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Chronic Fever and Persistent Fever (Low-Grade, Mild, High)

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
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  • Andyliang

    Hi Doctor, I am enquiring about the fever of unknown origin.
    My son is 20 months old, he got toxoplasmic infections in the 3rd month, a serious diarrhea (lasted for 6 weeks) when he was 7 months old. Since 11th month, he got recurrent diarrhea for more than two months. He was treated in a teaching hospital, but no bacteria were found in the stool or blood. After moving to another hospital, he was treated similarly and his white blood cell count and CRP always showed opposite directions. If CRP was dropping, WBC would go up, vice versa. No other choice we moved to the best hospital in a big city. At the time of almost cured, respiratory syncytial virus contracted my son and got serious pneumonia. Doctors prescribed antibiotics and glucocorticosteroid. In 10 days, my son looked ok and was discharged. In 3 days, he got another fever, but it varied from 37.4 to 38.6 in the same day without other symptoms except little appetite. We brought the baby to the hospital again, but they did not work out the possible cause. Regarding his long period of infection, do you think he has been cured before or he got a new infection? What will be the possible causes? Since he has been sick because of respiratory infection or gastroenteral infection, do you think there will be some anatomical anormalies or abscess somewhere? What will be the possible test for investigation? We are really tired of his sickness for such a long time. Hope this will help you to propose a possible diagnosis. By the way, his WBC, CD3,CD4,are normal, CD8 a little bit high, IgA/G/E/M are normal. Complements are in normal range. Millions of thanks. Andy

  • Dr. Chris

    Hi Andyliang

    It is difficult to say whether he was “cured” initially through this type of platform. Suspicions are that the infection may not have resolved completely and your son may have had a reaction to some medication he was prescribed – cannot go into detail without knowing the exact case. May not have been the fault of the doctors who were treating him. I would not like to interfere with the doctor’s approach by offering online advice which may be totally misguided. You should take him to a pediatrician (preferably a private pediatrician) and just stick with that doctor. An immunologist may also need to be considered. Try not to move from one doctor to another or among different hospitals. Each doctor may start from scratch and there is a lack of continuity. Unfortunately when you use public health services, this is not always an option. Rather consult with a private pediatrician who can follow this case through all the different investigations and communicate with relevant specialists as well.

  • Leigh

    For the past 6 months, my 8 year old daughter consistently runs an afternoon low grade fever of 37.3 c. Is this something to be worried about? She has had a blood test and everything is fine. Our family doctor does not seem concerned.

  • Dr. Chris

    Hi Leigh

    This temperature (37.3 C) does not fall out of the norm so I am not sure how you are defining fever. However, if she is showing other symptoms that you are assuming is fever, you should rather report that to your doctor then the actual temperature reading.

  • Leigh

    Thanks for the quick answer on the 37.3 c fever. I didn’t know it was a fever and I didn’t know it was normal. There are no other symptoms so I will stop worrying now.