Reoviruses, Rotaviruses and Caliciviruses

1
Reoviruses, Rotaviruses and Caliciviruses

• Chapter 37

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Properties of Reoviruses

• Structure
• Icosahedral, 60-80 nm, double capsid shell
• Segmented, double-stranded RNA 16-27 kb
• 9 structural proteins, several enzymes packaged
  in virus
• Nonenveloped
• Cytoplasmic replication
• Extremely stable
• Heat resistant
• Remain infectious after treatment of pH 3-9
• Classification
• Respiratory, enteric, orphan
• Family Reoviridae
• Nine genera

3
Reovirus Replication

• Receptor-mediated attachment and endocytosis
• Vesicle is targeted to lysosome (fusion)
• Drop in pH allows virus to shed its outer capsid
• This results in inner capsid escape into the
  cytoplasm
• The RNA transcriptase (a polymerase) is activated
• Inner capsid uncoats, releasing RNAs into
  cytoplasm
• Viral mRNA synthesis is initiated
• Viral polypeptides are synthesized
• A viral replicase synthesizes second RNA strand
• Assembly occurs in cytoplasm
• Mature virus exits when cell dies from lysis

4
Rotaviruses

• Major cause of diarrhea
• Similar to reoviruses in morphology and
  replication strategy
• Classification
• Five serological groups (A-E)
• Group A can be distinguished from others by
  electophoretic mobility of gene segments
• Animal susceptibility
• Rotaviruses are highly promiscuous
• Major cause of disease in livestock industry
• Many young animals can become persistent
  carriers, shedding virus to other animals
• Difficult to propagate in cell culture systems
• Often requires infection of animals, then
  harvesting virus from diarrhea

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Rotavirus Gene Segments
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Rotavirus Pathogenesis

• Infect villi of small intestine
• Multiply in enterocytes
• Distrupts cellular transport systems
• Endoplasmic reticulum in particular
• Damaged cells slough off and shed virus
• Enterocytes are replaced by crypt cells
• Crypt cells have different sodium and glucose
  transport features that lead to fluid loss in the
  small intestine
• Diarrhea ensues

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Rotavirus Pathogenesis

• Clinical findings and laboratory diagnosis
• Principally a disease of children
• Can be fatal if not treated (hydration)
• Diagnostic by ELISA or PCR
• Epidemiology and Immunity
• 3 to 5 billion cases per year
• As many as 5 million deaths
• 50 of childhood gastroenteritis are caused by
  rotaviruses
• IgA controls infection and provides immunity of
  differential durability
• Treatment and control
• Treatment is replacement of fluids and
  electrolytes
• Vaccine licensed in US in 1998 but was withdrawn
  because of bowel obstructions in some children
• New vaccine has recently been approved

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Caliciviruses

• Gastroenteritis
• Norwalk virus and Norwalk-like viruses
• Recently infamous for cruise ship infections
• Extremely difficult to decontaminate
• Unculturable (propagated in animals)
• Family Caliciviridae
• Clinical Findings and Laboratory Diagnosis
• Diarrhea
• No defined demographic distribution
• In the absence of bacterial etiology, such
  diarrheal disease is presumed a calicivirus
• Diagnosic is by PCR
• Epidemiology and Immunity
• Clustered outbreaks
• Food and water-borne
• Communicable
• Immunity is not durable

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Other Reoviradae Members

• Orbiviruses
• Naturally infect insects
• Can be transmitted to vertebrates
• Problem in livestock industries
• Bluetongue virus of sheep
• African horse sickness
• Astroviruses
• Single-stranded plus sense RNA
• Diarrhea
• Children and institutionalized elderly
• Can establish persistent infection in
  immunocompromised