Virus Features & Classification

Virus Features

• Viruses reproduce by commandeering a host cell and using its machinery
• Viruses reprogram host cells to become virus-making "factories"
• Viruses shapes and structures differ, have different genomes, and infect different hosts
• The components of a virus include a DNA or RNA genome inside a protein shell, called a capsid
• Some viruses also possess an external membrane known as an envelope
• Viral sizes range from 10-100nm, about 1/10th the size of bacteria
• Genomes can be either RNA or DNA, but never both
• Viruses are obligate intracellular parasites and can only multiply in a living host cell

Virus Classification

• Viruses are classified via symmetry and type of nucleic acid
• Symmetry include: Icosahedral, Filamentous, Head-tail
• Nucleic acid can be either DNA or RNA
• Viruses can have all possible combinations of strandedness and nucleic acid type
• Types include: double-stranded DNA, double-stranded RNA, single-stranded DNA, single-stranded RNA

Key Structures

• Key structures of viruses include: Capsid, Viral Nucleic acid, Nucleocapsid, Envelope, Viral Protein

RNA Viruses

• RNA viruses generally exhibit significantly higher mutation rates compared to DNA viruses

Viral Replication and Dissemination

• Steps may be merged or omitted for some viruses
• The period between infection and creation of the new virion can be 3 hours or several months
• Steps of viral replication and dissemination are:
• Adsorption: Virus particle attaches to specific host cell receptors, positive/firm attachment requires presence of receptors on host plasma membrane
• Penetration: Virus or its genome enters the host cell through endocytosis, fusion (direct fusion of viral envelope & host cell), or translocation (non-enveloped virus passes directly through host membrane)
• Uncoating/eclipse: No detection is possible during this pause in infectivity
• Eclipse phase: There is uncoating of the lipid membrane & protein capsid surrounding nucleic core
• Eclipse: Viral nucleic acid turns free, and acts as template for mRNA
• Transcription: mRNA synthesis of enzymes initiates early replication
• The virus uses the existing cell structures to replicate, such as integrating in the host DNA
• Synthesis of viral components: Viral proteins are structural or non-structural (enzymes)
• Components are built on ribosomes
• Viral assembly: Nucleic acid incorporates into capsomeres
• Assembly can occur in host nucleus, cytoplasm or at plasma membrane
• Release:
  • Lytic replication - host cell dies
  • Lysogenic replication - host cell lives and continues to function

Viral Terms

• Viral Persistence: Infections in which the virus isn't cleared and remains in specific cells of infected individuals
• Examples: HIV, Epstein-Barr, Human Cytomegalovirus, Human Herpesviruses 6 and 7, Varicella-Zoster Virus, Human Papovaviruses, and Hepatitis B Virus
• Persistent infections encompass stages of silent and productive infection excluding quickly killing / excessive damage of the host cells
• Viral Latency: A pathogenic virus can lie dormant (latent) within a cell, denoted as lysogenic
• Latent viral infection: A persistent viral infection type which contrasts chronic viral infection

Common Viruses Relevant to Dentistry

• HIV
• Hepatitis B
• Varicella Zoster
• Herpes
• Influenza

Viral Tropism

• Viral tropism: The ability of a virus to infect a particular cell, tissue, or host species
• This includes cellular tropism, tissue tropism, and host tropism