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MH Malignant Hyperthermia

Name: MH Malignant Hyperthermia
Brand: Equigerminal
Price: 35.0 EUR

Test that shows if your horse carries the gene change that makes its body overheat during anaesthesia or hard work....

€35.00 Incl. VAT

About

This test shows whether your horse carries the gene change behind Malignant Hyperthermia (MH), an inherited muscle disease. A horse with the variant can look normal for years and then suffer a sudden, dangerous attack.

It is meant for Quarter Horses and related breeds, above all horses facing surgery or used for breeding.

Why test?

Before anaesthesia. This is the main reason. Your veterinarian can plan the anaesthetic for a positive horse. A horse that has never had a problem gives you no warning.

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.

A horse with signs. Think of this test when a horse has unexplained or repeated muscle breakdown, locked muscles, stiffness, overheating or sweating. It also applies after sudden unexplained death following such signs, and after muscle damage or death linked to gas anaesthesia.

Clinical signs

Signs usually appear in adult horses. Two kinds of attack are on record.

Attacks under anaesthesia:

Body temperature climbing fast, recorded between 40 and 45 degrees Celsius. In one monitored case it rose about 0.12 degrees every minute.
A racing heart that loses its rhythm, and high blood pressure
Heavy sweating. The anaesthetic tubing becomes hot to the touch.
Muscles locking up: first in one place, such as the ears drawn in or the jaw clamped shut, then all over, with the third eyelid showing and the eye pulled back into the socket
The blood turning acid, with carbon dioxide building up even though the horse is on a ventilator with pure oxygen
Blood salts out of balance, thickened blood, high blood sugar, struggling kidneys and muscle breakdown products rising fast
Death, with the body stiffening unusually quickly afterwards

Attacks with no anaesthetic involved:

Severe, sudden muscle breakdown with a body temperature recorded between 40.2 and 43.3 degrees Celsius
Episodes that look like colic, locked muscles, a stiff way of going, tiring quickly, sweating on and off, a fast heart and fast breathing
Extremely high muscle enzymes on a blood test, and acid blood
Muscle enzymes that stay raised between attacks, in horses that otherwise look well
Sudden death, including death out in the field on an unusually hot day

What sets off an attack: every gas anaesthetic of the halogenated family (halothane, isoflurane, desflurane, sevoflurane) and muscle relaxants such as succinylcholine. Also exercise, the stress of transport or a new place, the start of training, the excitement of breeding, extreme heat, and another illness or muscle disease at the same time.

An attack is a veterinary emergency. What is reported to save horses: spotting it early, stopping the anaesthetic gas or muscle relaxant at once, cooling the body actively, and giving dantrolene.

None of these signs is unique to MH. They also happen under anaesthesia and after hard work for other reasons. Your veterinarian makes the diagnosis from the history, the examination, blood results and this genetic result.

Transmission

MH 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. Through a positive parent, the variant can pass into other breeds and lines.

What each mating can produce:

n/n x n/n gives 100% n/n
n/n x n/MH gives 50% n/n, 50% n/MH
n/MH x n/MH gives 25% n/n, 50% n/MH, 25% MH/MH

No horse with two copies has been found yet. The researchers put this down to how rare the disease is, not to any evidence that two copies would be lethal.

Prevention

There is no cure. The result guides two things.

Managing a positive horse. This part saves lives, and it belongs to your veterinarian:

Ask your veterinarian to record the result in the horse's clinical file, so it is known before any general anaesthesia and in an emergency
Medication can be given before anaesthesia to reduce the severity of an attack
Avoid the other triggers listed under Clinical signs where you can
A horse that also carries PSSM1 needs that condition managed as well

Breeding. Breeding from positive horses is discouraged. Test breeding stock before mating, and if a positive horse is used, choose a partner that tested negative.

Results

n/n: negative. No copy of the MH variant. Cannot pass it on.

n/MH: positive, one copy. The horse is susceptible to attacks. Can pass the variant on.

MH/MH: positive, two copies. The horse is susceptible to attacks. Passes the variant to every foal.

The report is a DNA certificate that identifies the horse and the genotype found.

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.

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

🛒 Buy the test: select and buy the test online.

📧 Receive the instructions: after payment is confirmed, you receive the sample collection instructions by e-mail.

📄 Complete the form: print and complete the submission form with the animal’s identification.

✨ Collect the sample: pull 20 to 40 hair roots with the bulb and stick them in the marked area of the form, or ask your veterinarian to collect blood in a K3-EDTA tube.

📮 Send it to the laboratory: Equigerminal, S.A., HIESE, Rua da Quinta do Sobreiro, 25, Quinta Vale do Espinhal, 3230-343 Penela, PORTUGAL.

📊 Receive the report: the certified report is issued as soon as the analysis is validated.

FAQs

At what age can a horse be tested?
Any age. The genotype is fixed at conception, so one test lasts for life.

The variant is rare. Should I still test?
Yes. Rare is not the same as low risk, because an attack can kill.

Does a negative result mean my horse is safe under anaesthesia?
No. This test looks only for the MH variant. It does not detect PSSM1, HYPP or any other muscle disease, and anaesthetic problems and overheating have other causes. Your veterinarian judges the anaesthetic risk.

Is this the malignant hyperthermia seen in people and pigs?
People, some pig breeds and dogs get malignant hyperthermia too. This test is for the horse variant only.

For Professionals

Genetic and clinical detail for veterinarians, geneticists and laboratories.

Gene / locus: RYR1 (ryanodine receptor 1), ECA10.
Variant: missense Arg to Gly. HGVS (EquCab3.0): NC_009153.3:g.9678680C>G; XM_023649662.1:c.7363C>G; XP_023505430.1:p.(Arg2455Gly).
Alternative designations: published as c.7360C>G, p.(Arg2454Gly) (R2454G; Aleman et al., 2004). OMIA restates it against transcript XM_023649662.1, giving c.7363C>G and R2455G; both designations refer to the same genomic change. Laboratories differ in which they print, so quote both when comparing reports.
Discovery: candidate-gene approach from the homologous disease in pigs and humans; the change was found in two affected horses (Aleman et al., 2004). The homologous position is altered in two of the five known human malignant hyperthermia susceptibility types.
Mechanism: RYR1 encodes the calcium release channel of skeletal muscle. In susceptible horses the channel can be forced open, flooding the cell with calcium; hypermetabolism follows, with intense heat production and acidosis, and often death.
Variant identifiers: no rs/EVA identifier listed in OMIA; OMIA variant 159.
Variant classification: not currently evaluated (ISAG/AVCG, per OMIA).
Inheritance: autosomal dominant (OMIA code D), inferred from two heterozygous affected horses; no formal pedigree analysis has been published (Aleman et al., 2004, as cited in OMIA).
Penetrance / expressivity: estimated mortality 34% in susceptible horses under halothane anaesthesia (Aleman et al., 2009). PSSM phenotype is more severe in horses carrying both RYR1 and GYS1 variants (McCue et al., 2009).
Breeds with documented carriers or cases: Quarter Horse, Appaloosa, American Paint Horse (OMIA variant record). All horses in the first clinical series were Quarter Horses of halter lines. The clinical syndrome has also been reported in Thoroughbreds, Arabians and ponies, not all attributed to this variant.
Allele / carrier frequencies: among 159,227 horses of 16 breeds, the variant was found in 391 Quarter Horses, 18 Paint Horses, 1 Appaloosa and 1 Quarter Horse x Clydesdale; allele frequency 0.13% in Quarter Horses and 0.12% in Paint Horses and Appaloosas (Aleman, McCue and Bellone, 2025).
Clinical notes for veterinarians: consider concurrent GYS1 (PSSM1) testing in Quarter Horse-related breeds.

Key references:

Aleman M, Riehl J, Aldridge BM, LeCouteur RA, Stott JL, Pessah IN (2004). Association of a mutation in the ryanodine receptor 1 gene with equine malignant hyperthermia. Muscle Nerve 30:356-365. doi:10.1002/mus.20084. PMID 15318347.
Aleman M, Nieto JE, Magdesian KG (2009). Malignant hyperthermia associated with ryanodine receptor 1 mutation in Quarter Horses. J Vet Intern Med 23:329-334. doi:10.1111/j.1939-1676.2009.0274.x.
McCue ME, Valberg SJ, Jackson M, Borgia L, Lucio M, Mickelson JR (2009). Polysaccharide storage myopathy phenotype in Quarter Horse-related breeds is modified by the presence of an RYR1 mutation. Neuromuscul Disord 19:37-43. doi:10.1016/j.nmd.2008.10.001.
Aleman M, McCue M, Bellone RR (2025). Allele frequencies and genotypes for the ryanodine receptor 1 variant causing malignant hyperthermia and fatal rhabdomyolysis with hyperthermia in horses. J Vet Intern Med 39:e70081. doi:10.1111/jvim.70081.
Sperandio LMS, Lago GR, Albertino LG, Araujo CET, Ferreira C, Borges AS, Oliveira-Filho JP (2024). Allele frequency of muscular genetic disorders in bull-catching (vaquejada) Quarter Horses. J Equine Vet Sci 136:105052. doi:10.1016/j.jevs.2024.105052.