With the increased numbers of horses in urban and suburban areas, it is now common for fire-rescue departments across the country to respond to calls involving horses trapped in mud, fallen through surface ice or trapped in swimming pools.
During most equine emergencies, initial assistance is concentrated on removing (rescuing) the victim from the incident site and taking care of critical medical needs later. In the case of hypothermia, such prioritization of the rescue effort sometimes has fatal consequences. Also, once the equine victim is removed from the hypothermia-causing environment, well-meaning but misunderstood efforts to restore body temperature can be counterproductive, resulting in a more severe hypothermic state. The fact that the horse may stand and eat some hay after the rescue can be interpreted as a sign that the animal is “OK,” but the horse may die a few hours later.
The purpose of this article is to assist professional emergency responders with information that may help save the life of the hypothermic horse.
Body Temperature
Body temperature is really “body temperatures.” The body core (organs, large blood vessels, central nervous system) can have a different temperature from shell or surface temperature (muscles, fat, skin, hair coat) , and these temperatures vary under the influence of several factors. Fortunately for us mammals, the thermoregulatory center of the brain receives input from temperature sensors across the organism and performs two main functions:
- It sets the average body temperature (body core temperature) and allows it to oscillate only ± 0.2 °C (0.36 °F), whereas surface (shell) temperature is allowed to oscillate up to ±4.0 °C (7.2 °F) before corrective mechanisms kick in.
- Normal core temperature is essential to preserve body functions. Therefore, the brain will do everything it can to maintain normal core temperature, even at the expense of surface/peripheral temperature, to maintain life.
The most practical way to measure body core temperature in a horse is by measuring rectal temperature. It is important to keep the tip of the thermometer in good contact against the wall of the rectum for the necessary amount of time, and not inserted into the feces.
Maintenance of Core Temperature
Horses have the ability to maintain normal core temperature in extreme cold climates. The thick haircoat and subcutaneous fat allows the Yakut horses in Siberia to tolerate temperatures of -68 °C (-90 °F) in their natural habitat. The peripheral shell acts as an insulator and is composed of muscles, fat, skin and haircoat helps to maintain a constant core temperature by releasing or preventing the loss of heat from the core. The rate of heat loss from the core to the environment will vary depending on the insulating capacity of the shell, including skin thickness, amount of subcutaneous fat, length and density of the hair and piloerection (raised hair trapping air).
Heat loss greater than heat production equals hypothermia. The normal body (core) temperature in a mature horse is 37.5-38.0° C (99.5-100.4° F). Any temperature below 37.5° C (99.5° F) is considered hypothermia in a mature horse. In humans, hypothermia is defined as a drop in core temperature below 35° C (95° F).
Studies on immersion hypothermia in humans have shown that death will occur in 70% of cases with a core body temperature of 30° C (86° F), and in 90% of cases with a core body temperature of 26° C (78.8° F). The ultimate cause of death during hypothermia appears to be cardiac failure with asystole or ventricular fibrillation.
Vasoconstriction will help isolate the thermal core to preserve normal core temperature. When core temperature continues to drop, vasoconstriction can no longer compensate for the periphery’s heat loss and shivering will appear to help maintain core temperature.
Water is an excellent conductor of temperature. Wetness of body surface will contribute to loss of core temperature through conduction more than any other mechanism. Water will conduct heat away from an object 25 times faster than air. A human body will lose heat in 26.5° C (80° F) water at the same rate as it does in 5.5° C (42° F) air.
Horses rarely suffer from hypothermia under normal conditions. It is when a large percentage of their body surface is in direct contact with a conduction agent like water that the loss of heat will exceed heat production by the body. The rate of body temperature loss in a horse immersed in water or mud is not known. Horses under general anesthesia for more than 45 minutes lost a mean body temperature of 0.8±0.6° C (1.44±1.08° F) per hour (range 0-2.4° C, 0-4.32° F). Wetness of body surface strongly contributed to loss of body temperature during general anesthesia.
Risk Factors
Risk factors affecting the onset and severity of hypothermia include:
- Dehydration. Most horses suffering from acute hypothermia also suffer from dehydration.
- Poor body condition. Horses in poor body condition are more susceptible to hypothermia due to a reduced layer of insulating subcutaneous fat.
- Age. The ability to generate heat decreases with age in all mammals studied.
- Body surface/volume (mass) ratio. The larger the body surface relative to body mass, the more core temperature will be lost across the body surface. This explains in part why donkeys and small horses (foals, ponies) are more susceptible to hypothermia than larger horses.
- Drugs. General anesthetics (anesthetic-induced vasodilation), sedatives will aggravate hypothermia through their action on the brain.
- Rough handling. Rough handling of a profoundly hypothermic patient after the rescue can trigger ventricular fibrillation and cardiac arrest.
A further decrease in core temperature, or continued cooling, after the rescue is known as the “afterdrop effect,” and is a phenomenon of conductive heat loss. The afterdrop can occur during shivering, which stimulates a shift of blood to the musculature, or during surface warming, which will stimulate peripheral vasodilation
Restoring Body Temperature
The main focus of rewarming the hypothermic horse should be to restore core temperature. Peripheral warming without core rewarming can be counterproductive. A horse with a rewarmed and dry coat can be suffering from more severe hypothermia than a horse with a wet and “cold” feeling coat.
To implement core rewarming, contact a large-animal veterinarian as soon as possible. No treatment is more effective in hypothermia than core rewarming, which is achieved most effectively through the administration of warm fluids intravenously and through a warm enema.
A large-animal veterinarian should initiate administration of warm intravenous fluids to the hypothermic horse as soon as possible, even before the horse is rescued. Intravenous fluids are important not only to restore core normal core temperature, but just as important to rehydrate the horse. Intravenous fluids should be warmed before administration, which can be achieved by placing the fluid bags in a water bath.
Ideally, fluids should be administered at a temperature of 40° C (104° F), but no less than 33° C (91.4° F). Fluid temperatures at or above 47° C (116.6° F) can cause thermal damage to red blood cells. Warm the fluids by placing the IV fluid bag (5-6 liter bag) in a warm water bath. Do not microwave.
Surface Rewarming
Surface (convective) rewarming can be used as an adjunct to core rewarming through the use of forced hot air, heat lamps, heating pads, blankets and protecting the victim from the wind. If possible, take the horse to a small barn/building and heat it with a forced-air heater. The warm air will help dry the coat without touching it, and the horse will inhale warm air to help in core rewarming. Vigorous rubbing should never be used for surface rewarming because it can damage frozen cells.
Careful handling of a hypothermic horse is extremely important to help prevent cell damage and heart dysfunction (arrhythmia/heart attack).
What To Do
- As soon as you are dispatched to this type of call, find out whether a large-animal veterinarian has been contacted and is responding.
- Use normal protocol upon arriving at the scene, including size-up and implementing the Incident Command System.
- It is safe to assume the horse is already hypothermic and dehydrated. In most cases, the horse has been immersed for a few hours before it is discovered and you are called.
- If a veterinarian is on scene, warm IV fluids (life support) should be started.
- Perform the rescue using appropriate technical large-animal-rescue techniques.
- Handle the horse as gently as possible.
- If the horse can walk, bring him to a small enclosed building out of the weather and warm the building with a forced-air heater.
- If the horse is lying down (recumbent), place him on a rescue glide or plywood sheet and bring him indoors. Place insulating material (wood shavings, straw, blankets, cardboard) between the horse and the floor.
- Blankets and towels are OK, but DO NOT rub.
- Warm packs on the neck against the jugular veins may help restore core temperature.
- Check rectal temperature periodically.
- Even a horse that looks “OK” should be taken to a veterinary facility for follow-up.
Hypothermia And Death
Under field conditions, a hypothermic horse should not be declared dead unless all efforts to restore core temperature have failed. Core temperature is not a reliable indicator of survival as demonstrated by many cases of humans with profound hypothermia. The only definite criterion of death is failure to respond to core rewarming. The following quote is well known in human hypothermia resuscitation: “A patient is not dead until he is warm and dead.” A human with a core temperature of 9° C (48.2° F) recovered after one hour without cardiac activity or respiration.
The bottom line: Hypothermia in a horse is a serious condition that must be managed in a timely and appropriate manner.
