Fever of Unknown Origin in Cats

By Tammy Hunter, DVM, Malcolm Weir, DVM, MSc, MPH; Ernest Ward, DVM

What is a fever of unknown origin?

Fever 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, 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 produced within the body (endogenous) or from outside the body (exogenous). 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, within the brain. This activates physiologic responses within the body to elevate the temperature.

If this is a natural process, why is it a problem?

A fever is beneficial to the body because it hampers the ability of viruses and bacteria to reproduce and improves the immune system response to foreign invaders. However, if the body temperature remains above 105°F (40.5°C) for more than a day or two, the patient becomes lethargic and anorexic, and can rapidly become dehydrated. If the temperature persists above 106°F (41.1°C), cerebral edema (swelling around the brain), bone marrow suppression, and clotting disorders (disseminated intravascular coagulation, or DIC) may develop.

"When a fever persists, several consequences occur within the body."

When a fever persists, several consequences occur within the body. Among these are increased metabolic demands to maintain the higher temperature, which causes increased fluid and caloric requirements and increased breakdown of muscle tissues. A persistent high fever (one that lasts for more than 48 hours) is considered serious and potentially life threatening.

What are the clinical signs of fever of unknown origin?

Most cats with a fever are lethargic, reluctant to move, have a loss of appetite, have increased heart and breathing rates, and are dehydrated. They may also be shivering or showing stiffness. With a fever of unknown origin, these clinical signs are present but there is no obvious cause for them.

How is FUO diagnosed?

If your cat has a fever, your veterinarian will perform a thorough physical examination, looking specifically for evidence of bite wounds, lacerations, punctures, or other signs of trauma. Diagnostic blood tests will usually include a complete blood cell count (CBC), a serum biochemistry panel, feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) testing, and a urinalysis. Urine cultures are often performed to search for an underlying urinary tract infection.

Based on your cat's clinical signs, additional tests may be recommended, such as feline infectious peritonitis (FIP) and Toxoplasmosis titers, blood cultures, or thyroid function tests. Diagnostic imaging (such as X-rays or ultrasound), or cytology (looking at the cell types aspirated from swellings or lumps) may also be needed to establish a firm diagnosis.

What causes FUO?

As the name suggests, FUO 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 these are usually accompanied by an obvious wound or swelling."

Bacterial infections can also cause a fever, but these are usually accompanied by an obvious wound or swelling. Unusual bacterial infections that are secondary to bites wounds include Yersinia, Mycobacteria, Nocardia, 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, and any other information that might be important.

How is FUO treated?

It is important to understand that the diagnostic work-up for FUO may be quite involved. Your veterinarian will search for a specific cause of the fever so that it can be correctly treated and so that an accurate prognosis can be given. In cases where a diagnosis is unobtainable, it is reasonable to make a diagnosis of FUO and initiate treatment based on the available information.

Antibiotics are often prescribed to treat any underlying bacterial infection or to prevent bacterial infections from occurring as a secondary problem. Dehydration must also be treated by either subcutaneous or intravenous fluids.

"It is important to understand that the diagnostic work-up for FUO may be quite involved"

On rare occasions, medications to reduce the fever will be given. In people, a fever is often treated with a non-steroidal anti-inflammatory drug (NSAID). However, there are only a few, recently developed non-steroidal anti-inflammatory drugs (NSAIDs) that cats can tolerate. Acetylsalicylic acid (Aspirin®) and acetaminophen (Tylenol®) are extremely toxic to cats and should never be administered by a pet owner without the explicit direction of a veterinarian.

What is the prognosis for a cat diagnosed with FUO?

An accurate prognosis can only be given when the cause of a condition is known. By definition, with FUO the exact cause is unknown; therefore, an accurate prognosis cannot be given. Most cats respond well to basic supportive care, such as keeping them warm and dry, providing plenty of water and nourishment, and administering antibiotics when indicated. Cats that have persistent fever or a fever that waxes and wanes must undergo a thorough work-up so that the cause of fever can be discovered and treated before irreversible damage occurs. In cases where this pattern repeats more than once, it is imperative that a thorough diagnostic work-up is undertaken.

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