Viruses
Aims and Objectives
After reading this document, you should be able to:
- Discuss what constitutes a living organism.
- Define what constitutes a virus.
- Have a knowledge of the principles of structure, classification and replication
of major viruses of medical importance.
- Understand the principles of diagnosis of virus infections.
- Know the mechanisms of action of antiviral drugs used in clinical practice.
General Characteristics
Viruses have the following properties:
- Obligate intracellular parasites
- Small- filterable through bacteriological filters
- Contain a single type of nucleic acid, either DNA or RNA
- Contain a protein coat (the capsid) consisting of individual protein units (capsomeres)
- May contain a host-derived lipid membrane (the envelope) through which may be inserted viral proteins (spikes or peplomers)
- Multiply inside living cells by using the biosynthetic machinery of the host
Virus Morphology
Viruses can be classified into several morphological types:
- Helical, e.g. bacteriophage M13
- Polyhedral/cubic, e.g. poliovirus
- Enveloped - may have poyhedral (e.g. herpes simplex) or helical (e.g. influenzavirus) capsids
- Complex, e.g. poxviruses.
This is on the basis of nucleic acid type and morphology. In future, it will be more likely to be based on nucleic acid sequence.
Major Virus Families That Infect Humans
| Family |
Nucleic Acid |
Envelope |
Capsid |
Example |
| Adenoviridae |
dsDNA |
No |
Cubic |
Adenovirus |
| Arenaviridae |
ssRNA |
Yes |
Complex |
Lassa fever virus |
| Bunyaviridae |
ssRNA |
Yes |
Helical |
Hantaan |
| Caliciviridae |
ssRNA |
No |
Cubic |
Norwalk virus |
| Coronaviridae |
ssRNA |
Yes |
Helical |
229E |
| Filoviridae |
ssRNA |
Yes |
Helical |
Marburg |
| Flaviviridae |
ssRNA |
Yes |
Cubic |
Yellow fever |
| Hepadnaviridae |
dsDNA |
No |
Cubic |
Hepatitis B |
| Herpesviridae |
dsDNA |
Yes |
Cubic |
Cytomegalovirus |
| Orthomyxoviridae |
ssRNA |
Yes |
Helical |
Influenza |
| Papovaviridae |
dsDNA |
No |
Cubic |
Papillomavirus |
| Paramyxoviridae |
ssRNA |
Yes |
Helical |
Respiratory syncytial |
| Parvoviridae |
ssDNA |
No |
Cubic |
B19 |
| Picornaviridae |
ssRNA |
No |
Cubic |
Rhinovirus |
| Poxviridae |
dsDNA |
Yes |
Complex |
Molluscum contagiosum |
| Reoviridae |
dsRNA |
No |
Cubic |
Rotavirus |
| Retroviridae |
ssRNA |
Yes |
Complex |
Human immunodeficiency virus |
| Rhabdoviridae |
ssRNA |
Yes |
Helical |
Rabies |
| Togaviridae |
ssRNA |
Yes |
Cubic |
Rubella |
Replication of Viruses
There are 5 major steps in the replication cycle of all viruses:
- Attachment
- Penetration
- Nucleic acid and protein synthesis
- Assembly of virions
- Release/egress
Prions - proteinaceous infectious particles, e.g. CJD agent
Viroids - RNA only, e.g. hepatitis D
Clinical Features of Viruses
These are protean, and range from the common cold to AIDS and cancer. See course textbook.
Diagnosis of Viral Infections
This can be by:
- detection of virus, e.g. electron microscopy
- detection of viral antigen, e.g. immunofluorescence
- detection of effect of virus, e.g. cytopathic effect on cells
- detection of virus nucleic acid, e.g. by PCR
- detection of anti-viral antibody, e.g by ELISA
Management of Viral Infections
Management of more trivial infections depends on control of symptoms, as there are few antiviral agents available. Principal drugs are:
- Acyclovir/Famciclovir/Valaciclovir - used against herpesvirus infections
- Ganciclovir - used in cytomegalovirus infections
- Ribavirin - used in respiratory syncytial virus infections
- Azidothymidine/Dideoxyinosine and other nucleoside analogues - used in HIV infection
- Rimantidine/Amantadine - used in influenza virus infections.
© AJC 2007.