Backcountry health & safety

Hyperthermia (heat illness)

by Susan Bradford, Ph.D., Richard Fantozzi, M.D., and Bill Fantozzi, Wilderness Remote and ER Instructor

Heat exhaustion and heat stroke can occur when high ambient temperatures overcome the body's natural ability to dissipate heat. Heat illness is primarily a problem when the temperature exceeds 90°F and when the relative humidity exceeds 80% (13). The level of physical exertion and poor acclimatization are factors in heat illness. Use of certain medications and heavy clothing may increase chances of heat illness (13).

Heatstroke is the most severe form of the heat-related illnesses and is defined as a body temperature higher than 41.1°C (106°F) associated with neurologic dysfunction.

Exertional heatstroke generally occurs in young individuals who engage in strenuous physical activity for a prolonged period of time in a hot environment. Nonexertional heatstroke is more commonly affects sedentary elderly individuals, persons who are chronically ill and very young persons during environmental heat waves.

Morbidity and mortality from heatstroke are related to the duration of the temperature elevation. When therapy is delayed, the mortality rate may be as high as 80%; however, with early diagnosis and immediate cooling, the mortality rate can be reduced to 10%. Older adults, young children, and persons with chronic medical conditions are particularly susceptible to these illnesses and are at high risk for heat-related mortality.


Heat exhaustion, caused by excessive loss of body fluids, causes symptoms that include weakness, nausea, anxiety, profuse sweating, and fainting. The body temperature ranges from 102-104F (11). The victim’s skin is grayish and clammy. The victim’s pulse is slow and their blood pressure is low.

Heat stroke is caused by failure of the body’s heat loss mechanism and affects the brain (11). Victims will exhibit an altered sensorium (inability to think clearly or concentrate). Symptoms can include headache, weakness, lack of sweating (anhidrosis) and sudden loss of consciousness. The victim’s skin is hot and red; pulse is rapid with high body temperature (>104F). This temperature criterion is relative in cases when the measured temperature is lower than 104F because the victim was allowed to cool down prior to measurement of the temperature, as may occur during transportation in a cool ambulance or evaluation in an emergency department. Similarly, some patients may retain the ability to sweat, removing anhidrosis as a criterion for the diagnosis of heatstroke.

Recognizing heat illness in dogs

Handlers of SAR K9s must be vigilant to prevent heat illness in their canine partners. According to the New Mexico Mountain Canine Corps, a medium size dog will drink more than a human during wilderness activities. (14)

Heatstroke or hyperthermia may be defined as moderate if the dog’s body temperature is between 104 to 106F according to Drs. Foster & Smith (15). The dog may recover readily if promptly treated.

Severe hyperthermia, defined as temperatures exceeding 106F, may cause permanent organ damage or death (15). A dog that has had hyperthermia may be more prone to subsequent heatstroke events in the future.

Symptoms of heat stroke may include panting and thick salvia. The dog’s gums may be either red or pale, and its tongue may be red in color (15). More severe symptoms may include vomiting, dizziness, diarrhea and coma.


Management of heat exhaustion includes treating the victim for fainting by keeping their head down and replacing lost electrolytes and fluids.

Management of heat stroke includes emergency cooling with water and ice and if necessary hospitalization.

Management of hyperthermia in canines includes carefully reducing the dog’s temperature (normal temperature ranges from 100-102.5F). Do not overcool the dog using ice cold water. If possible check the rectal temperature to determine the efficiency of cooling. Veterinary care may be required after hyperthermia.


Prevention of heat illness may include not hiking or climbing when the temperature and humidity is high. It is important not overexert oneself. Watch for signs and symptoms including dry mouth and thirst. Heat cramps can also occur and are the result of electrolyte depletion (13). The victim may also have spasms in their extremities (13). The importance of proper nutrition and physical conditioning prior to wilderness activities cannot be overstressed to help avoid medical emergencies. Preparation of both the handler and dog as a team is essential.

Prevention of hyperthermia in canines is similar to that prescribed for humans. In the wilderness, water with electrolytes should be provided every 15 minutes. The amount of fluid taken in should be noted. It is useful to know the dog’s normal body temperature and rate of respiration. Watch for signs and symptoms of heatstroke, and be prepared to quickly begin to cool the dog down.


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San Diego, California