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. Withdrawal and analysis of joint fluid is an important diagnostic test in horses and other large animals because infectious polyarthritis (arthritis involving 2 or more joints) is a common cause of fever.

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.

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. 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 (see Metabolic Disorders of Dogs: Fever of Unknown Origin in Dogs).

Last full review/revision July 2011 by George M. Barrington, DVM, PhD, DACVIM; David L. Evans, BVSc, PhD; Katharine F. Lunn, BVMS, MS, PhD, MRCVS, DACVIM; Donald C. Sawyer, DVM, PhD; Ivan W. Caple, BVSc, PhD, MACVSc, MRCVS; Sharon J. Spier, DVM, PhD, DACVIM