Non-influenza respiratory viruses may overlap and obscure influenza activity

Abstract

Objective: To report the number and timing of influenza A isolates, as well as overlapping respiratory viruses. Co-circulating 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 veterans 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, influenza 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 statewide data, we suspected an influenza outbreak; however, 26 isolates of parainfluenza virus type 1 were cultured with no influenza. The potential importance of culturing at the end of the season was demonstrated in 1991-1992 when an outbreak of respiratory syncytial virus (RSV) overlapped and extended beyond influenza A activity.

Conclusions: When interpreting new clinical respiratory illnesses as a basis for declaring an outbreak of influenza A, clinicians should realize that co-circulating respiratory viruses can account for clinical illnesses. Clinicians might utilize healthcare dollars efficiently by performing cultures to focus the timing of influenza A chemoprophylaxis. Cultures could be performed when clinical outbreak criteria are approached to confirm an outbreak. Culturing of new respiratory illness could begin again before the anticipated discontinuation of prophylaxis (approximately 2 weeks).

Figure 1

Thirty‐two influenza A infections occurred over 25 days. One parainfluenza 2 infection occurred 25 days earlier. Eight RSV infections overlapped and extended 14 days beyond the influenza period.

Figure 2

One hundred two influenza B infections occurred over 86 days, with five overlapping infections: RSV, parainfluenza 3, and three rhinovirus.

Figure 3

Sixty‐eight influenza A infections occurred over 47 days. A single overlapping parainfluenza 1 and 2 rhinovirus infections occurred 5 days after the influenza period.

Figure 4

Twelve influenza A infections occurred over 32 days. Seven rhinovirus, one parainfluenza 4, one parainfluenza 2, and two parainfluenza 3 infections occurred before the influenza period. Four parainfluenza 3 infections overlapped and extended beyond the influenza period.

Figure 5

A single influenza B infection occurred on 12/21. Twenty‐six parainfluenza 1 infections occurred over 43 days. Two parainfluenza 4 infections overlapped the parainfluenza 1 period, followed by 11 RSV and two rhinovirus infections.

Figure 6

Six influenza A infections occurred over 86 days, followed by nine influenza B infections over 14 days. Six parainfluenza infections (Types 2, 3, and 4) occurring between 12/02 and 12/30 overlapped the influenza A period. Four RSV and three rhinovirus infections overlapped the influenza B period.

Figure 7

One hundred fifty‐four influenza A infections occurred over 123 days. Twenty‐two parainfluenza 1 infections occurred before the influenza period. Ten rhinovirus infections occurred between 11/14 and 5/04, and a single RSV infection overlapped the influenza A period.