Hyperthermia
An animal’s core body temperature must be maintained within narrow limits to sustain normal metabolic functions. When it rises above these limits the condition is known as hyperthermia; when it falls below the limits the condition is known as hypothermia. Both situations are life-threatening. Conscious animals use a range of temperature-regulating mechanisms to maintain thermal stability, e.g. shivering when cold and panting when hot. However, many anaesthetic drugs interfere with these thermoregulatory mechanisms and tigers anaesthetised in extreme hot or cold conditions are vulnerable to dangerous changes in core temperature.
Hyperthermia during anaesthesia can result from high environmental temperatures, exposure to direct sunlight, convulsions, excessive pre-anaesthetic excitement or activity, bacterial and viral infections or even being confined in in poorly ventilated spaces such as box-traps in high ambient temperatures. Often, several of these factors operate at the same time in the conditions of field anaesthesia. For example, an animal held in a snare for any length of time on a hot day, with no shade or water may become seriously hyperthermic, especially if further excited by too much noise or disturbance immediately prior to an anaesthetic. As anaesthetic drugs may abolish normal thermoregulation, an animal which is moderately hyperthermic immediately prior to anaesthesia may very rapidly become dangerously overheated shortly after induction unless steps are taken to cool it down, especially on hot, windless days. Obese, very young and very old individuals are more prone to developing hyperthermia. Prior to capture attempts, be sure that necessary equipment and supplies are available to treat hyperthermia. Untreated, hyperthermia can lead to brain damage and death from cellular hypoxia. In less extreme cases a prolonged recovery from anaesthesia can be expected.
Signs:
The normal rectal temperature of a tiger lies between 37.8 degrees C. and 39.4 degrees C. (100 – 103 degrees F). Hyperthermia is considered to exist if the body temperature is above 40.0 degrees C (104 F). Hyperthermic animals will feel hot to the touch (especially the extremities); exhibit rapid, shallow breathing; and in severe cases, have a rapid and/or irregular heartbeat and may convulse.
Action:
- Do not give any further anaesthetic drugs. Consider reversing any alpha-2 agonists such as medetomidine or xylazine with atipamezole intravenously, but be aware of the consequences. Reversal may allow the animal to restore its natural temperature regulating mechanisms.
- The most effective way to lower body temperature is to immerse the tiger in water. However, this will generally not be practical for adult animals.
- Move the animal into the shade or create shade over it.
- Place the tiger on its back, and spread the legs apart.
- Cool the body (especially the abdomen, groin and feet) with water or alcohol. (Alcohol is more effective but shorter acting). Apply the water and/or alcohol frequently to maintain a large temperature difference between the animal’s core and its surface. In winter snow can be used instead of water. Alternatively pack cold water bags against the skin of the abdomen, groin and head.
- Fan rapidly – use anything at hand to create a breeze close to the wet skin
- Control any convulsions with small doses of midazolam intravenously
If these procedures do not bring the animal’s temperature down:
- Give a cold-water enema. Gently insert a soft plastic tube (such as a foal gastric tube) 10 – 20 cms into the rectum. Lubricate it well to avoid damaging the lining of the rectum. Introduce cold water into the rectum by gravity using a funnel attached to the free end of the tube and raising it above the animal. Leave water in rectum for 5 minutes, then drain. Repeat until 10 litres of water have been used. Wait 5 minutes and recheck the temperature.
- Repeat enema if temperature does not go back down to the normal range.
For the more experienced field vet other treatment options include:
- Slow administration of intravenous fluids at a temperature lower than normal (minimum 35 degrees C advised)
- Instilling cool water into the bladder via a urinary catheter. Leave for 5 minutes, drain and repeat a number of times. NB: Catheterisation of males is far easier than females.
NB: To avoid further increases in body temperature, overheated animals should never be enclosed in a box.
