The purpose of this study was to determine whether acute pulmonary exacerbations of cystic fibrosis associated with nonbacterial infections are clinically distinguishable from other exacerbations. Eighty exacerbations in 54 patients were studied. Exacerbations associated with influenza (n = 8) were compared with those associated with other nonbacterial infections (n = 15) and those in which no nonbacterial infection was detected (n = 57). Patients with influenza had lower Shwachman scores and were more likely to be seropositive for C-reactive protein than patients in the other two groups. Patients with influenza had a mean decrease in forced expiratory volume per second of 26%, compared with test results obtained before the exacerbation. In contrast, the mean decrease in forced expiratory volume per second was 6% for other nonbacterial infections and 12% for the group without nonbacterial infection (p less than 0.05 for both comparisons). The forced expiratory flow in first 25% of vital capacity decreased 44% in the influenza group compared with 13% and 17% in the other two groups, respectively (p less than 0.01 for both comparisons). The influenza group also had a higher proportion of patients with at least a 20% decrease in forced expiratory volume per second and forced expiratory flow in first 25% of vital capacity than the other two groups had (p less than 0.05 for all comparisons). These data suggest that influenza is associated with severe exacerbations in patients with cystic fibrosis and support recommendations for efforts to prevent influenza in this population.

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