Fever of Unknown Origin in Dogs

By Tammy Hunter, DVM, Malcolm Weir, DVM, MSc, MPH

What is a fever of unknown origin?

Fever refers to an elevated body temperature. The normal body temperature range for dogs 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. 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 dogs 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 dog has a fever, your veterinarian will perform a thorough physical examination, including looking for problems with the neurological system, the limbs and joints, and your dog’s eyes. Diagnostic blood tests will usually include a complete blood cell count (CBC), a serum biochemistry panel, and a urinalysis. Urine cultures are often performed to search for an underlying urinary tract infection.

Based on your dog's clinical signs, additional tests may be recommended, such as radiographs (X-rays) or ultrasound of the chest or abdomen; radiographs of swollen or sore limbs and joints; cytology (samples from swellings or lumps to look at cell types and to check for infectious agents); echocardiography of the heart (to look for inflamed heart valves); blood cultures (to look for bacteria in the bloodstream); fecal cultures (such as Salmonella infection); serological tests for various infectious agents; and immunological tests looking for immune-mediated diseases.

What causes FUO?

As the name suggests, FUO is a fever without a demonstrable cause. As a result, determining the cause of an unknown fever in dogs can sometimes be difficult.

As outlined, there are many potential causes of a fever of unknown origin. There may be a viral, bacterial, fungal, or parasitic infection in any number of the body’s systems as the root cause of the problem. There may be inflammation due to an immune-mediated problem in the dog’s body. The dog may be reacting to a drug or vaccination or may have an underlying cancer.

It is important to tell your veterinarian about any recent travel or boarding, any potential exposure to unknown or infected animals, any exposure to fleas or ticks, any recent vaccinations, whether your dog has been bred, whether your dog is getting supplements or medications, and any other information that might be helpful to determine what may be causing their fever.

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.

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

Antibiotics are often prescribed to treat any underlying bacterial infection or to prevent bacterial infections from occurring as a secondary problem. Ideally, however, if antibiotics are used, it is best to perform diagnostic testing before to prevent interference with, or masking of test results. Dehydration must also be treated by either subcutaneous or intravenous fluids.

On some occasions, medications to reduce the fever will be given. In people, a fever is often treated with a non-steroidal anti-inflammatory drug (NSAID). For the most part, such drugs do not treat the fever itself, and should be withheld until diagnostic tests are completed. In some cases, however, such as panosteitis or hypertrophic osteodystrophy (inflammatory bone diseases), NSAIDs may be the treatment of choice.

What is the prognosis for a dog 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 dogs respond well to basic supportive care such as keeping them warm and dry, providing plenty of water and nourishment, and administering antibiotics when indicated.

Dogs 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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