Fever Of Unknown Origin (FUO)

High fever is one of the most prevalent symptoms, and it could occur due to numerous diseases and conditions. However, in some cases, high fever might occur without a clear reason, and this is called fever of unknown origin (FUO).

Fever of unknown origin (FUO)

We say that a fever is of unknow origin if it meets one of these conditions:

• The temperature has been higher than 38.3 ˚C (101 ˚F) on several occasions.
• The fever continues for more than 3 weeks.
• The condition was undiagnosed after one week in the hospital.
• Three outpatient visits because of the fever or three hospital days without a certain diagnosis.

Fever of unknown origin can be categorized into:

Classical fever of unknown origin

It’s characterized by high fever for more than three weeks, three outpatient visits, or three days in the hospital without a certain diagnosis. Some examples include endocarditis, complicated urinary tract infections, abscesses, and tuberculosis.

Nosocomial fever of unknown origin

It’s characterized by three days in the hospital without a certain diagnosis. Some examples include drug fever, post-operative complications, venous thromboembolic disease, and malignancies.

Neutropenic fever of unknown origin

Mostly due to infections.

HIV-related fever of unknown origin

Fever might occur in untreated chronic conditions.

Causes of fever of unknown origin

In most cases, the reason for a high fever without reason is an atypical condition. The causes can be divided into:

Noninfectious inflammatory causes

The most common: Giant cell arteritis, adult Still disease (juvenile rheumatoid arthritis), lupus, microscopic polyangiitis (periarteritis nodosa), and rheumatoid arthritis, in addition to other diseases.

Infectious causes

The most common: Tuberculosis, Q fever, brucellosis, HIV, abdominopelvic abscesses, cat scratch disease, Epstein-Barr virus infection, cytomegalovirus infection, typhoid fever, toxoplasmosis, and extrapulmonary tuberculosis, in addition to other infections.

Neoplastic and malignancy causes

The most common: Lymphoma, renal cell carcinoma, myeloproliferative disorder, and acute myelogenous leukemia, in addition to other neoplastics and malignancies.

Other causes

The most common: Cirrhosis, drug fever, thyroiditis, Crohn's disease, and pulmonary emboli, in addition to other causes.

Symptoms of fever of unknown origin

The main reason is fever, and the longer the fever, the less likely that the cause is an infection.

Other symptoms could manifest, such as:

• Joint pain.
• Fatigue and exhaustion.
• Rash.
• Chills.
• Headaches.
• Pain in different parts of the body.

Diagnosis of fever of unknown origin

Complete blood, urine, liver, metabolic, and other tests are performed, in addition to HIV and tuberculosis tests, imaging tests, and biopsies.

Management of fever of unknown origin

There isn’t a specific protocol to manage FUO. The patient is given antipyretics in addition to managing other symptoms, if any. The patient must also stop taking any medications. In some cases, the patient is given antibiotics, and sometimes antifungals, as precautions until a diagnosis is confirmed.