Virology

Viruses: what are they and what do they do?

• Viruses are microscopic organisms that exist almost everywhere on earth. They can infect animals, plants, fungi, and even bacteria

• Viruses vary in complexity. They consist of genetic material, RNA or DNA, surrounded by a coat of protein, lipid (fat), or glycoprotein.

• Viruses cannot replicate without a host, so they are classified as parasitic.

Viruses

• Non-cellular form of life

• Obligate intracellular parasites

• The fundamental characteristic is their absolute dependence on a living host organism for their reproduction, no metabolic activity outside host

• Exist as inert particles (virions) outside the cell

• Virions harbor viral genome protected by protein shell

Virion vs virus:

• Virion is the infectious particle

• composed of nucleic acid, protein capsid, +/- envelope

• may be extracellular or intracellular

• Virus is any stage of infection

Transmission of Viruses

• Touch/direct contacts

• Exchanges of saliva, coughing, or sneezing

• Sexual contact

• Contaminated food or water

• Insects that carry them from one person to another

Viral Epidemiology

• Viral epidemiology is the scientific discipline concerned with the study of the incidence and spread of viruses in populations over time.

• Host, virus and environmental factors are monitored to determine the dynamics of viral infections, the ultimate goal of which is to devise intervention strategies.

Basic Structures of Viruses

General Structure of Viruses

• Capsid

  • Definition: Protein coat surrounding the virus

  • Function: Protects genetic material

  • Composition: Made of protein subunits called capsomers

• Nucleic Acid

  • Types: DNA or RNA (single-stranded or double-stranded)

  • Role: Carries genetic information for replication

• Envelope (in some viruses)

  • Definition: Lipid membrane surrounding the capsid

  • Origin: Derived from host cell membranes

  • Function: Aids in entry into host cells

III. Virus Shapes and Symmetry

• Shapes

  • Helical: Rod-shaped elongated nucleocapsid, e.g., Tobacco Mosaic Virus

  • Icosahedral: Cubic or Spherical, e.g., Adenovirus

  • Complex: Combination of shapes, e.g., Bacteriophage, Hepatitis B virus, HIV, Ebola

• Symmetry

  • Importance in classification

  • Types: Helical symmetry, icosahedral symmetry, complex symmetry

IV. Classification of Viruses

• By Host Type

  • Animal viruses

  • Plant viruses

  • Bacterial viruses (bacteriophages)

• By Nucleic Acid Type

  • DNA viruses

  • RNA viruses

• By Morphology

  • Enveloped vs. non-enveloped viruses

V. Virus Replication Cycle

• Attachment

  • Binding to host cell receptors

• Entry

  • Mechanisms: Endocytosis or direct fusion

• Replication and Assembly

  • Use of host cellular machinery

• Release

  • Lysis of host cell or budding process

Based on:

• the disease they cause, e.g. poliovirus, rabies virus

• the type of disease, e.g. murine leukemia virus

• geographic locations, e.g. Sendai virus, Coxsackie virus

• their discovers, e.g. Epstein-Barr virus (human herpesvirus 4 (HHV-4)

• how they were originally thought to be contracted, e.g. dengue virus (“evil spirit”), influenza virus (the “influence” of bad air)

• combinations of the above, e.g. Rous Sarcoma virus

Viral Classification

Based on one or all of the following:

• Nucleic acid

• Morphology

• Strategy of replication

Routes of ENTRY:

Virus Host range:

• •Host range: each virus type can only infect a specific range of host organisms

• •Some viruses infect only one host; others have a broader range

Virus Specificity

• Viral specificity: molecules on the surface of a virus determine whether it can attach to a particular cell type; cell must have correct surface receptors, plus other internal factors, for virus to infect it.

• Some viruses infect only one cell type in a single host species; others can infect many cell types

Viral Pathogenesis: 
Elements of Virus-Host Interaction

• Viral strain

• Inoculum size

• Route of exposure

• Susceptibility of host

• Is there pre-existent immunity from past exposure or vaccination?

• Host genetic factors

• Immune status and age of host

Viral Pathogenesis:
Net Result of Virus-Host Interaction

• No infection

• Abortive infection with limited viral replication

• Asymptomatic infection

• Symptomatic infection

• Persistent, latent or self-limited infection

• Depending upon the agent and immune competence of host

• Influenced by availability of effective prophylaxis or therapy

The Influenza Virus

• Has an RNA genome (8 genes)

• Highly variable virus

• Lipid envelope with protein spikes

• Relatively unstable at room temperature (half life = a few hours)

• There is a species barrier due mostly structure of HA protein

HEMAGGLUTININ (HA) - The “H” in influenza names

• On the surface of the virus

• Functions as the receptor for the virus to bind to the host cell

• There are 17 different subtypes of HA (representing the numbers, H1, H5, etc. in influenza naming)

• HA elicits an immune response and is part of the influenza vaccine

NEURAMINIDASE (NA) - The “N” in influenza names

• On the surface of the virus

• Functions as an enzyme to let the new viral particles out of the host cell

• There are 10 different NA subtypes

• NA is also part of the influenza vaccine

The species which different types of influenza viruses are able to infect are determined by HA receptor binding to different forms of the receptor present on the host cell

This provides a considerable species barrier between birds and humans which is not easily overcome.

Pigs provide a "mixing pot" - able to be infected by both types of virus & thus allowing the passage of avian viruses to humans.

How the FLU virus changes: antigenic shift and drift

• Flu viruses constantly change and mutate.

• Antigenic drift refers to changes to the flu virus that happen slowly over time.

• Antigenic shift results when two different flu strains combine and infect the same cell and their genomes combine. This results in a sudden change in the virus. This can result in a pandemic strain.