Influenza A, B, and C viruse - MegaMicro

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Influenza A, B, and C viruse

Micro > 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
b. RT-PCR
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
Flu
Drift
Shift
Pandemic
 
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