Pyrexia in pets

What It Is

Pyrexia is a rise in core body temperature above the normal range for that species, driven by the body deliberately resetting its internal thermostat higher to help fight infection or inflammation. This differs from heatstroke or hyperthermia, where the body overheats from environmental heat or exertion rather than from an internal immune signal, and the treatment for the two is very different. Normal temperature varies by species, so dogs and cats typically run warmer than adult humans, birds run warmer still, and reptiles depend heavily on their environment rather than generating fever in the same way mammals and birds do.

Common Causes

Bacterial and viral infections are the most frequent cause of fever across species, followed by inflammatory conditions, abscesses, and immune system disorders that trigger fever without a clear infection. Vaccination can cause a mild, short-lived temperature rise in the day or two afterward. In livestock and poultry kept in groups, a fever in one animal often signals the start of a contagious problem that could spread through the rest of the herd or flock.

When It Is an Emergency

A very high fever, a fever that will not come down, or fever combined with weakness, breathing difficulty, or collapse all need same-day veterinary care. Young animals and small mammals such as rabbits and guinea pigs can decline quickly once a fever sets in and should be seen promptly rather than monitored at home. Any suspected fever in a reptile is unusual enough, given how these animals regulate temperature, that it should be checked rather than assumed to be environmental.

What the Vet Will Do

The vet will take an accurate temperature, examine the animal for a source of infection or inflammation, and run bloodwork to look for signs of infection or organ involvement. Further testing, such as imaging or specific infectious disease tests, depends on what the exam and initial results suggest. Fluid support and temperature management may start before a definitive diagnosis is reached if the animal is unwell.

Home Care and Prevention

Do not give human fever medication to a pet, since many common over-the-counter drugs are toxic to animals even at low doses. Offer fresh water and a cool, quiet space, and take a rectal temperature if you have a suitable thermometer and know your pet's normal range. Keep vaccinations current where relevant and contact a vet for guidance rather than waiting to see whether a fever resolves on its own.