A fever is a body temperature higher than the normal range. In dogs and cats, normal body temperature is approximately 100-102.5 degrees Fahrenheit. Fever of unknown origin (FUO) is diagnosed when the fever is chronic or recurs frequently and physical examinations and routine laboratory tests have not identified the source of the fever.

The most common causes of fever are infection, immune disease, inflammation and cancer. Infectious causes include, but are not limited to, tick related infections, viruses, bacterial or fungal infections, and infections with protozoal agents and blood parasites. Inflammatory immune-mediated causes include drug or vaccine reactions and immune-mediated diseases of joints and blood. Any type of cancer can cause fever from release of inflammatory chemicals or destruction of tissues by tumor cells.

Clinical signs depend largely on the underlying cause, but are often vague and nonspecific. Many patients are lethargic and have a poor appetite; they may lose weight and may become dehydrated. These patients tend to be ill for several weeks as the fever persists and the underlying cause goes unidentified. Often multiple tests are necessary to determine the cause of the FUO. These can include x-rays, blood cultures, abdominal ultrasounds, blood tests, and specialized tests for common tickborne, fungal and protozoal diseases in your geographic area.

Once the cause is identified, specific treatment is aimed at the underlying cause. In some cases of FUO, even after many diagnostic tests, a specific cause for the fever is not identified. In these cases, a trial may be started with broad-spectrum antibiotics, antifungal agents or anti-inflammatory drugs. Follow-up visits and repeat testing depend on the underlying cause, the type of treatments instituted and the response to that treatment. Prognosis is good if a treatable cause is identified and the animal responds to treatment. Certain causes of FUO are untreatable and may pose a health risk to humans.


Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.

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