Viruses

• Virions - complete viral particles
• Chickenpox - acute, febrile, viral infection with fever and skin rash.
• Varicella is the cause of chickenpox.
• Reye syndrome may follow chickenpox if aspirin is given to children younger than 16 years of age.
• Chickenpox is the leading cause of vaccine-preventable death in the United States.
• Shingles, caused by herpes zoster virus, is the reactivation of the varicella virus.
• Shingles involves inflammation of sensory ganglia, producing fluid filled blisters, pain, and paresthesia.
• For chickenpox and shingles, use Airborne and Contact Precaution for hospitalized patients until lesions become dry and crusted.
• Herpes simplex virus (HSV) causes cold sores on the lips or genital herpes.
• For rubella, use Droplet Precaution until 7 days after onset of rash.
• Viruses


  Infectious agents that are too small to be seen (even when you use a light microscope) and that are not cells
• Viruses


  • When they invade susceptible host cells, viruses displays some properties of living organisms and so they appear to be on the borderline between living and nonliving
  • They can replicate ONLY inside a living host cell, making them an obligate intracellular parasites
  • Individual virus particles contain only one kind of nucleic acid – either DNA or RNA but never both. However, a DNA-RNA hybrid virus was found
• Viral replications


  Virus particle infect a cell and program the host's cell machinery to synthesize the components required for the assembly of new virus particles
• Nucleic acids


  The genetic information of the virus. It is used to replicate themselves in host cells
• Viral replication


  Depends on the expression of the viral genome for the formation of viral proteins and the replication of new viral genomes within the infected host cell
• Capsids


  • Protects and determines the shape of the virus
  • Play a key role in the attachment of some viruses to host cells
  • Each capsid is composed of protein subunits called capsomeres
  • The number of proteins and the arrangement of viral capsomeres are characteristic of specific viruses and thus can be useful in virus identification and classification
• Nucleocapsid


  Viral genome + capsid
• Envelopes


  • Typical bilayer membrane outside the capsids
  • Such viruses acquire their envelope after they are assembled in a host cell as they bud, or move through, one or several membranes
  • Viruses with only a nucleocapsid and no envelope are known as naked viruses
  • The composition of an envelope generally is determined by the viral nucleic acid and by the substances derived from host membranes
  • Depending on the virus, projections referred to as spikes may or may not extend from the viral envelope
  • Spikes attach virions to the specific receptor sites or susceptible host cell surfaces
  • In certain viruses, spikes causes various types of RBC to hemagglutinate (clump)
  • Because envelopes are acquired from and are therefore like host cell membranes, viruses may be "hidden" from attack by the host's immune system
• Size and shapes of viruses

  • Most viruses are too small to be seen with a light microscope, but they have a variety of sizes
  • Shapes of viruses are determined by their capsids
• Sizes of viruses

  • Megavirus chilensis (1,200 - 1,500 nm)
  • Bacteriophages (65 - 200 nm)
  • Enteroviruses (<30 nm)
• Shapes of viruses

  • Helical – ribbon like protein that forms a spiral around the nucleic acid
  • Polyhedral – many sided
  • Icosahedron – have 20 triangular faces
  • Complex – combination of helical and icosahedral shapes
  • Spherical – most viruses with envelopes
• Host range


  The spectrum of hosts that a virus can infect
• Most viruses are limited to only one host and to only specific cells and/or tissues of that host
• Viral specificity


  The specific kinds of cells a virus can infect. It is determined by the ability of the virus to attach on a cell, availability of appropriate host enzymes and other proteins the virus needs inside the host cell, and ability to release the replicated viruses from the cell to spread infection to other cells
• Polioviruses can be grown in the lab in monkey kidney cells but have never been observed to cause a natural infection in any animal other than humans
• Classification of viruses based on host

  • Bacterial viruses (bacteriophages)
  • Plant viruses
  • Animal viruses
  • Dermotropic – infects the skin
  • Neurotropic – infects the nerve tissues
  • Viscerotropic – infects the organs of the digestive tract
  • Pneumotropic – infects the respiratory system
• As more was learned about the structure of viruses at the biochemical and molecular levels, classification of viruses came to be based on the type and structure of their nucleic acids, method of replication, host range, and other chemical and physical characteristics
• The need for a single, universal taxonomic scheme for viruses led to the establishment in 1966 of the International Committee on Taxonomy of Viruses (ICTV)
• The ICTV requires that the English common name, rather than a Latinized binomial term, be used to designate a viral species
• Formal taxonomic designations

  • Family: Rhabdoviridae; genus: Lyssavirus; species: rabies virus
  • Family: Retroviridae; genus: Lentivirus; species: human immunodeficiency virus (HIV)
• The ICTV has assigned more than 5,000 member viruses to 108 families and 203 genera, plus 30 genera that have not yet been assigned to families
• Major groups of viruses distinguished by nucleic acid content

  • RNA viruses
  • DNA viruses
• RNA viruses

  • Can be single-stranded (ssRNA) or double-stranded (dsRNA)
  • Because most eukaryotic cells do not have the enzymes to copy viral RNA molecules, the RNA viruses must either carry the enzymes or have the genes for those enzymes as part of their genome
  • Positive (+) sense RNA - RNA acts like mRNA and can be translated by the host's ribosomes
  • Negative (-) sense RNA - RNA acts as a template during transcription to make a complementary (+) sense mRNA after a host cell has been entered
  • In order to perform the transcription step, (−) sense RNA viruses must carry an RNA polymerase within the virion
• DNA viruses


  Can also occur in single-stranded or double-stranded form
• DNA viruses

  • The human adenoviruses, responsible for some common colds, and the herpesviruses are double-stranded DNA (dsDNA) viruses
  • Only one single-stranded DNA (ssDNA) virus is currently known to produce human disease (parvoviruses)
• RNA virus families

  • Picornaviridae
  • Togaviridae
  • Flaviviridae
  • Retroviridae
  • Paramyxoviridae
  • Rhabdoviridae
  • Orthomyxoviridae
  • Filoviridae
  • Bunyaviridae
  • Arenaviridae
  • Reoviridae
• Picornaviridae


  • Very small (30 nm in diameter), naked, polyhedral, (+) sense RNA viruses
  • Interrupt all functions of DNA and RNA in the host cell
  • Divided in to several groups, including the Genera: Enteroviruses – include the polioviruses, Hepatoviruses – causes Hepatitis A, Rhinoviruses – responsible for common colds
• Togaviridae


  • Small, enveloped, polyhedral, (+) sense RNA viruses
  • Multiply in the cytoplasm of many mammalian and arthropod host cells
  • Transmitted by mosquitoes and cause several kinds of encephalitis (plural: encephalitides) in humans and in horses
  • The rubella virus (causes German measles) belongs to this family
• Flaviviridae


  • Enveloped, polyhedral, (+) sense RNA viruses
  • Transmitted by mosquitoes and ticks
  • Produce a variety of encephalitides or fevers in humans
  • The yellow fever virus is a flavivirus that causes a hemorrhagic fever—in which blood vessels in the skin, mucous membranes, and internal organs bleed uncontrollably
  • Hepatitis C and the dengue virus are also caused by Flaviviruses
• Retroviridae

  • Enveloped viruses that have two complete copies of (+) sense RNA
  • Contain the enzyme reverse transcriptase, which uses the viral RNA to form a complementary strand of DNA, which is then replicated to form a dsDNA
  • DNA migrates to the host cell nucleus and become incorporated into chromosomes of host cells (hence its referred as a provirus)
  • Cause tumors and leukemia in rodents and birds, as well as in humans
  • Human T cell leukemia viruses (HTLV-1 & HTLV-2) invade T lymphocytes and is associated with malignancies (leukemia and other tumors)
  • Human immunodeficiency virus (HIV-1 and HIV-2) causes acquired immune deficiency syndrome (AIDS)