In both animals and people, fever may indicate infection, inflammation, immune-mediated disease, or cancer. Determining the cause of a fever requires a history, physical examination, and sometimes laboratory or other diagnostic tests. Details about vaccinations, deworming, travel, response to any medications, and exposure to other (possibly sick) animals should be discussed with your veterinarian.
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 a case series of horses with fever of unknown origin, 43% had infectious disease, 22% had tumors, 6.5% had immune-mediated disease, 19% had miscellaneous causes, and in 9.5% the cause was not determined. If a joint or joints are found to be swollen, sampling and analysis of joint fluid is an important diagnostic test in horses and other large animals because infectious arthritis is a common cause of fever.
The key to diagnosis of fever of unknown origin is to develop and follow a systematic plan to discover the common or uncommon cause. Patience and persistence may be needed to discover the cause of the fever. First-stage evaluations may include history, physical examination, eye and nervous system examination, and blood and urine tests. Second-stage tests may repeat some of the initial tests for confirmation or to determine whether anything has changed, and additional testing may be suggested based on abnormalities found in initial tests. Secondary testing may include imaging (with x-rays or ultrasound), infectious disease testing of blood or other body fluids, and aspirating (removing) cells from any masses discovered for pathologic examination.
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 confirmed diagnosis. Options include antibiotics, antifungal agents, and anti-inflammatory or immunosuppressive therapy. Although trial therapy can resolve the clinical signs or may confirm a tentative diagnosis, it can also carry significant risk, and careful monitoring is needed.
With true fever, the high body temperature is being regulated, so cooling efforts work against the body's own regulatory systems. It is likely that the fever has some beneficial effects, especially in infectious diseases. However, fever can lead to loss of appetite, loss of energy, and dehydration, so animals may benefit from fluid therapy and fever-reducing medications.
Also see professional content regarding fever of unknown origin in animals.