Lung - nontumor
Influenza pneumonia

Topic Completed: 1 September 2011

Revised: 29 January 2019, last major update September 2011

Copyright: (c) 2003-2018,, Inc.

PubMed search: influenza pneumonia [title]

Elliot Weisenberg, M.D.
Page views in 2019: 1,328
Page views in 2020 to date: 1,335
Cite this page: Weisenberg E. Influenza pneumonia. website. Accessed May 28th, 2020.
  • 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)
  • Preexisting 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 hemagglutinins and neuraminidases, which allow the virus to evade most host antibodies
  • Antigenic shift leads to pandemics, and is caused by recombination of hemagglutinin 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
Case reports
  • 43 year old man in India with with fever for 3 days and sudden onset of shortness of breath (Case of the Week #459)
Clinical features
  • 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 nonspecific; the diagnosis can be confirmed with molecular testing, viral culture, DFA, IHC, serology, antigen detection assays
Microscopic (histologic) images

Images hosted on PathOut server:

Contributed by Dr. K.V. Vinu Balraam


Pulmonary lymph node


Board review style question #1
Which histological feature is highly specific to influenza A/H1N1 infection in diffuse alveolar damage?

A. Neutrophilic aggregation
B. Organizing pneumonia
C. Alveolar hemorrhage
D. All of the above
E. None of the above
Board review answer #1
E. No histological finding specific to influenza infection has been identified. The findings in A, B and C are quite common in diffuse alveolar damage of any etiology.
Board review style question #2
This image is from an autopsy of a 43 year old man with influenza A/H1N1 infection. Which histological feature is highly specific to this disease?

A. Neutrophilic aggregation
B. Organizing pneumonia
C. Alveolar hemorrhage
D. None of the above
Board review answer #2
D. None of the above
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