Fever of Unknown Origin (FUO) is a term used when a child has a prolonged or recurring fever and no clear cause is found, even after appropriate medical testing. Traditionally, FUO meant a fever over 38.3°C (101°F) lasting more than three weeks without a diagnosis after hospital evaluation. In more recent practice, FUO may also be considered when a fever lasts over 8 days with no clear explanation, even if the child is being seen as an outpatient.
FUO can be caused by many different conditions, including:
- Infections (such as Epstein–Barr virus),
- Autoimmune or autoinflammatory diseases (such as systemic juvenile idiopathic arthritis),
- Cancers (such as leukemia or lymphoma),
- Drug reactions or very rare conditions.
- Other rare medical conditions
In many children, FUO is eventually linked to a mild, self-limited viral infection. In fact, up to 30–50% of pediatric FUO cases may remain unexplained but get better on their own with careful observation and supportive care.
If the child has had a fever for more than a week with no clear cause, it’s important to consult a doctor for further evaluation.