Most cases of a fever are acute, lasing less than 4 days and are frequently due to an infection. A fever does not usually persist for a prolonged period of time without any other signs or symptoms (non-specific fever) and failure to attend to the cause can lead to complications or even result in death. However, there are instances where a fever can be prolonged, usually episodic but less frequently it may be continuous.

What is a Chronic Fever?

A chronic fever is a term commonly used to describe a fever of unknown origin (FUO) although it can be used to describe a fever that is persistent or episodic despite not meeting with the criteria for FUO. It is not uncommon for an  acute fever to last for more than 4 days in severe infections but usually does not persist past 7 days.

A chronic fever may not always be persistent and is often intermittent with short episodes of a few hours/days  of elevated body temperature spread over a long period of time, usually weeks or months or even years. This may also be related to periodic fever syndrome where the fever is usually accompanied by clinical features associated with inflammation like abdominal pain, joint pain and swelling. Periodic fever syndromes are not associated with an infection and are therefore not contagious but are often due to genetic factors as with Family Mediterranean Fever. Alternatively, the fever may be non-specific with no associated signs and symptoms.

Persistent Fever

The term acute and chronic refer to the duration of the fever, while the term persistent usually refers to the nature of the fever. A persistent fever is one that is ongoing (continuous) and may or may not respond to anti-pyretic medication and may or  may not settle although the suspected cause is resolving. A persistent fever may be acute or chronic.

Low-Grade, Mild or High Fevers

As with any fever, a chronic fever may be low-grade, mild/moderate or high.

  • A low-grade fever is an elevation in the body temperature above the norm but is usually below 38.3 C/101 F.
  • A mild/moderate fever or common fever is a body temperature above 38.3 C/101 F but less than 40 C/104 F.
  • A body temperature equal to or above 40 C/104 F is considered as high with temperatures exceeding 41 C /106 F referred to as very high/extreme fever.

In most cases, a chronic fever is either low-grade or mild.

There are many causes of a chronic fever, including infections, neoplasms, rheumatological causes and other unknown causes. If a chronic fever meets with certain criteria as listed below then it is diagnosed as a fever of unknown origin (FUO).

Fever of Unknown Origin (FUO)

A fever of unknown origin or FUO is essentially a chronic fever but as the name suggests, the exact cause cannot be diagnosed despite investigations. A fever of unknown origin (FUO) is diagnosed based on the following criteria :

  1. Body temperature greater than 38.3 C (101 F) either continuous (persistent) or recorded on multiple occasions
  2. Continuous fever or episodes of fever that persists for more than 3 weeks
  3. No conclusive diagnosis despite more than 3 days of inpatient care or after 2 or more outpatient visits.

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 20, 2010