17 February 2012
Myocardial Injury and Bacterial Pneumonia Contribute to the Pathogenesis of Fatal Influenza B Virus Infection
On 30th January 2012, the Journal of Infectious Diseases in the advance access page published a report by Paddock et al. which evaluated the characteristics of autopsy tissue specimens collected from patients with confirmed influenza B virus infection and examined the pathologic spectrum of cardiopulmonary injury and the frequency and composition of concomitant bacterial pneumonia. This study was based on the data submitted to the United States Centers for Disease Control and Prevention during May 2000-February 2010 for diagnostic evaluation. The data included the mandatory notified reports of fatal pediatric influenza, which started in 2004.
From a total of 45 patients (29 female and 16 male), the median age of death was 11 years (range, 10 months to 55 years) and 34 (76%) of them was aged <18 years. Most patients had rapid clinical progression: 50% of all case patients died within 3 days and 72% died within 4 days after the onset of illness. Seventeen (38%) patients had pathologic evidence of a concomitant bacterial pneumonia predominantly by Staphylococcus aureus. Age >18 years was the only significant characteristics associated with a higher likelihood to develop bacterial pneumonia (82% among age >18 years vs 24% among those <18 years). Myocardial injury was identified in 20 (69%) of 29 patients with cardiac samples available and 90% of patients were aged <18 years. Of those, 17 patients had myocyte damage including 10 with unequivocal myocarditis. The authors concluded that bacterial pneumonia and cardiac injury contributed to death after infection with influenza B virus infection and the frequency of these manifestations may be age related.
The impact of influenza B virus has long been overshadowed by influenza A virus as hospitalized cases are more often due to influenza A. As a result, the severity of influenza B virus infection has been considered to be milder than seasonal influenza A virus infection. However, recent studies [4, 5] found that influenza B virus infection accounts for substantial percentage of influenza associated hospitalizations and deaths for patients aged <18years. During the 2010-2011 season influenza B comprising 26% of all circulating influenza viruses caused 38% of all influenza associated pediatric deaths in the United States . In addition, as Li WC et al revealed that 24% of the hospitalized cases associated with influenza B virus infection in Taiwan had encephalitis/encephalopathy and 52% of them had influenza associated myositis. Those cases are sufficient to raise awareness of fatal outcomes with influenza B virus infection and preventive strategies. As McCullers et al.  also suggested in editorial of the Journal of Infectious Diseases that further studies are needed specifically for influenza B virus to understand differences in outcomes relative to influenza A virus.
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