FEVER OF UNKNOWN ORIGIN IN CATS
Description:
A fever is characterized by an elevated body temperature. For cats, the normal temperature range is between 100.5°F and 102.5°F (38.1°C and 39.2°C). A condition is classified as a fever of unknown origin (FUO) when the body temperature exceeds 103.5°F (39.7°C) for an extended period without an identifiable cause based on the cat's history and physical examination.
Causes:
A fever is triggered by the presence of a pyrogen, a substance that induces fever. Pyrogens can be either endogenous (originating within the body) or exogenous (coming from outside the body). These pyrogens prompt the release of substances from white blood cells (leukocytes), such as interleukin-1, interleukin-6, and tumor necrosis factor. These substances, in turn, adjust the body's "thermostat," located in the hypothalamus of the brain, leading to an increase in body temperature.
Complications:
Fever benefits the body by inhibiting the reproduction of viruses and bacteria and enhancing the immune system's response to foreign invaders. However, if the body temperature remains above 105°F (40.5°C) for more than a day or two, the cat may become lethargic, lose its appetite, and rapidly become dehydrated. If the temperature stays above 106°F (41.1°C), complications such as cerebral edema (swelling around the brain), bone marrow suppression, and clotting disorders like disseminated intravascular coagulation (DIC) may arise.
Symptoms:
Cats with a fever often exhibit lethargy, reluctance to move, loss of appetite, increased heart and respiratory rates, and dehydration. They may also shiver or display stiffness. In cases of fever of unknown origin, these clinical signs are present, but no obvious cause can be identified.
Diagnosis:
If your cat has a fever, your veterinarian will conduct a thorough physical examination, specifically looking for evidence of bite wounds, lacerations, punctures, or other signs of trauma. Diagnostic tests 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 may also be performed to check for an underlying urinary tract infection.
Based on your cat's clinical signs, additional tests such as feline infectious peritonitis (FIP) and Toxoplasmosis titers, blood cultures, or thyroid function tests may be recommended. Diagnostic imaging (such as X-rays or ultrasound) or cytology (examining the types of cells aspirated from swellings or lumps) may also be necessary to establish a definitive diagnosis.
Underlying Conditions:
As the name implies, FUO is a fever without an identifiable cause. Most fevers in cats are due to viral infections like FeLV, FIV, FIP, feline panleukopenia virus, herpesvirus, or calicivirus. Many viral infections may fluctuate before resolving. For instance, it is common for a cat with a viral infection to appear healthy and then experience a relapse a week or two later.
Bacterial infections can also cause fever, but these typically involve an obvious wound or swelling. Unusual bacterial infections secondary to bite wounds include Yersinia, Mycobacteria, Nocardia, Actinomyces, and Brucella. The infection may be located in the chest cavity (pyothorax), kidney (pyelonephritis), abdominal cavity (from a penetrating intestinal injury resulting in low-grade peritonitis), mouth (from a tooth root abscess), etc.
Less frequently, fever may result from inflammation caused by blunt trauma, lymphoma, other tumors, or a systemic fungal infection. All these potential causes must be ruled out before diagnosing FUO. It's important to inform your veterinarian about any recent travel, possible exposure to unknown or infected animals, supplements, or medications your cat is receiving, and any other relevant information.
Treatment:
The diagnostic work-up for FUO can be extensive. Your veterinarian will attempt to identify a specific cause for the fever to provide the correct treatment and an accurate prognosis. If a diagnosis cannot be made, it is reasonable to diagnose FUO and begin treatment based on the available information.
Antibiotics are often prescribed to treat any underlying bacterial infection or to prevent secondary bacterial infections. Dehydration should be addressed with subcutaneous or intravenous fluids.
In rare cases, medications to lower the fever may be administered. Although fever in humans is often treated with non-steroidal anti-inflammatory drugs (NSAIDs), there are only a few, recently developed NSAIDs that are safe for cats. Aspirin (Acetylsalicylic acid) and Tylenol (Acetaminophen) are highly toxic to cats and should never be given without direct veterinary supervision.
Prognosis:
An accurate prognosis can only be provided when the cause of the condition is known. With FUO, the exact cause is unknown, so a precise prognosis cannot be given. Most cats respond well to basic supportive care, such as keeping them warm and dry, ensuring they have plenty of water and nourishment, and administering antibiotics when necessary. Cats with persistent or fluctuating fevers require a thorough examination to determine and treat the cause of the fever before irreversible damage occurs. If this pattern repeats, it is crucial to undertake a comprehensive diagnostic work-up.
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