Virology: Enteroviruses

Enteroviruses

• Enteroviruses are transient inhabitants of the human alimentary tract and can be isolated from the throat or lower intestine.
• There are four types of enteroviruses:
  • Enterovirus 68: isolated from respiratory tract of children with bronchiolitis or pneumonia.
  • Enterovirus 70: causes acute hemorrhagic conjunctivitis, highly contagious, and transmitted person-to-person via fingers, fomites.
  • Enterovirus 71: isolated from patients with meningitis, encephalitis, and paralysis resembling poliomyelitis, and causes CNS disease and diarrhea.
  • Enterovirus 72: also known as Hepatitis A virus.

Echoviruses (Enteric Cytopathogenic Human Orphan viruses)

• There are 33 serotypes of ECHO viruses.
• They are not pathogenic for mice, unlike polioviruses, and do not cause diseases in monkeys.
• Incubation period: 2-9 days.
• They can cause opportunistic infections and diseases such as:
  • Aseptic meningitis
  • Rash, common in young children
  • Conjunctivitis
  • Muscle weakness and spasms
  • Infantile diarrhea
  • Congenital and neonatal disease
  • Cardiac disease

Rhinoviruses

• Rhinoviruses are isolated chiefly from the nose, conjunctiva, and throat, rather than the lower respiratory tract.
• They are acid-labile, killed by gastric acid when swallowed, which explains why they do not infect the gastrointestinal tract.
• The virion consists of a capsid shell of 60 submits, each of four protein (VP1-VP4) arranged in icosahedral symmetry around the ssRNA positive sense genome.
• More than 100 serotypes exist.
• Incubation period: 2-4 days.
• Mode of transmission:
  • Direct: person-to-person via aerosols of respiratory droplets.
  • Indirect: via contaminated hands or surfaces.
• Replication cycle:
  • Replicates better at 33°C than 37°C.
  • Replication is similar to that of poliovirus.
  • Cell surface receptor: ICAM-1 (Intercellular adhesion molecule-1).
• Clinical findings:
  • Associated with acute, febrile upper respiratory disease (common cold).
  • No specific antiviral therapy available.
  • Vaccines appear impractical due to the large number of serotypes.
  • Prevention: use paper tissues impregnated with citric acid and sodium lauryl sulfate.

Reoviruses

• Reoviruses are respiratory and enteric viruses that were initially isolated from the respiratory and enteric tracts.
• The family of reoviridae is divided into twelve genera.
• Rotavirus is the most important cause of infantile gastroenteritis worldwide.

Rotavirus

• Rotavirus was first identified by electron microscopy in 1973 from duodenal biopsies of children with diarrhea.
• Electron microscopic appearance: a wheel with radiating spokes (Latin, Rota = wheel).
• Important properties:
  • Medium in size (60-80nm in diameter).
  • Double-stranded linear, segmented RNA genome (10-11 segments).
  • Surrounded by a double-layer icosahedral capsid (outer capsid, inner capsid) without an envelope.
  • Replication occurs in cytoplasm, virions not completely uncoated.
  • Acid-labile but relatively stable in the environment.
  • Susceptible to disinfection with 95% ethanol, 'Lysol', and formalin.
• Proteins:
  • Six viral proteins (VPs): VP1, VP2, VP3, VP4, VP6, and VP7.
  • Six nonstructural proteins (NSPs): NSP1, NSP2, NSP3, NSP4, NSP5, and NSP6.
• Types of rotavirus:
  • Five species: A, B, C, D, and E.
  • Humans are primarily infected by species A, B, and C, most commonly by species A.
  • Within rotavirus A, there are different strains, called serotypes.
• Mode of transmission: fecal-oral route.
• Replication cycle:
  • Attach to the cell surface at the site of B-adrenergic receptor.
  • RNA-dependent RNA polymerase synthesizes mRNA from each of the segments within the cytoplasm.
  • mRNA are translated into structural and non-structural proteins.
  • One of these proteins, RNA polymerase, synthesizes minus strands that will become part of the genome of the progeny virus.
  • Capsid proteins form an incomplete capsid around the minus strands, and then the plus strands of the progeny segments are synthesized.
  • The virus is released from the cytoplasm by lysis of the cell.
• Clinical features:
  • Most common cause of severe diarrhea among infants and young children.
  • Nearly every child in the world has been infected with rotavirus at least once by the age of five.
  • Immunity develops with each infection, so subsequent infections are less severe; adults are rarely affected.
  • Incubation period: 1-4 days.
  • Symptoms: fever, nausea, vomiting, watery diarrhea, and low-grade fever.
  • Rotavirus replicates in the mucosal cells of the small intestine, resulting in excess secretion of fluids and electrolytes into the bowel lumen, leading to watery diarrhea.
• Diagnosis:
  • Electron microscopy for detection of rotavirus in stool.
  • Culture: group A rotaviruses can now be cultured in monkey kidney cells.
  • Latex agglutination for rapid diagnosis. Several kits are commercially available.