Malignant hyperthermia is a genetic disorder that rears it's ugly head chiefly during general anesthesia with certain types of drugs. Symptoms of the disease is the anesthetized or post-anesthetic recovering horse undergoes a run away fever that can go high enough to be life threatening. This hypermetabolic response is precipitated by potent volatile anesthetic gases such as halothane, sevoflurane, desflurane and the depolarizing muscle relaxant succinylcholine. In some horses it may exhibit with stress such as vigorous exercise and heat stress. Malignant hyperthermia genes also potentiate the problems associated with other genetic diseases of the muscle like the GYS1 mutation that is responsible for some types of Polysaccharide Storage Myopathies (EPSM and PSSM).

Though a single gene is implicated, ryanodine receptor 1 gene (RyR1), there may be many different types of mutations at this point. The best described is the nucleotide C7360G substitution and has a autosomal dominant form of inheritance. There are other types who's characteristics are less well defined.

A Rapid Detection Method for the Ryanodine Receptor 1 (C7360G) Mutation in Quarter Horses. J Vet Intern Med. 2009 Mar 6. Nieto JE, Aleman M. Departments of Surgical and Radiological Sciences and Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA. Background: Anesthetic-induced malignant hyperthermia has been documented in Quarter Horses and is caused by a single-point mutation in the ryanodine receptor 1 gene at nucleotide C7360G generating a R2454G amino acid substitution. An accurate, faster molecular test that is less prone to contamination would facilitate screening for the mutation in horses intended for breeding, in those undergoing surgical procedures, and in those with clinical signs compatible with malignant hyperthermia. Objective: To report a rapid and accurate method for the detection of the ryanodine receptor 1 C7360G mutation. Animals: Eleven diseased, 10 healthy, and 225 randomly selected Quarter Horses. Methods: This study included horses with the ryanodine receptor 1 C7360G mutation as detected by gene sequencing. Available genomic and complementary DNA extracted from whole blood, hair or skeletal muscle was used for genetic analysis. Real-time polymerase chain reaction (RT-PCR) melting curve analysis was performed by equine specific primers and 2 hybridization probes (sensor and anchor probes) that contain the site of the mutation. Results from this method were blinded and compared with nucleic acid sequencing for validation. Results: A rapid genotyping assay with fluorescence resonance energy transfer probes and melting curve analysis was accurate (100% agreement, K= 1) for identification of affected horses. The prevalence of the mutation in a random population of Quarter Horses was 1.3%. Conclusions and Clinical Importance: Malignant hyperthermia in Quarter Horses can be rapidly and accurately detected by RT-PCR melting curve genotyping with hybridization probes.