The changing etiology of fever of intermediate duration.

BACKGROUND Fever of intermediate duration (FID) is a common condition. Currently, its most frequent causes are not well defined. METHODS Prospective study of FID cases attended at a hospital in 2 periods: 1983-1989 and 2004-2005. Blood cultures and serologic studies for Brucella melitensis, Coxiella burnetii, Rickettsia typhi, Rickettsia conorii, cytomegalovirus, and Epstein-Barr virus were performed on all patients. Other microbiological, serological, radiological, or invasive procedures were performed according to clinician-in-charge criteria. RESULTS A total of 505 patients were included from 1983 to 1989, and 179 from 2004 to 2005. A diagnosis was reached in 410 (81.1%) and 109 patients (60.9%), respectively. The cause of FID was an infectious disease in 389 patients from the first period (94.8% of those with a final diagnosis) and 92 from the second (84.4%). Most were systemic infections, 328 (80%) in 1983-1989 and 74 (67.8%) in 2004-2005, followed by focal infections, 9.5% and 16.5%, respectively. Q fever was the most frequent etiology in both periods. In 2004-2005, brucellosis decreased and HIV infection emerged as a cause of FID. The origin of FID was non-infectious in 5.1% and 15.5%, respectively. CONCLUSIONS Q fever is the most frequent cause of FID in southern Spain. Studies over time are needed to identify changes in the etiologic spectrum of this condition. Important viral etiologies, such as HIV infection, may be detected as causes of FID. Further studies are needed to determine the importance of other agents as causes of FID.