With the horrifically cold temperatures all over the United States this week, I wanted to focus on the prevalence and problems associated with accidental hypothermia. Two examples took place this week in the United Kingdom and here in the United States.
A condition that can occur in any animal (horse, dog, cow, human) that is exposed to hypothermic conditions (for example, falling into a mud hole or pond, or getting trapped in something while it's raining). Rewarming of the hypothermic victim should only be performed by personnel who have a clear understanding of the adverse effects of improper rewarming. The hypothermic horse may die due to improper handling during the extrication and/or rewarming procedures. Arrhythmia/ventricular fibrillation can occur as a result of rough handling during rescue even under mild hypothermia. This is also known as “post-rescue collapse” or ”re-warming shock” and should be carefully evaluated by the veterinarian.
There were so many cases of horses extricated from mud and pond scenarios in late 2010 thru early 2011 that it initiated our investigation of what was going on with the sudden deaths of animals that had otherwise been successfully extricated but then died within 24 hours. A 2011 paper by Dr. Tomas Gimenez highlighted the challenges for firefighters, veterinarians, and horse owners called to the scene of exposed horses. He pointed out that in these scenarios it is most important to try to initiate first aid, extricate the victim, then rewarm the core body temperature of the horse without attempting to warm the extremities or the cold skin.
Well meaning efforts (skin rubbing or heating) can be counterproductive and even kill the horse. Surface reheating should take place after the core is rewarmed. The body core (organs, large blood vessels, central nervous system) can have a different temperature from surface temperature (muscles, fat, skin, hair coat). Core temperature in mature horses is 99.5-100.4° F, any temperature below 99.5° F is considered hypothermia. Normal core temperature is essential to preserve body functions, so the body maintains core temperature at the expense of surface or skin temperature. This is why veterinarians want a rectal temperature for a better idea of what is going on deeper in the body than the surface temperature.
The brain works to dissipate or conserve heat in the body at all times. Horses rarely suffer from hypothermia under normal conditions, and the rate of body temperature loss in a horse immersed in water or mud is not known. Obviously a fatter, larger horse will lose temperature more slowly than a thin, skinny, or smaller one. When core temperature continues to drop, animals shiver to help maintain core temperature, however, water conducts heat away from a body 25 times faster than in air. When a horse’s skin is in direct contact with cold water or mud, it will eventually become hypothermic in spite of the body’s efforts (shivering, etc).
Restoring Core Temperature
The main focus of rewarming should be to restore core temperature. A horse with a rewarmed and dry coat can be suffering from severe hypothermia worse than a horse with a wet and “cold” feeling coat. Core rewarming can be achieved most effectively through the administration of warmed fluids intravenously and a warm enema. Ideally, fluids should be warmed at a temperature of 104° F but no less than 91.4° F. Core rewarming can also be enhanced by a warmed fluid enema, and warm fluid administration per nasogastric intubation. Surface rewarming can be used after the core has been rewarmed. Forced hot air is more effective than blankets, and vigorous rubbing should never be used for surface rewarming.
Hypothermia and Death
The only definite criterion of death in hypothermia is failure to respond to core rewarming. The ultimate cause of death during hypothermia appears to be cardiac failure with asystole or ventricular fibrillation. Often this is caused by overstimulation, or failure to maintain and then rewarm the core temperature and occurs within 24 hours of extrication.
Please share this information with your local veterinarian and fire department. If you have experiences with hypothermia in horses, please share your story and tell us how the horse survived.