Viral causes of influenza-like illness: Insight from a study during the winters 2004-2007

Limited information is available on the viral etiology of influenza-like illness in southern European countries, and it is still a matter of debate whether certain symptoms can be used to distinguish among the specific viruses that cause influenza-like illness. The main objective of the present study was to identify the demographic and clinical predictors of influenza-like illness due to specific viral agents. The study, which was observational in design, was conducted in Rome and Naples, Italy. Cases of influenza-like illness were defined as individuals with fever >37.5 degrees C and at least one systemic and one respiratory symptom, recruited during the winters of 2004-2005, 2005-2006, and 2006-2007. Influenza and other respiratory viruses were identified using the polymerase chain reaction (PCR), performed on throat swabs. Basic individual information was collected using a standard form. A total of 580 persons were included in the analysis. Viral pathogens were identified in fewer than 50% of the cases. Overall, 240 viral agents were detected: 22.8% were positive for influenza viruses, 10.9% for adenoviruses, 6.0% for parainfluenza viruses, and 1.7% for respiratory syncytial virus. The month of diagnosis, and muscle and joint pain were associated with influenza virus, though the positive predictive value (PPV) was low. Abdominal pain was associated with adenovirus infection. Although the PPV of symptoms for influenza virus infection was low, especially in low activity periods, these findings may help clinicians to improve their ability to perform diagnoses.