Clinical features of influenza A and B in children and association with myositis.

Influenza virus is among the most common causes of respiratory illness, which may manifest as a range of conditions, from mild upper respiratory tract infection to bronchiolitis and pneumonia. Acute childhood myositis associated with influenza occurs mostly in influenza B infection. In this retrospective study, we analyzed the characteristics of 197 children with influenza virus treated from January 2000 to December 2001. Among them, 73 children had influenza A infection and 124 had influenza B infection. Influenza A virus outbreaks occurred in January 2000, July 2001, and December 2001, while influenza B virus outbreaks occurred from March 2000 to May 2000 and from December 2000 to February 2001. The most common clinical manifestations of influenza A and influenza B virus infection included fever, cough, and rhinorrhea. These infections also frequently manifested as laryngo-tracheobronchitis, pneumonia, and unexplained fever, which led to hospitalization. The most common clinical diagnosis was upper respiratory tract infection. The rates of benign acute childhood myositis in influenza A and influenza B were 5.5% and 33.9%, respectively. Creatine kinase levels were elevated in most myositis cases and boys were more commonly affected. Acute childhood myositis was more commonly seen in influenza B infection.