What is Malignant Hyperthermia?
Certain horses are affected by malignant hyperthermia more often than others such as Arabians, Thoroughbreds, and Quarter horses. This inherited disease is caused by a mutated gene that mistakenly mixes up glycine and arginine in your horse’s system. This is a dominant gene so only one mutated gene is needed from the parents to pass on the gene to their foals. In other words, only one of the horses (father or mother) needs to have this mutated gene for their offspring to have malignant hyperthermia. If your horse has the symptoms of malignant hyperthermia and you are not at the veterinarian’s office, you should treat this as a medical emergency. Once your horse reaches the stage of the disease called malignant hyperthermia crisis, it is usually fatal.
Malignant hyperthermia in horses is a genetic disease that causes a life threatening condition in certain situations such as anesthesia, stress, or extreme exercise. These triggers create an increase of calcium, which eventually causes hyperthermia and possibly death. Although this condition is uncommon in horses, it is best to have your horse tested for the MH gene before administering any kind of anesthesia. Testing positive does not mean that your horse is unable to be anesthetized. Be sure to let the veterinarian know about the MH mutation and ask for your horse to get dantrolene or an equivalent medication before anesthesia is given for any reason.
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Symptoms of Malignant Hyperthermia in Horses
Luckily, the disorder usually shows itself while your horse is in the care of an equine veterinarian that should be knowledgeable in this type of situation. If your horse starts to show signs of malignant hyperthermia, the veterinarian should recognize the problem right away and should have the drug needed to counteract the anesthesia. The signs that your horse may have malignant hyperthermia are as follows.
- Increase in body temperature
- Excessive sweating
- Rapid heart rate and breathing
- Rigid muscles
- Rhabdomyolysis (tying up)
- High blood pressure
- Shallow breathing
- Fatal hypermetabolic state known as an MH crisis
Causes of Malignant Hyperthermia in Horses
Malignant hyperthermia is caused by an autosomal dominant variation of the RYR1 gene that encrypts the ryanodine receptor of the cells in the bones and muscles. This is most commonly caused certain kinds of anesthesia such as isoflurane and halothane. It is most prevalent in:
- Quarter horses
- America paint horses
Diagnosis of Malignant Hyperthermia in Horses
You should always use an equine veterinarian (or a veterinarian that specializes in horses) when getting a diagnosis for any kind of situation, and especially with serious illnesses such as malignant hyperthermia. Chances are pretty good that your horse will be in the presence of a veterinary professional of some kind when this condition becomes known since it is most often set off by anesthesia. In that case, your veterinarian will know what to do. However, if your horse has an episode of malignant hyperthermia following strenuous exercise or stress, you should treat it as a medical emergency. When you get to the veterinarian’s office (or hospital), let them know what you were doing when it happened and what symptoms you have noticed so far. Be sure to let the veterinarian know if your horse is on any kind of medication. The veterinarian will likely do an in vitro contracture test (IVCT) or DNA analysis to be sure it is malignant hyperthermia. This procedure consists of introducing a triggering agent like caffeine or halothane in a small controlled area to check for muscle fiber contractions.
The veterinarian will also perform a physical examination including reflexes, weight, height, body score condition, heart rate, respirations, body temperature, and blood pressure. If possible, you will be asked to walk your horse around while the veterinarian assesses the muscles in motion and watches stature, attitude, and lameness. Blood testing needed are blood cultures, complete blood count (CBC), bacterial culture, biochemical analysis, fungal culture, and a sample of blood will go to an outside laboratory where it can be tested for the RYR1 mutation. A CT scan, bone scan, and digital radiographs (x-rays) will be taken to look for abnormalities. Sometimes, if your horse had a severe reaction, small fractures can be caused by the muscle contractions.
Treatment of Malignant Hyperthermia in Horses
Since there are not many medications that can successfully treat malignant hyperthermia, it is best to have your horse tested before any kind of anesthesia is needed. That way your veterinarian can administer dantrolene before giving anesthesia for surgery. However, if your horse is in active malignant hyperthermia, the veterinarian will likely treat with medication and some form of cold therapy.
The veterinarian will want to focus on treating the symptoms, one of which is a dangerously high body temperature. Cold packs (ice packs) and alcohol baths are usually the treatment plan for the fever.
Recommended medications are lidocaine, and dextrose or insulin (whichever is needed). Sodium bicarbonate may be given to maintain the proper pH and intravenous fluid therapy will be given to stabilize the electrolytes
Your horse will be kept in the hospital for observation for the first several days because relapses are common.
Recovery of Malignant Hyperthermia in Horses
Your horse’s prognosis depends on the cause and how soon treatment was given. For example, if it was anesthesia that set it off, the veterinarian will be able to treat your horse immediately, giving a much better chance for recovery.