Hyperthermia or Pyrexia-why it matters.

Following on from this spectacularly sunny weekend there was a lot of discussion on FB about heatstroke and cooling. Let’s recap what Pyrexia is and what heatstroke is, because what we do varies a lot between the two.

First up Pyrexia a rectal temperature of 39.2°c or more, when at rest (Packman 2015)

Pyrexia is a result of the hypothalamus resetting the bodies thermoregulation point. A bit like the thermostat being on the blink. Inflammatory markers can alter this thermoregulation point so infections or immune mediated disease can affect the thermoregulation (Packman 2015)

Hyperthermia is a little different, still the body’s core temperature is too high but it’s normally because the animal has become hot. Through being stuck in a hot car or being walked during the hottest time of the day resulting in heatstroke. Exercised induced hyperthermia and malignant hyperthermia can also be life threatening. Similarly, hyperthermia can result from seizures, increased muscle activity raises the body’s temperature.

Heatstoke and Active Cooling

Signs of heatstroke can include panting, shivering, collapse, lethargy and/or inappetence. It is crucial to ensure owners are aware of these signs and are able to prevent heatstroke from occurring. Simple steps like avoiding the hottest part of the day and not leaving their dog in a car.

In cases of severe hyperthermia/heatstroke the patient needs to be cooled quickly, organ damage can occur, and heatstroke can be fatal. Prolonged temperatures above 41°c can result in blindness, ataxia or even a coma (Vite and Galban 2018)

Also, there is risk of organ damage, kidney, liver, gastric ulcers and even brain damage can result (Packman 2015) Due to this biochemistry should be performed once the patient is stable. On-going it is recommended bloods are rechecked a week after the event (Packman 2015) NSAIDS should not be used due to risk of renal damage and can worsen GI ulceration.

Methods to cool these patients include cold water enemas, cold baths and fans (Packman 2015) If using towels these must be replaced often and not allowed to warm up. Be cautious not too over cool and stop active cooling once the temperature is 39.4°c (Packman 2015)

Often in simple cases, such as a patient arriving after a long car journey during summer. Patients can be monitored and allowed to cool down in a cool and calm environment. Monitoring of these patients is vital, every 20 minutes where possible.

Pyrexic- What to do?

In contrast patients that are Pyrexic should not be actively cooled unless their temperature reaches 41.1°c. If you actively cool below this point you increase the body’s metabolism. This is because the thermostat is set wrong and the body believes it needs to be that temperature. So, you cause your patient to use up more energy.

Consider when you have the Flu, you feel hot, you have the shakes, but you aren’t going to go and sit in a cold bath because your temperature is your body’s natural reaction. However, when you are holiday, and you’ve sat in the sun for hours you go and jump in the pool.


Sarah Packman, 2015. Chapter 18, pp 202 – 206 BSAVA Manual of Canine Practice

Charles Vite and Evelyn Galban, 2018 ‘Neurological Emergencies’ in BSAVA Manual of Canine and Feline Emergency and Critical Care 3rdEd.

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