Respiratory virus infections of the lower respiratory tract elevate bronchoalveolar lavage eosinophil fraction: a clinical retrospective study and case review

Abstract Background: Eosinophilic airway inflammation caused by respiratory virus infection has been demonstrated in basic research; however, clinical investigations are lacking. To clarify the extent to which respiratory viral infection induces airway eosinophilic inflammation, we reviewed the results of bronchoalveolar lavage (BAL) and respiratory virus testing performed at our hospital. Methods: Among the BAL procedures performed at our hospital from August 2012 to September 2016, we collected cases of acute respiratory disease in which multiplex polymerase chain reaction (PCR) was used to search for respiratory viruses. The effect of respiratory virus detection on the eosinophil fraction of BAL was analyzed using statistical analysis. A case study of respiratory virus detection in which the eosinophil fraction of BAL was elevated was also conducted. Results: A total of 95 cases were included in this study, of which 17 were PCR-positive. The most common respiratory virus detected was parainfluenza virus (eight cases). The PCR-positive group showed a trend toward a higher eosinophil fraction in BAL fluid than the PCR-negative group (p=0.030), and more cases had a BAL eosinophil fraction >3% (p=0.017). Multivariate analysis showed that PCR-positive cases were associated with BAL eosinophil fractions >1% and >3%. There were nine PCR-positive cases with a BAL eosinophil fraction >1%, of which two cases with parainfluenza virus infection had a significantly elevated BAL eosinophil fraction and were diagnosed with eosinophilic pneumonia. Conclusions: A trend toward an elevated eosinophil fraction in BAL fluid was observed in cases of viral infection of the lower respiratory tract. The degree of eosinophilic fraction elevation due to respiratory viral infection was mild overall, whereas some cases showed prominent elevation and were diagnosed with eosinophilic pneumonia. Respiratory viral infection is not a rare cause of elevated BAL eosinophilic fraction and should be listed as a differential disease in the practice of eosinophilic pneumonia.