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Fever of Unknown Origin (FUO)

  • Fever is a very common feature of childhood illnesses.
  • In most childhood illnesses the fever resolves in a short period of time either because the illness goes away on it’s own (e.g., viral illness) or the cause of the fever is identified and specific treatment is given (e.g., antibiotics).
  • Sometimes fever may persist without an easily identifiable cause, and the child may be considered to have a fever of unknown origin (FUO).
  • The longer the febrile illness the more important it is for your child to be carefully reevaluated in order to avoid missing any illness that is easily treatable.
  • A detailed history and physical examination is very important in the evaluation of a child with prolonged fever.
  • This includes watching for any other signs and symptoms that may develop over time or that come and go during the illness (e.g., rash, joint pain).

What are the causes of FUO in children?

The following is a partial list of the most common causes of FUO in children.


  • Viral infections such as Mononucleosis and others we cannot test for
  • Hidden Bacterial infections such as:
    Urinary tract infection
    Bone, joint or disc space infections
    Infections of fluids around joints or the brain

Inflammatory diseases (less common than infections noted above):

Other very rare causes:

  • Leukemia or lymphoma
  • Inflammatory bowel disease
  • Fever due to medications

What questions may our pediatrician ask of us regarding our child’s prolonged febrile illness?

In addition to the typical health questions, consider the following questions:

  1. Has your child been around anyone else that has been sick?
  2. Are there any symptoms that your child had at the beginning of their illness that are not present anymore?
  3. Are there any symptoms that come and go during your child’s illness (e.g., rash)?
  4. Has your child traveled out of the country recently?
  5. Has your child been around any farm animals or wild animals?
  6. Do you have any pets?
  7. Has your child been bitten by a tick?
  8. Has your child been scratched by a kitten?
  9. Has your child eaten any raw or undercooked foods or drink unpasteurized milk or juice?
  10. Does your child have a heart murmur?
  11. Has your child been taking any medications?
  12. Does anything like this run in the family?
  13. Has your child had any weight loss, night sweats, joint pain, bone pain or rashes?
  14. How has the temperature been taken, what is the height of the fever, and does it occur at a predictable time of day?
  15. Has your child had FUO in the past or repeated known infections (e.g., urinary tract infection)?

How will our pediatrician evaluate our child’s prolonged febrile illness?

  • Your pediatrician will initially manage your child’s fever based on all their signs and symptoms and their physical examination.
    For example, a strep test may be all that needs to be done if your child has a straightforward throat infection.
  • It is likely that as your child’s fever continues, your doctor will want to perform further testing to reveal the source of the fever.
  • These tests will depend largely on the history you give the doctor concerning your child’s illness so it is important to keep track of the details.
  • Further testing might include a chest xray, a urine test, testing for specific infections (e.g., Mono), a blood count, a sedimentation rate (ESR) test to look for signs of inflammation, and an abdominal ultrasound or CT scan.

When should I contact the pediatrician when my child has a fever?

Parents should contact their child’s pediatrician if their child has fever with any one of the following signs or symptoms:

  • ear pain
  • sore throat
  • severe coughing, breathing fast and/or difficulty breathing
  • drooling or your child cannot swallow
  • stiff neck
  • lethargy or extreme irritability
  • poor fluid intake
  • signs of dehydration (i.e. less urination than usual, dry mouth, minimal tears when crying)
  • continuous vomiting or nonbloody diarrhea
  • any bloody diarrhea
  • persistent abdominal pain, especially if starts around the belly button and moves to their lower right
    side of the belly
  • red, swollen wound or redness of an area of skin
  • urinary complaints which may indicate a urinary tract infection (e.g., painful urination)
  • petechial rash (small, flat, purple spots that when pressed down on do not blanch, or fade for a second or two) (See Photo called Petechiae)Also, contact your pediatrician:
  • if your child is under 2 months and has a rectal temperature of 100.4 degrees Fahrenheit or greater (even a slight fever can be a sign of a potentially serious infection in very young infants)
  • if fever persists for more than 48 – 72 hours for mild illnesses (e.g., cold, occasional bouts of vomiting or diarrhea)
  • any time you are uncomfortable with how your child looks