Pandemic influenza and pneumonia due to Legionella pneumophila: a frequently underestimated coinfection

Secondary bacterial pneumonia is recognized as one of the most common causes of death in influenza cases. Coinfection has been found in ?30% of all influenza cases in persons with seasonal influenza, and the pathogens most often involved are Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenza. However, the role of bacterial coinfection in complicating pandemic flu is not well described, because of the scarcity of data. The cases described in this report do not derive from a systematic ascertainment of all pneumonia cases associated with pandemic influenza, and the number might be underestimated, because the decision to perform etiological diagnosis rests with the individual physician. However, 6 legionellosis cases in a total of 33 bacterial coinfections may indicate that Legionella is involved more often than expected. In addition, our findings highlight that cross-linkage of different surveillance systems can be a useful method to quantify and to describe pneumonia cases related to influenza. Clinical Infectious Diseases 2010;51(1):115.