Prolonged fever of unknown origin is not a rare problem for pediatricians. Using the criteria described above, McClung and Lohr showed that an unidentified fever was responsible for approximately 0.7% of all admissions to their hospitals. Feigin reported on 39 children with a fever of unknown origin lasting more than 2 weeks who were admitted to the children’s hospital for one year.
In addition, another 24 children were admitted with a fever of unknown origin lasting 1 week. According to Pizzo, approximately 130 children each year go to a children’s clinic at the Boston Medical Center (Harvard Medical Center) for a prolonged fever. Annually, an average of 16 children meet his criteria for fever of unknown origin.
A review of the literature by Feigin notes that 30% of all patients with fever of unknown origin, described from 1930 to 1976, were under the age of 21. Fever of unknown origin is observed in children of any age, but somewhat more often in children of younger age groups.
According to Feigin, 59% of the patients he observed were younger than 6 years. According to Pizzo and Lohr, approximately 50% of patients were under the age of 6 years. Boys and girls are equally susceptible to fever of unknown origin.
Many authors emphasize that fever of unknown origin is more often caused by common diseases that occur in an unusual way than by rare or exotic diseases.
There are many causes of fever of unknown origin. As McClung said, “anything can cause a fever of unknown origin”. If this is an exaggeration, then it is not far from the truth. Almost any disease that can cause prolonged fever, can occur in an atypical or latent form and manifest as fever of unknown origin.
The cause of fever of unknown origin in children can be 3 categories of diseases (in order of frequency): infectious diseases, connective tissue diseases, and neoplasms. Tumors in children are a less common cause of fever of unknown origin than in adults. In the work of some pediatricians, infectious diseases caused LDL in more than 50% of cases. The diagnosis could not be established in 21% of cases.
Below is a rather extensive, but not an exhaustive list of diseases that can manifest as fever of unknown origin. It was compiled on the basis of published consolidated works on fever of unknown origin, descriptions of individual cases and materials of the author himself.