Non-influenza respiratory viruses may overlap and obscure influenza activity

OBJECTIVE: To report the number and timing of influenza A isolates, as well as overlapping respiratory viruses. Cocirculating respiratory viruses may obscure the determination of influenza activity. DESIGN: Prospective clinical surveillance for the new onset of respiratory illness followed by viral cultures during seven separate influenza seasons. SETTING: The Wisconsin Veterans Home, a skilled nursing facility for vetera ns and their spouses. RESULTS: Influenza A isolates were encountered in greater numbers than non- influenza A isolates during three seasons. Seasonal variability is striking . In December 1992, we identified a large outbreak of respiratory illness. Influenza type B was cultured from 102 residents. In December 1995, influen za A was cultured from 285 people in Wisconsin. At that time, we identified outbreaks of respiratory illness in two of our four buildings. Based on st atewide data, we suspected an influenza outbreak; however, 26 isolates of p arainfluenza virus type 1 were cultured with no influenza. The potential im portance of culturing at the end of the season was demonstrated in 1991-199 2 when an outbreak of respiratory syncytial virus (RSV) overlapped and exte nded beyond influenza A activity. CONCLUSIONS: When interpreting new clinical respiratory illnesses as a basi s for declaring an outbreak of influenza A, clinicians should realize that co-circulating respiratory viruses can account for clinical illnesses. Clin icians might utilize healthcare dollars efficiently by performing cultures to focus the timing of influenza A chemoprophylaxis. Cultures could be perf ormed when clinical outbreak criteria are approached to confirm an outbreak . Culturing of new respiratory illness could begin again before the anticip ated discontinuation of prophylaxis (approximately 2 weeks).