Influenza A, B, and C viruse - MegaMicro

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Influenza A, B, and C viruse

Microbiology > Virology > RNA viruses > Single Stranded RNA viruses > Orthomyxovirus
1.       Structure
a. Size: 80-120 nm
b. Usully spherical but can be filamentous
c. Enveloped virus - buds from plasma membrane
d. negative stranded RNA
e. Viral envelope made up of two main types of proteins; virus strainse are categorized based on these proteins
i. HA = hemagglutinin; causes RBCs to agglutinate. Antibody to HA leads to protective antibody
ii. NA = neuraminidase; cleaves the glycosidic bonds of neuraminic acid
f. Influenza A and B genomes have 8 nuceocapsid segments, each coding for a different viral protein. Influenza C has 7 segments
i. the segmented nature of the Influenza A genome is critical for reassortment and pathobiology/epidemiology

2.       Pathobiology
a. Virulence factors: may be resistant to antiviral agents oseltamivir and amantadine
b. Tropism: epithelial respiratory cells
c. Host: aquatic birds, humans, pigs, horses, seals
d. Incubation period: 1-3 days  
e. Antigenic DRIFT
    • season to season point mutations in HA and/or NA that lead to slight changes in antigenicity of the strain
    • responsible for seasonal changes to dominant influenza type
    • some limited immunogenicity exists in the human population year-to-year.
    • Drift leads to need for yearly modification of influenza vaccine to address antigenic changes
f. Antigenic SHIFT
    • seen only in influenza A
    • results from reassortment of viral segments in a transfected cell
    • multiple animal viruses infect a cell in a permissive host (usually birds or pigs), leading to reassorment of viral RNA strands and a new viral type.
    • introduction of a new HA and/or NA into a human influenza has limited existing immunity in human populations, leading to a pandemic with the new strain.

3. Epidemiology
a. Present worldwide, more common in winter time in temperate climates, and year-round in tropical regions.
  • Summer months in Southern Hemisphere
  • Winter months in Northern Hemisphere
b. Transmission: respiratory infection by aerosols and droplets or from contact transmission from contaminated surfaces
    • fecal transmission in animals (e.g. birds)
c. Highly communicable
d. Occasional pandemic seen after significant SHIFT in HA and/or NA  (shift is seen only in influenza A, not in influenza B or C)
    • last pandemic 2009 from a reassorted avian/swine/human variant
d. Reservoir: humans are the principle reservoir of human influenza A viruses
i. non-human reservoirs (birds, other mammals) are infected by influenza strains and can contribute to pandemics
e. Animal strains, most notably H5N1 and other avian strains, can occasionally infect humans.
    • High mortality but limited transmission person to person
    • High potential to become a pandemic if virus can transmit efficiently from person to person

4.  Laboratory diagnosis
a. Rapid antigen tests, using Nasopharygeal secretions
c. Culture (typically only used by health departments to characterize predominant viral strains)
d. None of the rapid influenza diagnostic tests provide any information about influenza A virus subtypes

5.  Disease manifestations
    • Influenza A:
      • incubation period 1-4 days
      • abrupt onset of illness
        • high  fever
        • myalgias
        • URI symptoms
        • Nonproductive cough
        • weakness and fatigue
      • recovery in 3-8 days+
      • Occasional viral pneumonia
      • damage to respiratory epithelium may lead to loss of mucociliary clearance, predisposing to bacterial pneumonia, otitis, and sinusitis.
      • Inflammatory responses
        • myocarditis
        • encephalopathy
        • post-influenza encephalitis
        • Reye Syndrome  (acute encephalopathy and hepatitis associated with aspirin use)
    • Influenza B:
      • Similar to influenza A
    • Influenza C:
      • usually milder compared to influenza A or B

6. Prevention:
a. Vaccine:
  • multiple varieties of vaccine are available
    • live, attenuated influenza vaccine (LAIV)
    • inactivated influenza vaccine (3-valent, 4-valent, and high dose varieties)
    • subunit vaccine
  • recommended for most populations in USA
  • combined antigens from recent influenza A and B isolates
b. Hand hygeine
c. PPE in healthcare settings

7. Treatment:
a. Usually supportive therapy
b. Influenza A:
Antiviral drugs
        • M2 protein inhibitors: Amantadine and Rimantadine
          • inhibit uncoating
          • most circulatingn influenza A is now resistant to amantadine and rimantadine
        • NA inhibitors
          • Zanamivir, Oseltamivir, Peramivir
          • Prevents cleavage of sialic acid on cell surface, thus preventing release of virus after budding
        • Inhibition of mRNA 5' cap of viral polymerase (PB2)
          • Baloxivir
c. Influenza B:
Antiviral drugs as above
d. Influenza C:
No approved agents

Related concepts
Segmented Genome
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