Fever of Unknown Origin

Most of the times, the cause of a fever can be readily explained: the patient has recent contact with sick or infected people, ate contaminated food or water, or has tick bites or scratch that can introduce microbial infections.

However, when there are:

  • No ready explanation for the fever
  • The fever hovers at 101° F (38.3 °C) or higher
  • The fever persists for days or weeks

then it is classified as Fever of Unknown Origin or FUO.

Causes of Fever of Unknown Origin

The possible causes of fever of unknown origin are:

  • Infections (30% to 40%)
    These include generalized infections, such as endocarditis and tuberculosis (or TB) and localized infections, such as liver and pelvic abscesses. In the tropics, malaria infection is often the cause of FUO.

    In children, infection is the most common cause of FUO.
  • Tumors or cancers (20% to 30%)
    These include leukemia, lymphomas and other cancers.

  • Collagen vascular diseases (10% to 20%)
    These include periarteritis nodosa (PAN) and rheumatoid arthritis.

  • Other diseases (15% to 20%)

Fever can also be a side effect of medications, such as antibiotics.

Despite the best medical efforts to diagnose some fever of unknown origin, the underlying cause of the fever is cannot determined in about 5% to 15% of the cases.

How is Infection Detected?

Two standard tests to detect infectious agents:

  • Microbial culture
    In this test, the presence of an infectious organism (such as bacteria and viruses) is detected in a throat culture, as well as urine, blood, and stool samples.

  • X-Ray or CT Scan

There are many reasons that sometimes these two tests do not produce conclusive results. The test may not work well or lack specificity. Also, there may not be a readily available testing method to detect some microorganisms.

Who Often Gets Fever of Unknown Origin?

Two age groups often have this condition - children and the elderly. In children, the most common cause of fever of unknown origin is an infection, whereas in the elderly, they are cancers and collagen vascular diseases.

How Long Does the Fever Usually Last?

The fever can last days, weeks, and even years!

In fever that persists for a year or more, the prognosis is usually good - this reduces the risk of the fever being a symptom of a serious condition. In many cases, a fever that goes on for 1 year or longer are caused by granulomatous diseases, such as sarcoidosis, Crohn's Disease, and granulomatous hepatitis.

Can Genetics Play a Role?

Yes. For example, in Mediterranean Fever, a blood protein called pyrin or marenostrin, which help to regulate fever, does not work properly. People with this inherited condition may have sudden, high fevers.

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