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Fever of Unknown Origin (FUO) – List of Possible Causes

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A fever of unknown origin is explained in the article on What is Fever of Unknown Origin?

The causes of a fever of unknown origin (FUO) can be divided into four categories :

  1. Infections
  2. Rheumatological /Autoimmune
  3. Neoplastic/Malignancies
  4. Miscellaneous (Other Causes)

Causes of a Fever of Unknown Origin

Infectious Causes of a Fever of Unknown Origin

An infection is caused by pathogenic microorganisms. These are the more common cause of a fever of unknown origin.

  • Abscess
    • Especially abdominal, pelvis and dental abscesses.
    • Consider if there is a history of trauma, gynecological procedures, peritonitis, diverticulosis.
  • Cytomegalovirus (CMV)
    • Consider if there is a history of blood transfusion or if HIV-positive.
  • Epstein-Barr virus (EBV)
  • Human Immunodeficiency Virus (HIV)
    • Consider if there is a history of  risky behavior like multiple sexual partners, unprotected sex, IV drug use.
    • Refer to Stages of HIV/AIDS.
  • Endocarditis
    • Inflammation of the inner lining of the heart and heart valves. In an FUO, this is usually due to an infection and is referred to as infective endocarditis.
  • Travel-Related Fevers
    • Consider if there is a history of recent travel.
    • Refer to Traveler’s Fever for a list of causes of a non-specific fever after travel.
  • Osteomyelitis
    • Inflammation of the bone due to an infection.
  • Sinusitis
    • Inflammation of the paransal sinuses due to an infection.
  • Tuberculosis (TB) – especially extrapulmonary TB.
  • Other
    • Fungal infections
    • Parasitic infections

Rheumatological / Autoimmune Diseases

This include inflammatory conditions caused by the body’s immune activity against its own tissues.

  • Adult Still’s disease
  • Giant cell arteritis (temporal arteritis)
  • Inflammatory bowel disease
  • Polyarteritis nodosa
  • Polymyalgia rheumatica
  • Reactive arthritis
  • Rheumatoid arthritis
  • Systemic lupus erythematosus (SLE)

Neoplastic / Malignancy

This includes malignant tumors.

  • Colon cancer
  • Liver cancer (hepatoma)
  • Leukemia and myeloma
  • Lymphoma
  • Renal cell carcinoma
  • Stomach cancer
  • Cancer of the pancreas
  • Metastatic cancer

Miscellaneous (Other Causes)

  • Alcoholic cirrhosis
  • Drug fever
  • Factitious fever – high body temperature due to manipulation by the patient.
  • Deep vein thrombosis (DVT)
  • Pancreatitis (inflamed pancreas)
  • Inflammatory bowel disease
  • Familial Mediterranean Fever (refer to periodic fever syndrome in Chronic Fever)
  • Thyrotoxicosis (hyperthyroidism) and thyroiditis
  • Sarcoidosis

Related Articles

  1. What is a Fever (Pyrexia)? Normal and High Body Temperature
  2. Sudden Fever – Causes of Acute Fever in Adults
  3. Childhood Fever – Causes of Fevers in Babies (Infants) and Children

Article reviewed by Dr. Greg. Last updated on July 21, 2010