Reviewer: Elliot Weisenberg, M.D. (see Reviewers page)
Revised: 6 September 2011, last major update September 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.
● Most common cause of viral pneumonia
● RNA virus, nucleoprotein determines type of virus: A, B or C (Virol J 2009 Mar 13;6:30)
● Envelope contains the viral hemagglutinin and neuraminidase that establish the viral subtype, e.g., H1N1 (Wikipedia)
● Pre-existing antibodies to specific hemagglutinins and neuraminidases prevent or amerliorate future infections
● Type A: causes most severe disease; major cause of epidemics and pandemics; also infects pigs, horses, birds; one subtype predominates at any given time
● Antigenic drift leading to epidemics is caused by small mutations in hemagglutins and neuraminidases, which allow the virus to evade most host antibodies
● Antigenic shift leads to pandemics, and is caused by recombination of hemaglutinin and neuraminidase RNA with animal influenza RNA, leading to absence of human immunity
● Antigenic drift and shift do NOT occur with influenza types B and C, which usually cause mild, self-limited illness in children
● References: World Health Organization Influenza update
● Cause mild acute lung injury to necrotizing pneumonia to BOOP-like changes
● May cause bronchiolitis or diffuse alveolar damage
● Viral cytopathic changes do NOT occur
● Airways show mucosal hyperemia with infiltrate of lymphocytes, histiocytes, plasma cells
● Swelling and increased mucus production impair bronchociliary elevator, and may lead to secondary bacterial infection; small airways may become occluded causing focal atelectasis; in severe disease, this is more widespread with distal airways plugged by fibrinopurulent debris
● If process is prolonged, obliterative bronchiolitis with irreversible lung damage may occur
● Histologic findings are non-specific; the diagnosis can be confirmed with molecular testing, viral culture, DFA, IHC, serology, antigen detection assays
Heavy and firm lung
End of Lung-nontumor > Infections > Influenza pneumonia
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