Fever of Unknown Origin
Cornell Feline Health Center
Cornell University College of Veterinary Medicine
Ithaca, New York 14853
The normal feline body temperature ranges between 100.50 and 102.50 F, a few degrees higher than the normal range for humans. A prolonged reading significantly higher than that should be of concern to a cat’s owner, since a persistently elevated body temperature can result in severe dehydration, significant brain damage, and other lethal consequences.
A high temperature usually signals one of two conditions:
- the cat is experiencing hyperthermia, in which the abnormal reading is typically, though not always, caused by an external environmental influence, such as confinement within a closed automobile during hot weather
- the cat is running a fever, a rise in temperature generated by a portion of the brain as a beneficial natural defense against an invasion of pathogenic microorganisms—disease causing bacteria or viruses, for example, which have difficulty thriving in a warm environment
In most cases, the cause of either hyperthermia or fever can be discerned with relative ease by a veterinarian. And once the underlying cause is identified and appropriately treated, the temperature will return to its normal level.
There is, however, a third category of elevated body temperature—commonly referred to as “fever of unknown origin” (FUO)—that is more problematic. The clinical signs of FUO are the same as those accompanying any type of fever. That is, an afflicted cat will typically be lethargic, anorexic and dehydrated; may breathe rapidly or with its mouth open, perhaps even panting; and may have an increased heart rate. With most fevers, however, these clinical signs will soon dissipate and disappear once the source of the high body temperature is discerned and appropriately treated. In cases of FUO, however, the signs may persist for several weeks or more—with possible life-threatening consequences—because their source remains a mystery.
Diagnosing the cause of FUO presents a formidable challenge to any veterinarian, says Christine Bellezza, DVM, former Co-Director of the Cornell Feline Health Center at Cornell University’s College of Veterinary Medicine. “It can be a huge hunt,” she notes, “because to identify the source of the fever, you have to rule out every other possible cause.” The vast range of causes, she notes, includes bacterial, viral, and parasitic infections; inflammatory diseases, such as pancreatitis; adverse reactions to many drugs such as tetracycline; and the presence of such immune-mediated diseases as anemia, rheumatoid arthritis, and meningitis. Fever may be found in cats afflicted with cancer, and the ingestion of poisonous chemicals will also cause an alarming rise in a cat’s body temperature. Young cats with persistent or recurring fevers may have feline infectious peritonitis (FIP).
In searching for the cause of any dramatic rise in body temperature, a thorough veterinary examination will include a complete blood count and blood chemistry profile; urinalysis; fecal examination, and tests for the presence of a variety of infectious diseases, such as the feline immunodeficiency virus (FIV) and the feline leukemia virus (FeLV). The veterinarian will also examine the patient carefully for any signs of lacerations or bite wounds that might have become infected. Owners should be aware, however, that the diagnosis of FUO may require a great deal of time and that repeated and specialized testing may be necessary to identify the source of an animal’s fever. Tests to consider include blood and urine cultures; biopsy of suspicious swellings; bone marrow examination; serologic or PCR tests for toxoplasmosis, fungal diseases, tick borne diseases and heartworm disease; cerebrospinal fluid analysis; chest and abdominal radiographs; abdominal ultrasound examination and an echocardiogram. . In some cases, despite extensive testing, a conclusive diagnosis may not be reached. Despite the absence of such a diagnosis, a treatment program may be launched that could include intravenous fluids and a therapeutic antibiotic trial. The signs that a cat may be harboring a fever-causing ailment—lethargy, weakness, anorexia, troubled breathing, and so forth—call for immediate attention. Dr. Bellezza encourages owners to ask their veterinarians for instructions on monitoring a cat’s temperature at home if such signs become evident. Owners may also benefit by viewing an instructional video, produced by the Cornell Feline Health Center, which is available online at http://partnersah.vet.cornell.edu/pet/fhc/taking_temperature.
By Tom Ewing
December 20, 2010
