Influenza A virus infection complicated by fatal myocarditis

Influenza virus typically causes a febrile respiratory illness, but it can present with a variety of other clinical manifestations. We report a fatal case of myocarditis associated with influenza A infection. A previously healthy 11-year-old girl had malaise and fever for approximately 1 week before a sudden, witnessed fatal collapse at home. Autopsy revealed a pericardial effusion, a mixed lymphocytic and neutrophilic myocarditis, a mild lymphocytic interstitial pneumonia, focal bronchial/bronchiolar mucosal necrosis, and histologic changes consistent with asthma. Infection with influenza A (H3N2) was confirmed by virus isolation from a postmortem nasopharyngeal swab. Attempts to isolate virus from heart and lung tissue were unsuccessful. Immunohistochemical tests directed against influenza A antigens and in situ hybridization for influenza A genetic material demonstrated positive staining in bronchial epithelial cells, whereas heart sections were negative. Sudden death is a rare complication of influenza and may be caused by myocarditis. Forensic pathologists should be aware that postmortem nasopharyngeal swabs for viral culture and immunohistochemical or in situ hybridization procedures on lung tissue might be necessary to achieve a diagnosis. Because neither culturable virus nor influenza viral antigen could be identified in heart tissue, the pathogenesis of influenza myocarditis in this case is unlikely to be the result of direct infection of myocardium by the virus. The risk factors for developing myocarditis during an influenza infection are unknown.