From Janet Tobiassen Crosby, DVM,
Your Guide to Veterinary Medicine.
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Definition: Pyrexia is a fairly common symptom for both pets and people; it may be a life-threatening situation or not much to worry about.

Pyrexia is more commonly known as fever, and may be mild, like what is seen at the onset of a viral infection (i.e. human cold), or severe, causing seizures or organ shut down.

Normal body temperature for dogs and cats ranges from 100.5 - 102.5 degrees Fahrenheit (38 - 39.2 Celsius). Temperatures above 103F are considered to be a fever. It is important to note that extreme stress or excitement (such as being nervous at the veterinary clinic or from the car ride to the clinic) may push some animal's temperature to the range of 103F, even if they are not feeling ill at home. History and physical examination are important tools to rule out early fever or possible illness.

Infection, either by viral or bacterial pathogens, is the most common cause of pyrexia in dogs and cats. Other causes of elevated body temperature include heatstroke, certain drugs or toxins, seizures, or head injury. Some cancers are also treated by inducing a fever.¹

Pyrexia is determined in dogs and cats by taking a rectal temperature. Pyrexic animals may be depressed and suffering from a loss of appetite.

Treatment is directed at what is causing the pyrexia, i.e. antibiotics for a bacterial infection, etc. In the case of extremely elevated temperatures, such as heatstroke, a cool water bath and enemas may be administered.

As always, if you suspect that your pet is sick or has a fever, please call your veterinarian immediately to assess.

References:
¹ Hyperthermia (fever) - Chicago Manual Style (CMS): hyperthermia. Dictionary.com. The American Heritage® Science Dictionary. Houghton Mifflin Company. http://dictionary.reference.com/browse/hyperthermia (accessed: March 29, 2008).

Veterinary Glossary Index

Pronunciation: pie-rex-e-ya

Also Known As: fever, hyperthermia

Common Misspellings: pirexia

Examples: The dog became pyrexic after the owner stopped the antibiotics prematurely and the infection spread throughout the body.