We've all heard of people and animals that are "sensitive" to anesthesia. Monica Aleman, MVZ, PhD, Dipl. ACVIM, head of the neuromuscular disease lab at the University of California, Davis, at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004, reported on a genetic problem called equine malignant hyperthermia (EMH) that can make simple anesthesia deadly for some horses. Malignant hyperthermia-like episodes in the horse have been associated with drugs such as halothane, isoflurane, succinylcholine, and caffeine.

It has been shown that a mutation in the ryanodine receptor 1 (RyR1) gene causes a dysfunction in the sarcoplasmic receptors of skeletal muscles, and that results in the excessive release of calcium into the myoplasm. This triggers a series of events that result in a hypermetabolic state and/or death. Symptoms include hyperthermia (fever), hyperhidrosis (excessive sweating), tachycardia (high heart rate), dysrhythmias (abnormal heart rhythm), tachypnea (shallow breathing), muscle rigidity, and death with acute rigor mortis.

Her study was the first to identify a mutation in the RyR1 gene in the horse, but it is unknown if this mutation is responsible for all cases of EMH. Aleman said a benefit of this discovery is the creation of a faster and more direct screening test to identify susceptible horses.

She currently is accepting samples from horses that practitioners think might suffer from EMH. She can accept hair and whole blood, but prefers muscle. Practitioners should also send signalments and the pedigrees of affected horses. Aleman can be contacted at mraleman@ucdavis.edu