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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 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.