Equine Conditions: PSSM

We’re back with the equine conditions series, and this time, we dive into PSSM - an interesting condition with significant clinical signs that, when not managed, can significantly impact your horse’s happiness and performance. 

PSSM (polysaccharide storage myopathy) is a condition that is caused by abnormal sugar accumulation in the muscles. There are two general types of PSSM: PSSM1 and PSSM2. 

• PSSM1 horses have a gene mutation that causes the abnormal sugar accumulation. 

• PSSM2 horses have abnormal sugar accumulation without the genetic mutation.
  

PSSM1

PSSM1 is a genetic disorder that affects the muscle’s ability to manage simple sugars, where there is a mutation at equine chromosome 10, on the GYS1 gene.

This gene plays a role in glycogen production, with the PSSM1 mutation causing persistent glycogen synthase activity (Valberg et al, 2010). Glycogen is the form of sugar that gets stored in muscles as a backup energy source. Normally, muscles will use glucose in the blood for energy, while storing some as glycogen, but in PSSM horses, the muscle cells will instead excessively make glycogen, producing 1.5-4x higher glycogen concentrations than found in normal horse muscle (KER, 2011). This interferes with normal muscle function and energy metabolism during exercise.

This alteration contributes to the clinical signs commonly affecting light horse breeds:

• Soreness from muscle cramping/tying up.

• Lack of energy when under saddle. 

• Reluctance to move forward.

• Stopping and stretching as if to urinate.

• Sour attitude toward exercise.

Horses who recurrently tie up despite optimized and balanced nutrition may have the genetic mutation. However, having this genetic mutation does not mean that the horse is doomed to a life of tying up or muscle dysfunction, as some horses have the PSSM1 gene mutation but no clinical signs.

PSSM1 occurs in over 20 horse breeds, and as such likely a mutation that occurred prior to the foundation of most modern horse breeds, so is not attributed to any given stallion or pedigree (McCue et al, 2023). 

PSSM horses also tend to be more sensitive to insulin. Insulin is a hormone which controls blood sugar levels. Sugar in the diet can stimulate an insulin response, which can further worsen PSSM as more glycogen production is promoted.

PSSM2

Horses with PSSM2 will have abnormal muscle sugar stores but do not have the genetic mutation. This type of PSSM affects a larger variety of breeds, but may manifest differently in different breeds.

For instance, in quarter horses with PSSM2 typically have increased creatine kinase (indicative of muscle damage), and tying up is the most predominant clinical sign in affected quarter horses (McCue et al, 2023).

McCue, Ribeiro and Valberg (2010) have noted that different breeds show specifically different clinical signs, with QH-related breeds being more likely to experience exertional rhabdomyolysis but less likely to experience gait abnormalities than draught-breeds and warmbloods. QH were also less likely to have muscle atrophy than draught breeds. 

Warmblood breeds tend to represent a large proportion of PSSM2 affected horses, but it is often more difficult to pin down in these horses as their creatine kinase levels are often more normal than affected QH (Hunt et al, 2010; Lewis, Nicholson and Williams, 2017).

PSSM2 signs in warmbloods include (McCue et al, 2023): 

• Abnormal gaits.

• Poor performance.

• Shifting lameness.

• Unwilling to collect.

• Poor rounding over fences.

• Muscle soreness over back and hindquarters.

• Muscle atrophy.

The degree of excess sugar storage in muscle will impact the clinical signs shown, with higher levels of glycogen are more likely to cause poor performance in warmbloods with PSSM2  (Williams, Bertels and Valberg, 2018). 

Hunt et al (2010) also reported no differences between warmblood mares and geldings for PSSM, but that muscle atrophy was higher in male warmbloods.

Meanwhile, PSSM2 in draft horses often presents as (McCue et al, 2023; KER, 2014):

• Muscle breakdown and damage.

• Progressive weakness.

• Reddish brown urine.

• Excessive limb flexion.

• Trembling.

Interestingly, Schroder et al (2015) noted that Haflingers with PSSM did not have any clinical signs despite being on a carb-rich diet, which may be due to regular exercise routines (turnout and exercise). As such, breed seems to impact how PSSM presents. 

While clinical signs can present at any age, poor performance due to clinical signs (reluctance to collect/go forward) often begin around 6 years of age (Williams, Bertels and Valberg, 2018). That said, this could be due to the typical young horse's workload increasing around this time.

PSSM2 is less well understood but studies have reported improvements when treating them similarly to a Type 1 horse, in that diet, exercise schedule and environment all play a role. 

Management & Diet of the PSSM Horse

Management and diet changes can have great effects on the PSSM horse’s condition, while reducing likelihood, severity and/or frequency of tying up episodes (Harris and Rivero, 2017).

In fact, Hunt et al (2010) reported that owners who provided a recommended low-starch, fat-supplemented diet and regular daily exercise reported improvement in clinical signs in 68-71% of horses with PSSM or suspected muscle myopathies.

The management recommendations are based on the following strategies (Valberg et al, 2010; Harris and Rivero, 2017; Williams, Bertels and Valberg, 2018; McCue et al, 2023). 

• Reduce muscle glycogen synthesis.

• Promote glycogen clearance.

• Minimize stress and discomfort. 

Reduce muscle glycogen synthesis by minimizing sugar and providing fat as an alternative fuel which can also support regulated blood sugar. Hay testing, dry lots or use of grazing muzzles if on lush grass can help monitor and control sugar intake.

Promoting glycogen clearance by providing regular exercise without long periods of inactivity. Consistent exercise enhances glycogen utilization, increases turnover of structural proteins in the muscle and builds enzymes needed to burn dietary energy as fuel. 

• Minimize stall time and provide maximal turnout with compatible companions to minimize stress and enhance energy metabolism..

• Even 10-15 minutes of work per day, or 4x per week WITH ample turnout, can benefit muscle function.

Minimize stress and discomfort by exercising/feeding the affected horse first, providing a compatible equine company, and avoiding negative interactions. Start exercise gradually and do not push the horse if the horse is stiff or seems tired. PSSM2 horses will require longer warm up periods, and do better with lots of stretching and long & low work.

We have summarized diet and management recommendations below according to available research.

PSSM Diet Checklist 

• Feed more than 1.5% BW (DM basis) of appropriate hay per day.

• Keep the diet at <10-12% NSC.

• Protein supplementation can be benefical for horses with atrophy. Alfalfa can be useful for increasing protein in the diet but make sure to feed at no more than 50% of the hay ration.

• Drive energy from fat instead of grains (13-20% DE from fat). If feeding oil, note that 1 ml oil/kg BW per day is a general upper guideline unless working with a professional for guidance. It is recommended to add 1.0-1.5 IU of Vitamin E per ml of oil in diet.

• Additional antioxidant supplementation may also help the exercising horse: Vitamin E at 3-5 IU/kg BW, and Se intake at 0.006 mg/kg BW.

• Ensure adequate salt provision in diet to maintain electrolyte balance: 30 grams at maintenance, increasing 2-3x with exercise or sweating (hot weather).
  

PSSM Management Checklist 

• Maximize turnout, minimize stall time.

• Avoid lush pasture or use grazing muzzle when not possible.

• Consistent work (even 10-15 minutes/day).

• Lots of stretching during warm up.

• Minimize stress.

• Do not push the horse during work if they seem tired or stiff.

Research has repeatedly emphasized that implementing both diet AND management recommendations are necessary to support the horse’s wellbeing and yield an improvement in clinical signs. While PSSM is a condition which can cause discomfort, affected horses can still have a happy life when managed strategically. 

Need a hand formulating a balanced diet that fits these recommendations for your horse?

Let's connect! Book directly at www.honosnutrition.ca or email honosnutrico@gmail.com to learn what Honos service is best suited for your needs.

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References 

Graham-Theirs, PM., Kronfeld, DS. 2005. Amino acid supplementation improves muscle mass in aged and young horses. Journal of Animal Science 83(12): 2783-2788

Harris, PA., Rivero, JLL. 2017. Nutritional considerations for equine rhabdomyolysis syndrome. Equine Veterinary Education 29(8): 459-465. 

Hunt, LM., Valberg, SJ., Steffenhagen, K., McCue, ME. 2010. An epidemiological study of myopathies in Warmblood horses. Equine Veterinary Journal 40(2): 171-177. 

KER. 2011. Update: PSSM in Horses. Accessed online: https://ker.com/equinews/update-pssm-horses/

Lewis, SS., Nicholson, AM., WIlliams, ZJ. 2017. Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy. AVMA 78(11)

McCue, ME., Ribeiro, WP., Valberg, SJ. 2010. Prevalence of polysaccharide storage myopathy in horses with neuromuscular disorders. Equine Veterinary Journal 38(s36): 340-344.

McCue, M., Valberg, S., Schultz, N. 2023. Polysaccharide storage myopathy (PSSM) in horses. University of Minnesota Extension. Accessed online from: https://extension.umn.edu/horse-health/polysaccharide-storage-myopathy-pssm

Schroder, U., Licka, TF., Zsoldos, R., Hahn, CN., MacIntyre, N., Schwendenwein, Ile., Schwarz, B., Van Den Hoven, R. 2015. Effect of Diet on Haflinger Horses With GYS1 Mutation (Polysaccharide Storage Myopathy Type 1). Journal of Equine Veterinary Science 35: 598-605.

Valberg, SJ., Mickelson, JR., Gallant, EM., Macleay, JM., Lentz, L., De La Corte, F. 2010. Exertional rhabdomyolysis in Quarter Horses and Thoroughbreds: one syndrome, multiple aetiologies. Equine Veterinary Journal 31(s30): 533-538. 

Valberg, SJ., Macleay, JM., Billstrom, JA., Hower-Moritz, MAMickelson, JR. 2010. Skeletal muscle metabolic response to exercise in horses with ‘tying-up’ due to polysaccharide storage myopathy. Equine Veterinary Journal 31(1): 43-47.

Williams, ZJ., Bertels, M., Valberg, SJ. 2018. exercise regimes in Warmblood horses with type 2 Polysaccharide Storage Myopathy. PLos ONE 13(9): e0203467