RNA Viruses

• Properties of RNA viruses:
• RNA is labile and transient
• Most RNA viruses replicate in the cytoplasm
• Cells cannot replicate RNA
• RNA viruses must encode an RNA-dependent RNA polymerase
• The genome structure determines the mechanism of transcription and replication
• RNA viruses are prone to mutation
• The genome structure and polarity determine how viral mRNA is generated and proteins are processed
• RNA viruses, except for (+) RNA genome, must carry polymerases
• All (-) RNA viruses are enveloped
• Classification and Replication of RNA viruses:
• Picornaviruses and Togaviruses:

  • (+) sense RNA virus
  • Structure: single-stranded RNA genome; icosahedral capsid
  • Replication
• Reoviruses:

  • (+/-) sense RNA virus
  • Structure: Segmented, double-stranded RNA genome; icosahedral capsid with no envelope
  • Replication
• Picornaviruses:

  • Poliovirus
  • Morphology: single-stranded RNA genome; icosahedral capsid with no envelope
  • Transmission: Fecal-oral route
  • Spectrum of Disease: Poliomyelitis and Meningitis
• Coxsackie Virus (A and B):

  • Morphology: single-stranded RNA genome; icosahedral capsid with no envelope
  • Transmission: Fecal-oral route
  • Spectrum of Disease: Coxsackie A virus - Herpangina, Coxsackie B virus - Pleurodynia
• Echo (Enteric Cytopathic Human Orphan):

  • Morphology: single-stranded RNA genome; icosahedral capsid with no envelope
  • Transmission: Fecal-oral route
  • Spectrum of Disease: Aseptic meningitis
• Rhinovirus:

  • Morphology: single-stranded RNA genome; icosahedral capsid with no envelope
  • Transmission: Contact with respiratory secretions
  • Pathogenesis: Replicates better at 33°C than at 37°C, affects primarily the nose & conjunctiva
  • Spectrum of Disease: Common cold
• Hepatitis A Virus (Enterovirus 72):

  • Morphology: single-stranded RNA genome; icosahedral capsid with no envelope
  • Transmission: Fecal-oral route
  • Pathogenesis: Replicates in the GI tract and spreads to the liver
  • Spectrum of Disease: Hepatitis A infection
• Togaviridae - Rubella Virus:

  • Morphology: enveloped, icosahedral, non-segmented, (+ ssRNA)
  • Transmission: Respiratory droplets
  • Spectrum of Disease: Rubella/German Measles/3-Day Measles
• Reoviridae - Rotavirus:

  • Morphology: naked double-layered capsid with segmented dsRNA
  • Pathogenesis: Villous destruction with atrophy
  • Spectrum of Disease: Viral gastroenteritis (childhood: "white stool diarrhea")
• Hepeviridae - Hepatitis E Virus:

  • Morphology: Naked + ssRNA (icosahedral symmetry)
  • Transmission: Oral-fecal route
  • Spectrum of Disease: Hepatitis E
• Coronaviridae - Coronavirus:

  • Morphology: enveloped helical non-segmented, (+ ssRNA), with prominent club-shaped spikes
  • Reservoir: Horseshoe bat
  • Transmission: Respiratory droplet
  • Spectrum of Disease: Common cold, SARS, MERS-COV, COVID-19