Fever of Unknown Origin in Dogs

Fever, whether it is in animals or humans, is usually a symptom of an underlying infection or some disease conditions, including inflammations and cancers. It may result from autoimmune disorders too. In addition to physical examination and a detailed medical history, laboratory tests and other diagnostic techniques may be required to determine the underlying cause. Occasionally, when the cause of recurrent fever in certain animals cannot be diagnosed in spite of several tests, it may be referred to as fever of unknown origin. Undetected infections, cancer growth or immune related disease conditions may be causing this kind of fever in dogs.

Regulation of Body Temperature

The hypothalamus in the brain has a temperature regulating mechanism; it works like a thermostat, almost always keeping the temperature within the normal range. This thermoregulation is achieved by setting off metabolic activities in the body that result in either the production of heat or the loss of it. The thermostatic setting of the hypothalamus is changed to a higher setting at the time of true fever.

But there are certain conditions when temperature of the body is raised beyond the normal range without the resetting of the thermostat. Examples are heatstroke, a condition called malignant hyperthermia, and seizures that are not considered true fever, even though the body temperature is high. These conditions can result in dangerously high temperatures which go above 1060F or 41.10C in severe cases. On the other hand, body temperature during true fever remains within a 103 to 106°F or 39.5 to 41.1°C range.


Fever of unknown origin typically takes a long time to diagnose. It takes several laboratory tests and other diagnostic investigations because the diagnosis is reached after a process of elimination that rules out all other possible causes.

The veterinarian gathers the detailed medical history of the dog, and then conducts a thorough physical examination. Eyes are examined carefully, and the dog is observed closely for signs of neurologic involvement. A review of the vaccination status of the dog, the measures taken for parasitic control, history of travel, previous illnesses and medications, illnesses and disease conditions found in the animal and human members of the owner’s household etc., is done. Routine blood and urine tests, as well as x-rays of the abdomen and the chest, are conducted.

Depending on the results of routine tests, the veterinarian may decide on further tests and investigations to be conducted. The physical examination may be repeated, and the most probable reasons for a fever of unknown origin are explored. Specific tests for various infectious diseases, and testing of the fluid drawn from the joints are also done. Other tests include ultrasound scanning of the chest, abdomen and the heart, biopsies of tissue samples of lymph nodes and bone marrow, culture of blood, urine, joint fluid, cerebrospinal fluid etc. CT scans or MRI scans, or in some cases, exploratory surgery also may be done.


When the cause of the fever of unknown origin cannot be determined even after extensive testing, further investigations are stopped, and several experimental treatments are tried one after the other. Antibiotic therapy, antifungal treatment, immunosuppressive and anti-inflammatory medications etc., may be tried. In many cases, these experimental treatments may help resolve the fever, and in some cases, it may even lead to the confirmation of a tentative diagnosis. However, in view of the risk associated with experimental treatments, the dog is kept under observation, and its responses to the various trial treatments are closely monitored.

When the body temperature is controlled by the thermoregulatory mechanism of the body, as it is in the case of true fever, it may not be necessary to use cooling baths or other such measures to bring the temperature down. It may actually go against the natural healing mechanism of the body, as the elevated temperature may have some benefits in overcoming infections. But when the dog has fever, it may have other associated symptoms such as low energy levels or severe loss of appetite. Fever can have severe dehydrating effect too. Intravenous administration of fluids, as well as drugs to reduce fever, may be given to dogs having fever of unknown origin.

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