Topic 5

• Viruses


  Virus particles are called virions, composed of either RNA or DNA that is encased in a protein coat called a capsid, either naked or enveloped
• Viruses

  • Replicate only in living cells and therefore are obligate intracellular parasites, cannot be observed with a light microscope
• Viral genomes

  • Single stranded or double stranded, linear or circular, segmented or non-segmented
• Capsid

  Composed of structural units called capsomers, aggregates of viral-specific polypeptides, serves to protect the viral genome, initiate infection, stimulate antibody production, and act as antigenic determinants
• Viral envelope

  Surrounds the nucleocapsid of enveloped viruses, composed of viral-specific glycoproteins and host-cell-derived lipids and lipoproteins, contains molecules necessary for enveloped viruses to initiate infection, act as a stimulus for antibody production, and serve as antigens in serologic tests
• Steps of viral replication

  1. Adsorption (attachment)

  2. Penetration

  3. Uncoating

  4. Synthesis of viral components (mRNA, viral proteins, NA)

  5. Assembly

  6. Release
• Attachment

  Viral proteins on the capsid or phospholipid envelope interact with specific receptors on the host cellular surface, determines host range (tropism)
• Penetration

  Attachment to a specific receptor can induce conformational changes in viral capsid proteins or lipid envelope, resulting in fusion of viral and cellular membranes, some DNA viruses can also enter through receptor-mediated endocytosis
• Uncoating

  The viral capsid is removed and degraded by viral enzymes or host enzymes, releasing the viral genomic nucleic acid
• Replication

  Transcription or translation of the viral genome is initiated, leading to de novo synthesis of viral proteins and genome
• Assembly

  Viral proteins are packaged with newly replicated viral genome into new virions that are ready for release from the host cell
• Virion release

  Occurs through lysis (death of host cell) or budding (enveloped viruses, do not kill host cell)
• Types of viral infections

  • Asymptomatic
  • Acute
  • Persistent
  • Latent
• Acute infections

  Rapid production of infectious virus followed by rapid resolution and clearing of the infection by the host, clinical manifestations observed for a short time (days to weeks)
• Localized acute infections

  Viral multiplication and cell damage remain localized to the site of viral entry, short incubation time, may cause systemic clinical features, pronounced viremia usually does not occur
• Disseminated acute infections

  Virus spreads from entry site to target organ, involve a primary viremia and perhaps a secondary viremia, incubation time is moderate
• Latent infections

  Virus persists in the body in a non infectious form that can periodically reactivate to an infectious virus and produce clinical disease, synonymous with recurrent disease
• Persistent infections

  Infectious virus may be produced continuously or intermittently for months or years, infected individuals are known as carriers
• Antiviral agents

  Must selectively inhibit viral replication without affecting host cell viability or normal functions, work by inhibiting viral nucleic acid replication, penetration, uncoating, or specific viral enzyme function
• Viral diseases

  • Respiratory infections (colds, pharyngitis, otitis media, sinusitis)
  • Gastrointestinal infections (gastroenteritis)
  • Liver infections (hepatitis)
  • Rabies
  • Poliomyelitis
  • Childhood diseases (measles, mumps, rubella)
  • Latent/recurrent infections (herpes, varicella zoster)

  
• lysis results in the death of an infected host cell these types of viruses are referred to as cytolytic
• the types of viruses that do not usually kill the infected cells are termed cytopathic viruses