Microbiology · MU Sofia

88. Family Orthomyxoviridae – Influenza Virus

Covers the influenza virus within the family Orthomyxoviridae, from its segmented negative-sense RNA genome and the three types A, B and C to its surface proteins haemagglutinin and neuraminidase. It then explains antigenic drift and shift, epidemiology, pathogenesis, clinical features and complications, laboratory diagnosis with a comparison table of tests, antiviral treatment, and prevention through vaccination and infection control.

Inside: 10 sections · 4 labelled pictures · 2 tables · 14 flashcards · 3 min

Topic 88 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 10 sections, about 300 words (3 min of reading), with 14 flashcards.

In this note

  1. Family Orthomyxoviridae – Influenza Virus
  2. I. General Characteristics
  3. II. Structure and Antigenic Components
  4. III. Antigenic Variation
  5. IV. Epidemiology
  6. V. Pathogenesis
  7. VI. Clinical Manifestations
  8. VII. Laboratory Diagnosis
  9. VIII. Treatment
  10. IX. Prevention

I. General Characteristics

  • Belongs to Family Orthomyxoviridae.
  • Enveloped, single-stranded, negative-sense RNA virus.
  • Genome is segmented (8 segments in Influenza A & B).
  • Helical nucleocapsid, surrounded by lipid envelope.
  • Replicates in the nucleus (unique among RNA viruses).
  • Three main types:
  • Influenza A: infects humans, birds, pigs, etc. → causes pandemics.
  • Influenza B: only in humans → seasonal epidemics.
  • Influenza C: mild illness, rarely causes epidemics.

III. Antigenic Variation

  • Antigenic drift:
  • Minor mutations in HA or NA genes.
  • Occurs in Influenza A and B.
  • Causes seasonal epidemics.
  • Antigenic shift:
  • Sudden reassortment of gene segments between different Influenza A strains.
  • Leads to new HA and/or NA subtypes.
  • Responsible for pandemics (e.g., 2009 H1N1).

IV. Epidemiology

  • Reservoir: humans (A, B, C), animals (A: birds, pigs).
  • Transmission: respiratory droplets, direct contact.
  • Seasonality: Winter in temperate regions.
  • Incubation: 1–4 days.
  • High risk: elderly, children, immunocompromised, pregnant women.

V. Pathogenesis

  • Inhaled virus infects respiratory epithelial cells.
  • HA binds to sialic acid receptors → endocytosis.
  • M2 protein acidifies endosome → viral uncoating.
  • Replication in host nucleus → viral RNA polymerase synthesizes mRNA.
  • Destruction of respiratory epithelium:
  • ↓ mucociliary clearance → risk of secondary bacterial infections.
  • Inflammatory cytokine release → systemic symptoms (fever, myalgia).

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