fever of unknown origin, FUO

Fever of unknown origin ("FUO") refers to fever in a patient that has resisted efforts at diagnosis for a significant period of time. The usual rule of thumb is that an FUO is a fever for which not source is found after a week of focused investigation in the hospital or three weeks as an outpatient. There are very few true "FUO's" in pediatric practice. Most of them turn out to be typical or atypical cases of common diseases. Usually the cause is found and treated. In as many as 25% of cases, the fever disappears and the true cause is never determined.

The major causes of mysterious fever in children are a bewilderingly long list of infections and to a lesser degree, rheumatoid or autoimmune inflammatory diseases such as systemic lupus erythematosus or juvenile rheumatoid arthritis. Inflammatory bowel disease (Crohn disease or ulcerative colitis), rheumatic fever, and Kawasaki disease are also commonly found to be the causes of FUO. These diseases can be confusing because early on in the course of disease, fever may be the only symptom. Cancers may present as fever without other symptoms, but this is unusual. A drug fever is possible if the patient is taking a medication; drug fevers tend to be more unrelenting and disappear within a few days of stopping the offending medication. There are also recognized syndromes of periodic fever, meaning fevers that recur on a very regular basis without obvious cause.

Since there is such a broad range of possibilities, your child's doctor will spend a lot of time taking a thorough medical history. Some important points to reflect upon and review in your mind to help your child's doctor include:

  • any history of exposure to animals, wild or domestic pets or livestock
  • any ingestion by the child of wild rabbit or squirrel meat
  • tick bites or exposure to ticks
  • pica, eating of foreign substances
  • any foreign travel
    • what precautionary shots were taken
    • any souvenirs or artifacts brought back from abroad
    • what precautions were taken against contaminated food or water
  • all medications, including any over-the-counter medications
  • a thorough family history on both sides; some familial conditions have fever as a prominent symptom
  • as accurate a timeline for the appearance of symptoms as you can recall
These facts will help your child's doctor focus the search to the most promising areas of investigation.

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