In both animals and people, fever may indicate infection, inflammation, an immune-mediated disease, or cancer. Determining the cause of a fever requires a history, physical examination, and sometimes laboratory or other diagnostic tests. Often, a fever resolves on its own or in response to antibiotic therapy. However, in a small percentage of animals, the fever continues or keeps coming back and the cause cannot be determined. This is called fever of unknown origin. In cats, infections are the most common cause.
Body temperature is regulated by an area of the brain called the hypothalamus. The hypothalamus acts as a thermostat to maintain temperature as close to normal as possible, and it sets off activities in the body that influence heat production, heat loss, and heat gain. In true fever, the thermostat is reset to a higher temperature.
In certain conditions, such as heat stroke, seizures, or malignant hyperthermia, the body temperature is increased above the normal range. This it is not considered a true fever, however, because the thermostat is not reset. Depending on their severity, these conditions can potentially result in dangerously high body temperatures of 106°F (41.1°C) or higher. In comparison, most patients with true fever have body temperatures in the range of 103 to 106°F (39.5 to 41.1°C).
The diagnosis of fever of unknown origin can require considerable time and patience and demand numerous diagnostic tests.
Initial diagnostic efforts include getting a history and performing a detailed physical examination as well as eye and neurologic examinations. Vaccination status, parasite control, travel history, response to previous medications, and the presence of illness in other animals (including people) will be reviewed. Initial laboratory tests include a complete blood count (CBC) and other blood tests (for example, tests for feline leukemia virus and feline immunodeficiency virus), urinalysis, a urine culture, and usually x-rays of the chest and abdomen. Medications that can cause a fever may need to be discontinued.
To establish the diagnosis, some initial tests may need to be repeated (particularly the physical examination). Other specialized tests may also be needed, depending on results of the initial tests or to more specifically investigate the most common known causes of fever of unknown origin. These tests might include other blood tests; specialized tests for infectious diseases; an ultrasound of the abdomen, chest, or heart; aspiration or biopsies of the bone marrow, lymph nodes, or other tissues to examine the cells of these tissues; analysis and bacterial cultures of various body fluids; x-rays of the bones, joints, or spinal cord; computed tomography (CT) or magnetic resonance imaging (MRI); or exploratory surgery.
In some fever of unknown origin cases, a specific diagnosis cannot be found, or diagnostic testing is discontinued, and different treatments are tried without a diagnosis. Drugs that may be tried include antibiotics, antifungal agents, and anti-inflammatory or immunosuppressive therapy. Although trial therapy may improve the signs or confirm a tentative diagnosis, it can also carry significant risk, and careful monitoring is needed.
In true fever, the high body temperature is being regulated by the body, so cooling methods such as water baths work against the body’s own regulatory mechanisms. It is also likely that fever itself has some beneficial effects, particularly in infectious diseases. However, fever can lead to loss of appetite, loss of energy, and dehydration. Because of this, animals with fever of unknown origin may benefit from intravenous fluid therapy or from the use of fever-reducing medications.
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