Unknown fever (FUO)
Fever may appear as part of a harmless common cold or may be a sign of a serious illness. In some cases, the doctor cannot determine a cause of the fever despite a thorough examination. In the case of fever with no apparent cause of more than 3 weeks in duration with temperatures above 38.3 ° C and unsuccessful standard tests, one speaks in technical terms what is known as “fever of unknown cause" ("Fever of unknown origin", FUO).
Physicians distinguish 4 types of fever of unknown cause:
- Classic FUO
- Nosocomial FUO
- FUO in people with a weakened immune system due to a decrease in white blood cells (neutropenic FUO)
- FUO in people with HIV
The most common causes of FUO are undetected infections, tumor diseases or rheumatic diseases or autoimmune diseases. One possible cause of infection is tuberculosis or an abscess in an unusual place. Such accumulation of pus may result from broken appendicitis or diverticulitis, or from malignant tumors or injuries.
Especially in the elderly, a fever of unknown cause is often caused by malignant tumors that are difficult to identify and therefore remain undetected for a long time, such as Chronic leukemia, lymphoma cancer (lymphoma), kidney, colon or pancreatic cancer or daughter tumors (metastases) of tumors. Autoimmune diseases can cause fever, e.g. Still syndrome, systemic lupus erythematosus or inflammation of arterial blood vessels (e.g. temporal arteritis).
Medications can also be an explanation for a fever of unknown cause if the body is hypersensitive to certain active substances, such as those are contained in dehydrating agents (diuretics), pain relievers, antibiotics, anti-allergic agents (antihistamines) or sedatives or sleeping pills. These "hypersensitivity reactions" then express themselves as a fever. Here are some examples of active ingredients that can cause fever. In principle, however, any medication can be considered for a hypersensitivity reaction with fever.
Classic FUO
The classical Fever of unknown cause usually lasts longer than 3 weeks. It is caused by infections, malignant tumors or diseases of the connective tissue (rheumatic diseases, collagenosis) or the blood vessels (vasculitides).
Nosocomial FUO
As nosocomial Fever of unknown cause is referred to as fever that occurs after admission to a hospital in patients who did not have a fever at the time of admission. Nosocomial FUO is often caused by pulmonary embolism, an infection of the intestine with the bacterium Clostridium difficile or by phlebitis with the spread of the bacteria via the bloodstream. Medications can also cause such a fever. Inflammation of the sinuses (sinusitis) may be the cause of the fever in patients who are fed via a nasogastric tube.
Neutropenic FUO
The neutropenic fever of unknown cause occurs in people who have fewer than 500 so-called neutrophil granulocytes (white blood cells) per cubic millimeter of blood. Most often the fever in these patients is caused by an infection with bacteria. Infections with fungi (e.g. aspergillosis or Kandida infection) or with viruses (e.g. herpes viruses) can also cause neutropenic FUO. It is typical of these complications that often no infected organs are observed. On the other hand, bacteria are often found in the blood, but it is not clear how the pathogens entered the body.
FUO in people with HIV
An acute infection with the HI virus can be accompanied by a fever. However, fever can also occur during the course of the HIV disease. If the cause of the fever cannot be identified in an outpatient HIV patient after 4 weeks and in an inpatient HIV patient after 3 days, doctors speak of an HIV-associated FUO. It is usually caused by pathogens that do not cause infection in healthy people, for example cytomegalovirus, Pneumocystis jeroveci or Mycobacterium avium-intracellulare. Other causes of HIV-associated FUO are tumors that occur very rarely in healthy people, such as Kaposi’s sarcoma or lymphoma cancer (lymphoma). Medications can also trigger this form of fever.
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