Fever and the presence of moderate or large amounts of pus in the urine of infants or young children usually lead to the diagnosis of pyelitis. Based on these criteria, pyelitis is a common disease, but the frequency with which it is diagnosed clinically is in striking contrast to its extreme rarity at postmortem examination. Infants suffering from so-called pyelitis usually recover, but when death occurs, most often due to some intercurrent disease, inflammation of the pelvis of the kidney is rarely found on postmortem examination. Such experiences have led a number of observers to doubt the dependency of pyuria in the young on inflammation of the pelvis of the kidney, but suggest its origin in some other part of the urinary tract (Thiemich,1 Bugbee2 and Chown3).
During the past twelve years, we have had an opportunity to study postmortem material from forty-nine infants and young children
WILSON JR, SCHLOSS OM. PATHOLOGY OF SO-CALLED "ACUTE PYELITIS" IN INFANTS. Am J Dis Child. 1929;38(2):227–240. doi:10.1001/archpedi.1929.01930080003001