sugar in the bloodstream. Carbohydrates that are high in starch, such as sweet feed, corn, wheat, oats, barley, and molasses, should be avoided and extra calories can be provided in the form of fat. An important part of the management of PSSM horses is daily exercise. This suppresses glucose uptake, enhances glucose utilization, and improves energy metabolism in skeletal muscle. If only the diet is changed, we found that approximately 50% of horses improve. If both diet and exercise are altered, then 90% of horses have had no or few episodes of tying-up.

An old theory about tying-up is that it is due to too much lactic acid in the muscle. Many exercise studies have proven that this is absolutely not the case with PSSM. PSSM is actually a glycogen storage disease and there are several diseases in other species and in human beings that also result in the storage of too much glycogen in skeletal muscle. In these other diseases, glycogen accumulates because the muscle lacks an enzyme (protein) necessary to burn glycogen as an energy source. These similarities led us to test PSSM horses for the disorders in glycogen metabolism identified in human beings. We found that PSSM is a unique glycogen storage disease because the PSSM horses have all the necessary enzymes to burn glycogen as a fuel in their muscles. With exercise, PSSM horses show the expected decrease in muscle glycogen as it is burned as fuel.

The unique feature of PSSM is that the muscle cells in PSSM horses remove sugar from the blood stream and transported into their muscle at a faster rate, and make more glycogen than normal horses. Our recent research shows that the reason for this is that PSSM muscles are very sensitive to insulin beginning as early as 6 months of age. Insulin is a hormone released by the pancreas into the bloodstream in response to a carbohydrate meal. It stimulates the muscle to take up sugar from the bloodstream. Once inside the cell the muscle’s of PSSM horses make much more glycogen than a normal horse.

4. What should I do if a horse is stiff and reluctant to move?

• Stop exercising the horse and move it to a box stall. Do not force the horse to walk.
• Call your veterinarian.
• Blanket the horse if weather is cool. Hose the horse to remove sweat if the weather is warm.
• Determine if the horse is dehydrated, due to excessive sweating. Pinched skin will normally spring back and saliva should be wet, not tacky.
• Provide fluids: small frequent sips of water. Electrolytes (potassium, sodium, and chlorine) may be added to drinking water, if palatable to the horse. Plain water should always be available as an alternative. If the horse is dehydrated, intravenous fluids may be needed. Once cool, the horse may have free access to water.
• Relieve anxiety and pain. Your veterinarian may administer drugs such as acepromazine and flunixin meglumine (Banamine).
• Remove grain. Feed only hay until symptoms subside.
• Provide small paddock turnout once the horse walks freely, usually in 12-24 hours.

5. Does PSSM differ from HYPP in horses?

Hyperkalemic periodic paralysis HYPP is a completely separate muscle disorder in Quarter Horses from PSSM. The two diseases have different clinical signs, different causes and different treatments.

6. How do I know if my horse is having an episode of tying-up? (not PSSM testing)

A blood sample can be used to determine what is happening inside your horse’s muscles. Proteins measured in a blood sample can determine the extent of the muscle damage.

Two muscle proteins often used as a measure of muscle damage include creatine kinase (CK) and aspartate transaminase (AST). When muscle cells are damaged, they release these proteins into the bloodstream within hours. A blood sample taken to measure these proteins can determine how much muscle damage has occurred. This is not a specific test for PSSM as the presence of CK and AST in the blood occurs with any muscle damage. With many forms of tying-up, blood CK activity returns to normal within days if horses are rested. It is very common for horses with PSSM to have high CK activities even if they are rested for weeks after an episode of tying-up.

Quarter Horses frequently exhibit elevations of serum creatine kinase (CK) activity in association with clinical signs and possibly elevations of aspartate transaminase (AST) if rhabdomyolysis is chronic in nature. Persistent elevation in CK activity, despite an extended period of rest, is a common observation in PSSM Quarter Horses.

7. What if the episode occurred weeks ago and I want to know if my horse is susceptible to tying-up?

One way to see if tying-up occurs is to do an exercise test. This is useful if a horse shows no signs of muscle stiffness when a veterinarian performs a physical exam but still suspects the horse suffers from a form of tying-up. The horse should be worked in a round pen at a walk and trot for 15 minutes. If the horse shows any signs of persistent muscle stiffness, the test should stop. The idea here is not to produce signs of stiffness, but to see if this mild form of exercise cases slight muscle damage. In cooperation with your veterinarian, have a blood sample drawn 4-6 hours after the exercise test. A significant result indicating sub clinical muscle damage would be a serum CK activity greater than 800 U/L 4 hours after 15 minutes of exercise at a trot. The blood sample is not informative if taken right after exercise, as the CK protein has not had enough time to leak out of the muscle cells. This test is not specific for PSSM, but does indicate muscle damage is occurring with exercise. For more details on the exercise challenge test, see the recommended diagnostic work-up.

8. Can my horse's muscles heal after an episode of tying-up?

Muscle has a remarkable ability to repair itself. After an episode of tying-up, the muscle cells usually heal completely within three to four weeks without any scarring. If the damage is severe, you may notice that the horse's muscle mass decreases in size as the body removes the damaged proteins. The muscle mass usually returns within two to four months.

9. How is a diagnosis of PSSM established?

Currently, PSSM type 1 can be diagnosed with a genetic test, however, at present PSSM type 2 must be diagnosed with a muscle biopsy. Which test is most appropriate depends on the breed of your horse. Click here for an Adobe Acrobat document "Decision tree for genetic testing versus muscle biopsy".

Genetic Testing

Submission information for genetic testing for PSSM type 1 is available from the Veterinary Diagnostic Laboratory (800) 605-8787.

We strongly recommend your veterinarian be involved in genetic testing. We cannot make anything more than general recommendations about the disease as we have not evaluated your horse to know if there are any concurrent problems that would make our diet or exercise recommendations contraindicated.

See recent advances for more information about the genetic test.

***Dr. Mickelson, Valberg and McCue are holder of the patent for PSSM genetic testing and a portion of the profits from the genetic test directed to their research and patent royalties.

Muscle Biopsy

Veterinarians can obtain instructions on how to perform a muscle biopsy here, or by calling (612) 624-3611 and requesting information via fax. The sample must be specially prepared and shipped overnight to our laboratory. Results are usually available within 10 days.

The sample is taken from the semimembranosus muscle, which is part of the rear limb hamstring muscles. Sections of muscle are evaluated with a number of special stains. The periodic acid Schiff's (PAS) stain is used to look at the amount of sugar stored as glycogen in the muscle. With PSSM, the intensity of this stain is very dark indicating a large amount of glycogen is present in the horse's muscle. Measurements of glycogen are usually 1.5 to 4 X higher than normal horses. A large amount of glycogen, however, is not uncommon in trained horses, and not a basis for diagnosing PSSM. In addition to storing excessive normal glycogen, horses with PSSM have deep purple inclusions of an abnormal complex sugar stored in fibers. This is the classic diagnostic feature of PSSM muscle.

Biopsies are often graded as mild, moderate or severe based on the amount of abnormal polysaccharide. The abnormal polysaccharide always remains within the muscle tissues and does not decrease in amount over time.

A normal biopsy (left) and a biopsy from a horse with PSSM (right) stained with PAS. Note the lack of a uniform texture in the PSSM biopsy. The darker areas in the PSSM biopsy indicate the accumulation of excess glycogen and abnormal polysaccharide.

We have observed the accumulation of abnormal polysaccharide in the muscle of PSSM horses from a few months of age; however, establishing a diagnosis of PSSM in horses less than a year of age can be difficult because they not have developed abnormal polysaccharide yet. To be certain, a biopsy from a foal may have to be repeated at a later age.

Horses with unusually large accumulation of glycogen in muscle cells with a granular appearance are diagnosed with mild PSSM. In this case, the amount of abnormal polysaccharide is considered low, which may be associated with the horse being less than two years of age. In horses older than two, we advise veterinarians to ensure no other lameness issues may be contributing to signs of muscle pain.

Moderate to severe PSSM reflects the degree of accumulation of amylase resistant abnormal polysaccharide. The amount to abnormal polysaccharide does not always match the severity of clinical signs or prognosis.

10. Is PSSM in horses an inherited disease?

Researchers at the University of Minnesota performed a small breeding trial to investigate the question of PSSM heritability. We bred mares with PSSM and followed their foals for five years. Many of their foals developed signs of muscle damage after a few months of age and signs of PSSM in muscle biopsies by age two. This research suggests there is strong evidence that PSSM in Quarter Horse-related breeds is inherited, likely as a dominant trait.

At present, evidence suggests that only one parent needs to pass the genetic mutation to its offspring for signs of tying-up to occur. Because this disease causes considerable pain, our laboratory does not recommend breeding horses that have been diagnosed with PSSM. No matter who is selected as the breeding partner there is a 50% chance or greater that a PSSM horse’s offspring will develop the disease.

11. Is there a genetic test for PSSM in horses?

Yes there is a DNA test to determine if a horse has the genetic mutation that causes PSSM type 1. Until we know more about the basic cellular mechanism involved in PSSM type 2 we will have difficulty identifying the gene with its mutation.  Genetic research into PSSM type 2 is on going and we welcome working with breeding farms with several related PSSM and healthy horses that would allow us to study their herd. We would welcome contact from such breeders. Please contact Dr. Molly McCue at the University of Minnesota (mccu0173@umn.edu) or call her at 612-624-9320.

12. How do I prevent another episode of tying-up in my horse?

It is very important to let horses move about as they choose as soon as their muscle stiffness has disappeared. Horses recover very slowly if left in a stall.  Hand walking is not advised, because it often leads to another episode of tying-up even if done for 10 minutes or less. Without additional changes in the daily exercise routine, horses with PSSM will continue to have problems, with or without a fat supplemented diet. Exercise in PSSM horses burns the stores of muscle glycogen, appears to decrease the muscles heightened sensitivity to insulin and increases the enzymes necessary to burn fat as a fuel. To prevent muscle stiffness, horses with PSSM should be turned-out as frequently as possible and exercised regularly. If they have been laid-up for more than a few days, they should be returned to work very gradually. Stall rest or an irregular exercise program may result in another episode of tying-up.

Minimizing stress and providing regular routines and daily exercise are highly beneficial. Turn out each day for as long as possible with other horses will keep the horse active. The amount of time the horse is left in the stall should be decreased as much as possible. If there has been a recent severe episode of tying-up, turn the horse out for two weeks on the diet recommended below. Once the horse has been on the diet for two weeks, begin longeing once a day for five minutes at a walk and trot. Gradually increase the time by two minutes each day.  If the horse seems stiff, slow to a walk and see if the stiffness persists. If the horse is stiff, stop there; if not, continue after a two-minute walk. When the horse can exercise for 15 minutes, give it a five-minute break at a walk, and gradually increase walking and trotting after this. Once the horse has reached 30 minutes of trotting on a longe-line (with a break at 15 minutes), ride the horse for 20 to 30 minutes while gradually increasing the length and intensity of exercise. At least three weeks of groundwork will precede any mounted exercise. Keeping the horse fit will change the muscle metabolism, and this seems the best prevention against further episodes of tying-up.

Adherence to a strict diet will also help horses with PSSM.  A high-quality grass or oat hay should form the basis of the diet. If feeding alfalfa hay, a mixture (half alfalfa and half grass or oat hay) may be best. A vitamin and mineral supplement that contains Vitamin E and Selenium is also beneficial, however, make sure that there is not already enough Selenium and Vitamin E in the feeds you are using.

Eliminate grain and sweet feed from the diet and replace these calories with a fat supplement. There are a number of fat supplemented commercial diets available, but it is very important to find out what the non-soluble starch (NSC) or starch content is in the diet before using them. In general, our research suggests that the more severe the signs of Exertional Rhabdomyolysis (ER), the lower the starch content of the diet should be. In this regard, Re-Leve™ is ideal because it is palatable, balanced for vitamins and minerals and very low in starch, high in fat.

Alternatively, most horses find the pelleted forms of rice bran palatable, but it has more starch than Re-Leve™. Rice bran products works well for many, but not all, horses. Rice bran is rich in calories, due to its 20% fat content, and may be fed at a rate of one pound per day for horses in light training and up to 4 pounds per day for horses in intense training. A vitamin/mineral supplement that balances the calcium to phosphorus ratio (1:1 or 2:1) should be purchased and fed with these rice bran feeds

Sources of fat for equine diets include:

Re-Leve™** made by Hallway Feeds (1-859 255-7602). Feed one to three pounds to horses in light to moderate work; six to 10 pounds can be fed to thin horses in heavy work, along with grass hay. This is also a good diet for young growing horses with PSSM. Weanlings can be fed 6.5 lbs of Re-Leve™ and mixed grass/alfalfa hay (8 lbs/day). Yearlings can be fed 8 lbs Re-Leve™ and a 50-50 alfalfa: grass hay (9 lbs/day). ** Do not feed additional selenium with this feed, as it is fully supplemented.

Purina Ultium™**, www.ultium.com, Beet pulp/rice bran in pelleted form. 1-888-ULTIUM1 [858-4861] ** Do not feed additional selenium with this feed, as it is fully supplemented.

Moorman's Natural Glo™ Rice Bran with GroStrong mineral supplements (1-800-680-8254). Less palatable than the pelleted form for some horses.

MoorGlo™ is a pelleted form of rice bran that may be more palatable to horses that do not like the loose rice bran (1-800-680-8254).

Equi-Jewel™ is heat stabilized rice bran, to be fed along with 0.25 pound per day of Micro-Phase supplement made by Kentucky Equine Research (1-800-772-1988).

Buckeye Ultimate Finish™ with Gro‘N Win™ mineral supplement and Harvest Salt (1-800-898-WINS)

Corn oil, gradually adding up to 2-3 cups per day over alfalfa pellets or in a mash.

Omegatin™, Kent Feeds. 1 800 367-2730

Alternatively, you could work with your nutritionist to design a balanced diet containing beet pulp, alfalfa pellets, rice bran and oil that meets the requirements in the attached table.

In addition to a salt block in the stall, an electrolyte supplement should be offered to horses in hot, humid weather.

Many horses with this muscle disorder have recurrent episodes of tying-up. At present, the best we can do is to manage the condition so that recurrence is kept to a minimum. Several of the horses on the rice bran supplement and this training regime have responded well and are competing in pleasure and hunter/jumper shows.

A portion of the profits from Re-Leve™ is contributed to the University of Minnesota and Stephanie Valberg to advance research of PSSM.

Table 1. Feeding recommendations for an average-sized horse (500 kg) with chronic exertional rhabdomyolysis at varying levels of exertion.

Daily requirements derived from multiple research studies (% NSC and % fat) and Kentucky Equine Research recommendations.

From: 2003. McKenzie EM, Valberg SJ and Pagan J. Nutritional Management of Exertional Rhabdomyolysis. In: Current Therapy in Equine Medicine 5. ed Robinson E Saunders, Philadelphia PA, 2003, pp 727-734.

Table 2. Potential rations for a 500-kg horse with polysaccharide storage myopathy.

From: McKenzie EM, Valberg SJ and Pagan J. Nutritional Management of Exertional Rhabdomyolysis. Current Therapy in Equine Medicine 5. ed E Robinson Saunders, Philadelphia PA, 2003, pp 727-734.

*Vitamin and mineral supplement required for nonfortified feeds. The mineral recommended for the specific rice bran product should be provided (not necessary for Re-Leve™).

Addition of 50-100 g of salt per day to all rations is recommended based on level of exertion.

13. Can my horse be cured?

When the described diet and exercise routines were followed we found that all horses improved, and >75% of horses stopped tying-up. PSSM horses, however, will always be susceptible to this condition and if their exercise schedule is disrupted. If they become ill from other causes, they may again develop clinical signs again. If this occurs, they should go back to the fitness program described above using longeing or round pen work. Many horses with this condition are happy trail horses, successful pleasure horses, and useful ranch horses. The greatest difficulty in owning a horse with PSSM is the time commitment to keep the horse fit and the moderate expense of special feeds.

14. Where can I find out more about PSSM?

Dr. Valberg and other members of the lab team have published their research on PSSM in many general interest and scientific articles.

Non-peer reviewed articles

1. Valberg SJ and Mickelson JR. Polysaccharide storage myopathy: One important cause of exertional rhabdomyolysis in horses. Journal of World Equine Health Network. 1997;2 (3):32-37.
2. Valberg SJ Polysaccharide storage Myopathy. Ipologica 1998;9. No 1.
3. Macleay J, Valberg SJ, De La Corte FD and Pagan J. Recurrent exertional rhabdomyolysis in Thoroughbred racehorses: Effect of Diet and Exercise Intensity. Proceedings Am Assoc Vet Pract 1999.
4. Valberg SJ, Mickelson JR, Gallant EM, MacLeay JM, Lentz L and De La Corte FD. Tying-up in Quarter Horses and Thoroughbreds; separate disease with common clinical signs. Proceedings Am Assoc Vet Pract 1999.
5. De La Corte FD, Valberg SJ, MacLeay JM and Mickelson JR. The effect of feeding a fat supplement to horses with polysaccharide storage myopathy. Journal World Equine Health 1999;4,2:12-19.
6. Valberg SJ, Douglas RB, Harris P, and Huntington P. On Tying-up. Journal of World Equine Health Network. 3(1) 1997.
7. Valberg SJ. And Brubacker K. Tying-up. Horseplay Magazine. November 1997.
8. Dr. Stephanie Valberg studies the baffling disease known as tying-up. Equine Care Watch Media Bulletin. March 1998.
9. Valberg SJ. Unraveling the mystery of tying-up. Horse'n Around. May 1998.Volume 8(5) page 6-8.
10. Valberg SJ. The art and science of training. Grayson Jockey Club Research Foundation News Letter. Vol 15(3) July 1998.
11. Frydenborg K (Valberg SJ). Tying-up : The latest research. Equus magazine March 1999 p 20-33.
12. Valberg SJ and Mickelson JM. Unraveling the mystery of tying-up. AQHA Journal July 1999.
13. Valberg SJ. Tying-up. AAEP answer line. The Horse Dec 1999 p 114.
14. Valberg SJ The horse as the ultimate athlete. Horse'n Around. 2000.
15. Valberg SJ. Skeletal muscle function and responses to training. Horse'n Around 2000.
16. Goer R and Valberg SJ. Tying-up. Management of exertional rhabdomyolysis. The Horse 2000: June pp 81-90.
17. Valberg. SJ. Tying-up more than one disease. International Equine Science. Jan 2000 p2-8.
18. Valberg. SJ. Strangles. Horse'n Around Magazine October 2000.
19. Valberg SJ. Tying up is split up. AAEP Forum, The Horse. Oct 2001, p 26.
20. Valberg, S. Tying-up. Practical Horseman. Sept 2002
21. Valberg, S. Tying-up. MyHorseMatters.com, Feb 2003.
22. Llewellyn, Mark, When Two Worlds Merge, EquiNews. Spring 2004.
23. Equus Column, The Source of PSSM. Aug 2005.
24. Hudson, B. Pictures of Health, Horse Sense. Fall 2005.
25. Smith-Thomas, H. Causes and Solutions for Tying-up. Thoroughbred Times. Sept. 2005
26. Piscopo, S., Inheritance of RER in Thoroughbreds. The Horse.com, Nov 2005
27. Pascoe, E. PSSM in Warmbloods. Practical Horseman. Oct 2005
28. Dobson, C. Untie the Solution, Quarter Horse Journal, Jan 2006.

Summary of our research: University of Minnesota researchers find tying up may be the manifestation of a dominant genetic trait. Equus Magazine. (MF) 338/10

1. Genrick R, Duncanson A and Valberg SJ. Low Sugar Hay and Pasture for Horses.  Forage Focuus. Nov/Dec 2006. pp 14-15.
2. SJ Valberg and Lisa Borgia. Tying up in Horses. Part I. Cross Country Magazine Spring 2007 12(38): 18-19.
3. SJ Valberg and Lisa Borgia. Tying up in Horses. Part II. Cross Country Magazine Spring 2007 13(38): 20-21.
4. Ron Genrick and SJ Valberg. Low sugar forage. U of Minnesota Extension Horse Newsletter. Vol 3 (7).
5. SJ Valberg. Tying up in horses. U of M Horse Newsletter. August 2007 Vol 3(8) pg 1.

Scientific articles

1. Valberg SJ, Cardinet III GH, Carlson GP, and DiMauro, S. Polysaccharide storage myopathy associated with exertional rhabdomyolysis in the horse. Neuromuscular Disorders. 1993, 2:351-359.
2. Valberg SJ. Exertional rhabdomyolysis and polysaccharide storage myopathy in Quarter Horses. American Assoc. Equine Pract. 1995;228-230.
3. Valberg SJ, Geyer CJ, Sorum S and Cardinet III GH. Familial basis of polysaccharide storage myopathy and exertional rhabdomyolysis in Quarter Horses and related breeds. Am J Vet Res 1996;57:286-290.
4. Valberg SJ, MacLeay JM and Mickelson JR. Polysaccharide storage myopathy associated with exertional rhabdomyolysis in horses. Comp Cont Educ 1997;19(9)10:1077-1086.
5. Perkins G, Valberg SJ, Madigan JE, Carlson GP, and Jones SL. Fluid, electrolyte and renal abnormalities associated with acute rhabdomyolysis in four neonatal foals. J Vet Int Med 1998;12:173-177.
6. Valberg SJ, Townsend D, MacLeay JM and Mickelson JR. Glycolytic capacity and phosphofructokinase regulation in horses with polysaccharide storage myopathy Am J Vet Res 1998;59:782-785.
7. Valberg SJ, MacLeay JM, Billstrom JA, Hower-Moritz MA and Mickelson JR. Skeletal muscle metabolic response to exercise in horses with polysaccharide storage myopathy. Equine Vet J 1999;31:43-47.
8. De La Corte FD, Valberg SJ, Williamson S, MacLeay JM and Mickelson JR. Enhanced glucose uptake in horses with polysaccharide storage myopathy (PSSM). Am J Vet Res 1999;60;458-462.
9. De La Corte FD, Valberg SJ and Hower-Moritz A. The effect of exercise on glucose clearance in horses with polysaccharide storage myopathy. International Conference on Equine Exercise Physiology , Equine Vet J Suppl. 1999 (accepted).
10. Valberg SJ, Mickelson JR, Gallant EM, MacLeay JM, Lentz L and De La Corte FD. Exertional rhabdomyolysis in Quarter Horses and Thoroughbreds; one syndrome, multiple etiologies. International Conference on Equine Exercise Physiology Equine Vet J Suppl. 1999 (accepted).
11. DeLaCorte FD and Valberg SJ. Treatment of polysaccharide storage myopathy. Comp. Cont. Educ. 2000;22(8):782-788.
12. Byrne E, Jones SL, Valberg SJ, Zimmel DN and Cohen N. Rhabdomyolysis in two foals with polysaccharide storage myopathy and concurrent pneumonia. Comp Cont Educ 2000;22:503-507.
13. DeLaCorte FD, Valberg SJ, MacLeay JM and Mickelson JR. Developmental onset of polysaccharide storage myopathy in 4 Quarter Horse foals. J Vet Int Med 2002;16:581-587.
14. Valberg SJ. A review of the diagnosis and treatment of rhabdomyolysis in foals proceedings Am Assoc Equine Pract 2002 pp117-121.
15. Firshman AM, Valberg SJ, Bender J, Finno C. Epidemiologic characteristics and management of polysaccharide storage myopathy in Quarter Horses. Am J Vet Res 2003;64:1319-1327.
16. Park, H.B Marklund, S. Jeon, J.T , Mickelson J.R. Valberg S.J, Sandberg K, and Andersson L. Molecular characterization and mutational screening of the PRKAG3 gene in the horse. Cytogenetics and genome research.  Cytogenet Genome Res 2003;102:211-216.
17. Annandale EJ, Valberg SJ, Mickelson JR and Seaquist ER. Insulin sensitivity and skeletal muscle glucose transport in Equine Polysaccharide Storage Myopathy. Neuromusc Disorders 2004;14(10):666-674.
18. Ribeiro W, Valberg SJ, Pagan JD and Essen Gustavsson B. The effect of varying dietary starch and fat content on creatine kinase activity and substrate availability in equine polysaccharide storage myopathy J Vet Int Med 2004;18:887-894.
19. Annandale EJ, Valberg SJ, Essen Gustavsson B The effect of submaximal exercise on adenine nucleotide concentrations in skeletal muscle fibers of horses with polysaccharide storage Myopathy Am J Vet Res 2005;66:839-845.
20. Firshman AM, Valberg SJ, Karges TL, Benedict LE, Annandale EJ, Seaquist ER. Serum Creatine Kinase Response to Exercise During Dexamethasone-induced Insulin Resistance in Four Quarter Horses with Polysaccharide Storage Myopathy. Am J Vet Res 2005;66:1718-1723.
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