Fever of Unknown Origin in Cats

What is a fever of unknown origin?

Fever is a term that refers to an elevated body temperature. The normal body temperature range for cats is between 100.5°F and 102.5°F (38.1°C and 39.2°C). To be classified as a fever of unknown origin (FUO), the body temperature must be above 103.5°F (39.7°C) for longer than a few days in duration, with no obvious underlying cause based on history and physical examination.


What causes a fever?

A fever is initiated by the presence of a pyrogen (a fever-producing substance). The pyrogen can be endogenous (produced from within the body) or exogenous (from the outside). This pyrogen causes the release of substances from the white blood cells (leukocytes) such as interleukin-1, interleukin-6, and tumor necrosis factor. These substances, in turn, reset the body's "thermostat", the temperature-regulating region located in the hypothalamus (located in the brain). This activates physiologic responses within the body to elevate the temperature.

What causes FUO?

As the name suggests, this is a fever without a demonstrable cause.

Most cases of fever in cats are caused by a viral infection such as FeLV, FIV, FIP, feline panleukopenia virus, herpesvirus or calicivirus. Many viral infections will wax and wane before resolution. For example, it is common for a cat with a viral infection to seem completely well and then to experience a relapse a week or two later.

Bacterial infections can also cause a fever, but this is usually accompanied by an obvious wound or swelling. Unusual bacterial infections that are secondary to bites wounds include Yersinia, MycobacteriaNocardia, Actinomyces, and Brucella. The infection may be located in the chest cavity (pyothorax), the kidney (pyelonephritis), the abdominal cavity (from a penetrating intestinal injury resulting in low-grade peritonitis), in the mouth, from a tooth root abscess, etc.

Less commonly, a fever may be secondary to inflammation caused by blunt trauma, lymphoma and other tumors, or a systemic fungal infection. It is necessary to eliminate all of these causes before the diagnosis of FUO is reached.

It is important to tell your veterinarian about any recent travel, any potential exposure to unknown or infected animals, any supplements or medications that you are administering or any other information that might be important.

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