They might be on opposite ends of the spectrum, but hyperthermia and hypothermia in horses are more alike than one might think. At the 2013 Western Veterinary Conference, held Feb. 17-21 in Las Vegas, Nev., Amelia S. Munsterman, DVM, MS, Dipl. ACVS, ACVECC, reviewed these two equine environmental emergencies and how to best manage affected horses.
Munsterman, a clinical lecturer in equine emergency and critical care at the Auburn University College of Veterinary Medicine, explained that both conditions result from circumstances that overwhelm horses' normal thermoregulatory mechanisms.
"Both conditions result in an aberrant core temperature, accompanied by an exaggerated acute phase inflammatory response and multi-organ failure," she said.
She noted that several factors can predispose a horse to developing hyperthermia and hypothermia, including:
- Age;
- Illness;
- Metabolic disorders;
- Injury;
- Electrolyte imbalances; and
- Lack of acclimatization in a new climate.
Munsterman reviewed each condition and described treatment options for a veterinary audience.
Hyperthermia
By definition, hyperthermia describes a core body temperature that exceeds a horse's thermoregulatory set point; essentially, it's an elevated body temperature. Extreme hyperthermia is referred to as heat stroke, Munsterman said, and occurs when the horse's body temperature rises to about 105.8°F. In compromised horses, she noted, the temperature at which an animal develops heat stroke can be lower. A combination of heat and humidity can place horses at risk for developing hyperthermia, Munsterman said.
Normally, when a horse becomes hyperthermic, his blood temperature rises, Munsterman said. This change alerts the hypothalamus (the part of the brain that operates much of the autonomic nervous system, including body temperature regulation) that the horse is hot and needs to be cooled, and the animal's natural cooling mechanisms—including sweating and exerting heat via the respiratory tract—kick in.
However, problems occur when the horse's compensatory mechanisms are overwhelmed and heat stroke sets in. At this point, and through a variety of physical pathways, the horse essentially develops an uncontrolled and overwhelming inflammatory reaction, called the systemic inflammatory response syndrome (SIRS). If left unchecked, SIRS can lead to multiple organ failure, including the kidneys, liver, and lungs.
Clinical signs of heat stroke include:
- Increased core body temperature;
- Central nervous system dysfunction (including depression, agitation, seizures, and coma);
- Tachycardia (increased heart rate);
- Tachypnea (increased respiratory rate);
- Systemic hypotension (low blood pressure);
- Electrolyte abnormalities; and
- Metabolic derangements.
Systemic complications of heat stroke include:
- Encephalopathy (brain disorders or disease);
- Rhabdomyolysis (tying up);
- Acute renal failure;
- Myocardial (heart muscle) injury;
- Liver dysfunction;
- Intestinal ischemia (lack of blood flow to parts of the intestines); and
- Coagulopathy (blood's ability to clot is impaired).
Munsterman suggested veterinarians perform a complete blood count, serum chemistry (including a measure of liver function), urinalysis, and coagulation monitoring on horses with heat stroke.
He said that when dealing with a hyperthermic horse, the No.1 treatment goal is to return the animal's body temperature to around 101.3°, at which point the hypothalamus can take over and continue the cooling process.
"First and foremost, move the horse out of the sun, and provide both free-choice water and an electrolyte solution of the horse has a normal mentation," Munsterman said. Next, attention should be turned to cooling the horse.
Munsterman cautioned that during cooling, it's important to reduce shivering and vasoconstriction (narrowing of the blood vessels) using massage or a tepid water bath, as these can be counterproductive to the cooling process.
"Cold water, even cool water can be recognized by the hypothalamus due to skin receptors, causing shivering to counter the cold the animal is feeling," Munsterman explained. "It is an effort to prevent simulation of the skins thermoreceptors. Shivering will only increase heat production."
Munsterman discussed several cooling methods that can be used on hyperthermic horses:
- External—One external cooling method involves applying cool water (much like a rider would when hosing a horse out after a ride) or cool wet towels to the horse. Because wet towels and hair coat can actually be insulators, change towels often and scrape water from the horse's coat regularly. Another external cooling method is using fans, often in conjunction with cool water application, Munsterman noted. Additionally, she cautioned that while adding isopropyl alcohol can aid in cooling, "the fumes can be extremely dangerous for both horses and humans."
- Internal—In cases of extreme heat stroke, Munsterman said an internal lavage is more effective than external cooling methods. Cooling lavages are often gastric or rectal in nature, although Munsterman noted that a peritoneal (abdominal) cooling lavage can be used in sterile conditions. She stressed that horses' electrolyte and protein levels should be monitored carefully during lavages.
Watch for and treat potential related problems and complications, such as hypotension, seizures, rhabdomyolysis (tying-up), or laminitis, she said.
Expect the horse to get cooler than you intended, Munsterman said, and also expect a fever several hours, days, or even weeks later. The fever, she noted, actually indicates the hypothalamus is working properly. Many horses that don't develop a fever eventually die, she cautioned.
"The prognosis after heat stroke is variable, based on the severity of the hyperthermia and organ systems involved," Munsterman said.
Munsterman noted that owners can reduce the risk of heat stroke by properly acclimatizing horses (or helping them adapt) to new or changing environmental conditions, a process that can take anywhere from a few weeks to several months.
Hypothermia
On the other end of the spectrum, hypothermia is defined as a core body temperature below normal limits, Munsterman said. It's caused by exposure to cold temperatures (be it air, water, or mud, she said) coupled with a predisposing factor that inhibits the normal physiologic response, such as:
- Old or very young age;
- Endocrinopathic disorders;
- Malnutrition;
- Neuropathies or trauma; and
- Sepsis.
Horses with hypothermia can be "grouped" into grades, based on their body temperature:
- Mild—a body temperature of 89.6-98.6°F;
- Moderate—a body temperature of 82.4-89.6°F; and
- Severe—a body temperature of less than 82.4°F.
When horses are exposed to cold, their hypothalamus sends signals to activate the body's warming mechanisms, such as shivering. As hypothermia progresses, the warming system is essentially overwhelmed and organ dysfunction sets in.
"The progression of multi-organ failure has been well-documented in other species, and these descriptions may apply to the horse, as well," Munsterman said.
Clinical signs of hypothermia include:
- Low body temperature;
- Cardiac dysfunction;
- Respiratory dysfunction;
- Central nervous system abnormalities;
- Renal dysfunction;
- Coagulopathy; and
- Ileus (lack of gastrointestinal motility), among others.
"Treatment for hypothermia revolves around returning the core temperature to closer to normal," Munsterman said. To achieve this, there are several methods veterinarians can choose from, depending on each patient's individual circumstances:
- Passive external rewarming—This method, which relies on the horse's ability to produce heat via shivering, is only effective in mild cases of hypothermia, Munsterman said. It generally involves wrapping the animal in a blanket and sheltering his body from further cooling with by a tarp, which helps prevent further heat loss. She noted that using this method alone is generally ineffective when dealing with severe hypothermia.
- Active external rewarming—This option uses forced warm air, warm water baths, heating pads, hot water bottles, or heat lamps to encourage warming, she said. However, these methods can be counterproductive in some cases, she said, as they can cause vasodilation and increased circulation to the extremities, pulling heat away from the core. She also noted that active external rewarming can burn the horse if not applied properly or carefully. This method is often used in conjunction with another rewarming option.
- Active internal rewarming—Munsterman said active internal rewarming is commonly combined with an external rewarming method. Active internal rewarming options include warmed intravenous (IV) fluids and body cavity lavage, similar to that used for hyperthermic horses.
Munsterman advised monitoring the horse's body temperature closely monitored via a rectal thermometer and discontinuing rewarming efforts when the body temperature reaches about 95°F. At that point, she said, the body can take over and the horse can warm up on his own, provided passive rewarming techniques are continued.
Munsterman noted that during rewarming, horses' organ function should be assessed regularly. Additionally, horses should be started on IV fluids to replace fluid losses. Most horses will require supportive care as well.
Complications from hypothermia include sepsis, pneumonia, and compartment syndrome (increased pressure in muscle compartments), Munsterman said.
Munsterman said the prognosis, while temperature dependant ("More severe cases will have more fatalities," she said), is often excellent for survival.
Take-Home Message
Hyperthermia and hypothermia are environmental emergencies that require aggressive and immediate medical attention. The prognosis is variable; however, many horses survive.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.
