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Introduction

A virus is ‘a minute parasitic microorganism’¹ that can only replicate by using living cells. More than 200 viruses have been identified as capable of causing disease in humans¹ and they result in some of the most communicable and dangerous diseases known. This paper takes a brief look at these fascinating parasites.

Definitions

There are three general factors that distinguish a virus:

They are so small many are only visible through an electron microscope,

They (generally²) posess RNA or DNA, but never both,

They cannot grow on lifeless media, only within living susceptible cells³.

A virion is a complete viral particle. It is a bloc of genetic material surrounded by a coat that protects it from the environment and serves as a vehicle for transmission from one cell to another⁴. Virions lack the materials needed for complete replication and growth, and must invade a host cell to replicate. The virion provides the genetic code for replication, and the host cell provides the necessary energy and raw materials. The virion effectively converts a cell into a virus factory.

Many virions contain special enzymes or virion enzymes not available in the host cell. These are used to aid in the manipulation of the host cell once invaded.

Viruses multiply by synthesis of separate components followed by assembly. This mode of replication accounts for obligatory intracellular parasitism⁴, that is the need to invade cells in order to reproduce.

There are two modes of action for an invading virus: lytic and lysogenic infection. Lytic infection sees the death of the host cell and lysogenic infection sees the virus lay dormant within the host cell for some time prior to activity. Most viruses are lytic².

Host Range

Virions only multiply in particular host cells and are subdivided into three main classes: animal viruses, bacterial viruses (bacteriophages) and plant viruses. Within each class the virion is only able to infect a certain species of cells.

The host range is determined by the specificity of attachment of the virions to the cells or by the availability of cellular factors required for viral replication.

Attachment specificity is dictated by the properties of the virion’s coat and receptors on the cells suface. These factors disappear when the infection is via naked viral nucleic acid as specificity receptors are not there.

Classes

Virions display a range of morphological types that is useful for classification, however, the number of distinctive morphological classes is small and classification is not based on morphology alone⁴. Instead classification is based on nucleic acid composition, size, symmetry, structure, sensitivity to ether, and the disease they cause³.

Within each host range viruses are grouped into families. For example there are 15 families of animal virus², some of which include: Parvoviridae, Papvaviridae, Herpetoviridae, and Poxviridae.

Morphology

There are four general morphologies identified in viruses⁴.

Icosahedral Virions - an Icosahedral* protein shell (capsid) surrounds nucleic acid and proteins (the core). The capsid and core form a nucleocapsid.

Helical Virions – long rods of nulceic acid are surrounded by a cylindrical capsid which has a helical structure.

Enveloped Virions – the nucleocapsid, either icosahedral or helical, is surrounded by a loose membranous envelope. The envelope is roughly spherical but makes varying shapes because it is not rigid.

Complex Virions – these do not possess clearly defined capsids, but have several coats around the nucleic acid core.

Nucleocapsids that are not enveloped are called naked virions (ie Iscosahedral or Helical virions).

Mulitiplication Cycle of a Poxvirus

The multiplication cylce refers to the sequence of events the virion goes through to replicate, the first stage of which is always invasion of the host cell. The general process of viral infection reflects the life cylce of the virus.

Small pox is caused by the varida virus and is so virulant and contagious that laboratory investigations of it have been limited. The closely related vaccinia virus, however, is the one of the most studied viruses to infect humans, and most of our knowledge regarding varida is derived from it⁴.

The varida virus has a complex architecture without obvious symmetry, the nucleic acid is DNA. Its host range is limited to humans and monkeys⁴.

The virion attaches to host cell membrane using undefined receptors, and it enters the cell using a process similar to phagocytosis. After penetration the viral DNA is released in two stages.

The first stage involves the partial breakdown of the coating of the virion by enzymes in the host cell to release the nulceoprotein core. This takes approximately twenty minutes. After another thirty to sixty minutes the nulceoprotein core is broken down to release the viral DNA. The liberated viral DNA is stable and able to transmit its genetic information for several hours.

Enzymes required for the replication process are generated by the virus early in the piece, but production of these is blocked three to four hours after infection. The viral DNA begins replication about two to three hours after infection, and stops after about seven hours.

Three hours after infection some of the proteins developed by the virus start to enclose patches of the viral DNA and begin to make immature particles. The nucloid then begins to take shape within the immature particles and an additional membrane encloses the condensing DNA. Finally the outer coat structures are laid down on the previously formed membrane completing assembly of mature virions.

The mature virions are released through cell villi by a process resembling phagocytosis in reverse, this process is inefficient compared to the standard virus practice of host cell lysis, as only 10% of formed viruses are released⁴.

Viruses and Cancer

There is evidence that viruses are responsible for certain forms of cancer². Lysogenic infection is seen as a way in which an animal cell can be predisposed to tumor formation. The presence of the viral nucleic acid within the cell may disturb the cellular metabolism causing the formation of cancer cells².

Cures for Viral Infection

Viruses pose a significant threat to health in society, for example the flu virus costs industry many thousands of dollars per year in lost working days. It is not an easy task to defeat viral infection, virions multiply within living cells, and it is difficult to tell the difference between an infected cell and a normal one. It has been found that the best defence against viral infections is vaccination.

Vaccination stimulates the host organism to generate antibodies, agents that attach to invading viruses which lead to their inactivation and destruction. The complex formed between the antibody and the virus is subsequently removed by macrophages.

The problem with vaccination is that the antibodies are very specific for the virus to which they attach, and it is possible for the surface of the virus to mutate so they are no longer recognised by the antibody. Such is the case with the flu virus, which seems to have a different strain every year.

There are many who disagree with the concept of vaccination owing to its side-effects, and a perceived weakening of the body’s natural immune system.

Conclusion

Viruses are simple cell parasites that can lead to deadly diseases and epidemics. As small as they may be they pose a significant threat to the well-being of society, and it is necessary to have a strong and well functioning immune system to defeat infection.

References

1 Moseby’s Medical, Nursing, and Allied Health Dictionary – 4^th Edition, Mosby, 1994.

2 Davies, J., Shaffer Littlewood, B., Elementary Biochemistry – An Introduction to the Chemistry of Living Cells, Prentice-Hall Inc, New Jersey, 1979.

3 Parker, M.J., Microbiology for Nurses – 5^th Edition, Balliere Tindall, London, 1978.

4 Davis, D., et al, Microbiology – 2^nd Edition, Harper& Row, Maryland, 1973.