About
This test shows whether your horse carries the gene change behind PSSM1, an inherited muscle disease. Affected horses can become stiff and sore and tie up after exercise.
It is relevant for horses of many breeds, including Warmbloods, and for any horse used for breeding or offered for sale.
Why test?
Studbook registration. Several Warmblood studbooks, including the KWPN, require a PSSM1 result for registration. Check your studbook's current rules.
Breeding. Knowing the result of both parents tells you what each mating can produce.
Buying and selling. The result is objective information you can show a buyer.
Early management. A positive horse can start the right diet and exercise before muscle damage builds up. In a horse that already ties up, the result explains why.
Clinical signs
Signs usually appear with exercise, often after a rest period in which the horse kept eating grain. They vary a lot from horse to horse.
- Stiffness, reluctance to go forward, a short or shuffling stride
- Repeated tying up: painful muscle cramps, often soon after work starts. Your vet may call this exertional rhabdomyolysis.
- Heavy sweating, fast breathing and pain that can look like colic
- Hard, painful muscles over the hindquarters and back
- Dark, coffee-coloured urine in severe episodes
- Raised muscle enzymes (CK and AST) on a blood test
- Poor performance, muscle tremors and, in long-standing cases, loss of muscle
None of these signs is unique to PSSM1. Your veterinarian decides whether a horse is ill or only carries the variant.
Transmission
PSSM1 is dominant. One copy is enough to make a horse susceptible. The gene is not on a sex chromosome, so colts and fillies are affected equally.
What each mating can produce:
- n/n x n/n gives 100% n/n
- n/n x n/P1 gives 50% n/n, 50% n/P1
- n/n x P1/P1 gives 100% n/P1
- n/P1 x n/P1 gives 25% n/n, 50% n/P1, 25% P1/P1
- n/P1 x P1/P1 gives 50% n/P1, 50% P1/P1
- P1/P1 x P1/P1 gives 100% P1/P1
Prevention
There is no cure. The result guides two things.
Breeding. Mate a positive horse only with a partner that tested negative, and test every foal. This keeps a valuable line while the variant is bred out.
Managing a positive horse. These measures are consistently reported to prevent episodes:
- A diet low in starch and sugar, with no grain or sweet feed
- Extra calories from fat instead of starch
- Daily turnout and steady exercise, built up gradually; avoid long stall rest
- No sudden jump in work after a rest period
- Diet changes spread over several weeks
Your veterinarian and nutritionist set the exact targets for your horse.
Results
n/n: negative. No copy of the PSSM1 variant. Cannot pass it on.
n/P1: positive, one copy. Can show signs, though expression varies and some horses never do. Can pass the variant on.
P1/P1: positive, two copies. Signs tend to appear earlier and be more severe. Passes the variant to every foal.
Sample
Hair roots: 20 to 40 hairs pulled (not cut) from the mane or tail, with the roots attached. Tape them inside the marked area of the printable sample submission form. Hairs without roots cannot be analysed.
Blood: 5 mL of whole blood in a K3-EDTA tube, collected by a veterinarian. Label the tube with the horse's name and send it with the submission form.
Keep samples dry and at room temperature, and do not freeze them. Do not send hair that is wet, mouldy or soiled with bedding or faeces.
Send your sample by regular mail or express delivery to:
Equigerminal Lab HIESE
Rua da Quinta do Sobreiro Nº25
3230-343 Penela, Portugal
Turnaround
Standard processing: results within 5 to 10 working days of the sample arriving at the laboratory. Shipping is arranged and paid for by the client, and transit time is not included in the 5 to 10 working days.
Samples that fail DNA extraction or amplification are repeated at no extra cost, which may add a few working days. We contact you if a new sample is required.
How it works
🛒 Purchase the test: select and buy the test online.
📧 Receive instructions: after payment confirmation you receive sample collection instructions by e-mail.
✨ Collect the sample yourself: pull 20 to 40 hair roots with the bulb attached, or ask your veterinarian to collect blood in a K3-EDTA tube.
📄 Complete the form: print and complete the submission form with the animal identification.
📮 Send it to the laboratory: Equigerminal, S.A., HIESE, Rua da Quinta do Sobreiro, 25, Quinta Vale do Espinhal, 3230-343 Penela, PORTUGAL.
📄 Receive your report: your certified report is issued as soon as the analysis is validated.
FAQs
At what age can a horse be tested?
Any age, from newborn foal to adult. The genotype never changes, so one test lasts for life. For young foals, tail hair is usually easiest.
Does a negative result mean my horse cannot tie up?
No. This test looks only for PSSM1. Tying up has other causes, including PSSM2 (also sold as MIM), recurrent exertional rhabdomyolysis, and electrolyte or management problems.
For Professionals
Genetic and clinical detail for veterinarians, geneticists and laboratories.
- Gene / locus: GYS1 (glycogen synthase 1, skeletal muscle), ECA10.
- Variant: R309H (Arg309His), missense. HGVS (EquCab3.0): NC_009153.3:g.19203501C>T; NM_001126125.2:c.926G>A; NP_001119597.2:p.(Arg309His).
- Alternative designations: Ensembl ENSECAT00000023453.3:c.926G>A / ENSECAP00000019426.2:p.Arg309His (same position as RefSeq). The original publication gave the protein change only; OMIA records its cDNA position as c.?G>A. Laboratory allele symbol: P1.
- Variant identifiers: rs1150416011 (EVA); OMIA variant 163; phene OMIA:001158-9796.
- Variant classification: Pathogenic (P), ISAG/AVCG classification as listed in OMIA.
- Mechanism: gain of function. The overactive enzyme leads to excess muscle glycogen plus an abnormal polysaccharide the muscle cannot break down. This stored material is poorly used during exercise, and the resulting energy shortfall causes stiffness, pain and muscle damage (McCue et al., 2008a).
- Inheritance: autosomal incomplete dominant (OMIA code AID). Segregation analysis excluded all single-locus modes other than autosomal dominant (Dranchak et al., 2005, as cited in OMIA).
- Penetrance / expressivity: incomplete penetrance and variable expressivity. Phenotype differs by breed type: atrophy and progressive weakness in draft breeds, soreness and gait abnormality in Warmbloods, acute exertional rhabdomyolysis in Quarter Horses (McCue et al., 2008a, as summarised in OMIA).
- Breeds with documented carriers or cases: OMIA variant record: Belgian Draft, Quarter Horse, Appaloosa, American Paint; OMIA phene record adds Haflinger and Noric. McCue et al. (2008a) found the His309 allele in Quarter Horse, Paint, Appaloosa, five draft breeds, three Warmblood breeds, Morgan, Mustang and Rocky Mountain Horse. Stanley et al. (2009) reported it in a variety of UK breeds.
- Allele / carrier frequencies: allele frequency 0.035 to 0.350 depending on breed (McCue et al., 2008a; 356 variant-positive horses, 15 breeds). Among 831 biopsy-diagnosed PSSM cases from 36 breeds, the proportion carrying the variant was 87% draft-related, 72% Quarter Horse-related, 18% Warmblood and 24% other light breeds (McCue et al., 2008b). Allele frequency in American Quarter Horses born 2020 to 2024: 38.6% in halter horses versus 1.2% in a random cohort (Brown et al., 2026; 300 performance and 300 random horses). Estimated allele age: mean 159 generations, roughly 1,200 to 1,500 years (McCue et al., 2008a).
- Clinical notes for veterinarians: muscle biopsy (subsarcolemmal vacuoles and PAS-positive inclusions, per OMIA) is a separate procedure and remains the reference for suspected PSSM2/MFM. Commercial PSSM2/MIM variants (MYOT P2, FLNC P3a/P3b, MYOZ3 P4) occurred at similar frequency in controls and biopsy-diagnosed cases (Valberg et al., 2021; Valberg et al., 2023).
Key references:
- McCue ME, Valberg SJ, Miller MB, Wade C, DiMauro S, Akman HO, Mickelson JR (2008a). Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics 91:458-466. doi:10.1016/j.ygeno.2008.01.011. PMID 18358695.
- McCue ME, Valberg SJ, Lucio M, Mickelson JR (2008b). Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy. J Vet Intern Med 22:1228-1233. doi:10.1111/j.1939-1676.2008.0167.x.
- Stanley R, McCue M, Valberg S, Mickelson J, Mayhew I, McGowan C, et al. (2009). A glycogen synthase 1 mutation associated with equine polysaccharide storage myopathy and exertional rhabdomyolysis occurs in a variety of UK breeds. Equine Vet J 41:597-601.
- Brown BN, Hughes S, Le TM, Tatar NP, Grahn JC, Carrillo-Alvarez M, Bellone RR, Finno CJ (2026). Allele frequencies of 7 inherited disorders in performance and random cohorts of American Quarter Horses (2020-2024). J Am Vet Med Assoc 1-7. doi:10.2460/javma.26.04.0256.