Certain capsular serotypes of Streptococcus pneumoniae are particularly likely to cause disease in people who have recently experienced a respiratory viral infection, study findings indicate.
Specifically, infrequently colonising invasive (ICI) serotypes were significantly more prevalent in patients with post-viral pneumonia than in those with primary pneumococcal pneumonia.
“It is not clear why ICI serotypes would be more transmissible compared to others”, admit the researchers, led by Joon Young Song (University of Alabama at Birmingham, USA), writing in PLoS ONE. “Further clinical and experimental studies are warranted to clarify the pathogenesis.”
Song’s team retrospectively identified 919 patients who were hospitalised at Korea University Guro Hospital with community-acquired pneumonia caused by S. pneumoniae over a 5-year period.
In all, 327 of the patients (35.6%) were classified as having post-viral pneumococcal pneumonia, based on a documented history of flu-like illness 0–7 days before pneumonia onset.
Compared with the 592 patients with primary pneumococcal pneumonia, those with post-viral disease were significantly older and had more comorbidities, including immunodeficiency, chronic lung disease and chronic liver disease. Rates of bacteremia and 30-day mortality were similar between the groups.
Serotypes 3 and 19A were the most prevalent overall, followed by serotypes 19F, 6A, and 11A/11E. The ICI serotypes (4, 5, 7F/7A, 8, 9V/9A, 12F, and 18C) were relatively uncommon, accounting for just 33 cases (3.6%).
Interestingly, however, 23 (69.7%) of the ICI serotypes were in patients with post-viral pneumococcal pneumonia whereas just 10 were in patients with primary pneumonia, a significant difference.
In multivariate analysis, ICI serotype was a significant independent predictor of post-viral disease, with an odds ratio (OR) of 4.66. Other significant predictors were immunodeficiency (OR=1.66) and chronic lung disease (OR=1.43).
Of the various ICI serotypes, 7A/7F was significantly more prevalent in patients with post-viral disease than in those with primary pneumonia. Of note, three-quarters of ICI-serotype pneumonia occurred during influenza epidemic periods, whereas other serotypes were evenly distributed between epidemic and non-epidemic periods.
Song et al say that their observation that ICI serotypes are more frequent in post-viral disease strongly suggests that “increased transmissibility may be the critical factor in humans.”
Noting that epidemic pneumococcal pneumonia outbreaks might be related to concurrent respiratory viral infections, they write: “Consequently, surveillance of respiratory viral infections in relation to pneumococcal outbreaks would be warranted”.
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