A 2-year-old with 4 weeks of daily fever

A 2-year-old female presents for evaluation of 4 weeks of daily fevers. When the fevers began, she had mild upper respiratory tract symptoms, which quickly resolved. The fevers persisted, however, with a maximum of 40°C. The child's review of symptoms was significant for a 1-kg weight loss over the past month. Ten months before presentation, she had moved from Saudi Arabia with her family. One week before the onset of symptoms, she had visited a petting zoo. During episodes of fever, the patient was ill-appearing and had an elevated heart rate and respiratory rate. On examination, she was found to be thin, febrile, tachycardic, and with scattered lymphadenopathy. Results of laboratory tests were remarkable for an elevated white blood cell count of 16,100 cells per uL with a neutrophilic predominance. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were elevated at 99 mm/h and 27 mg/dL, respectively. A chest radiograph indicated a small amount of fluid in the interlobar fissures. Our expert panel examines her case, offers a definition of fever of unknown origin, and makes diagnostic considerations.