- What are viruses?
- Where are viruses found?
- How are viruses classified?
- Predictions on a virus's properties based on taxonomy
Rabbit Biocontrol Advisory Group
Viruses
What are viruses?
Viruses were first distinguished from other infectious micro-organisms on the basis of their much smaller size as determined by filtration studies at the end of the 19th century.
Viruses range in size from 18 to about 400 nm (Nanometer) and, in their simplest form, consist of only a nucleic acid core {either DNA [D(eoxyribo)N(ucleic) A(cid)] or RNA [R(ibo)N(ucleic) A(cid), never both} surrounded by a protein coat. In addition, some viruses have a fatty covering which also has glycoproteins (a group of joined proteins that contain a carbohydrate as the nonprotein component).
Viruses can only replicate inside living cells and they actually divert the host cell's normal enzymes and sub-cellular structures for their own replicative purposes. While viruses remain infectious for a variable period in the environment outside living cells, they are unable to "grow" until they enter and take over a susceptible cell.
Where are viruses found?
Viruses have been found in a vast array of animal and plant life and it can be safely assumed that every species of animal and plant will have its own particular viral "flora"; a range of virus species. Viruses may also be detected in the environment, e.g. in water, but they are only present in such places because they have been shed into that place from the living host in which they have replicated.
The capacity of a particular virus naturally to infect is usually restricted to a single host species or group of related species. For example, poliovirus naturally infects man and a range of other primates, but not ruminants, birds, dogs, cats etc. This is often explained by proposing that viruses have co-evolved with their particular hosts or even, as some have suggested, been derived from the genetic material of that host. There are, however, some very well-known examples where viruses have a particularly broad host range and rabies virus, which is infectious for all warm-blooded animals, is a notorious case in point.
How are viruses classified?
The earliest classification of viruses was on the basis of the clinical disease that they caused and the host in which the disease appeared. We thus have such common names as "human acquired immunodeficiency syndrome virus", "bovine foot and mouth disease virus", "swine influenza virus" and "cattle plague virus".
A more objective and useful taxonomic system emerged once it became possible to work with viruses in the laboratory and to visualize them, determine their chemical composition including the nature and even the sequence of their nucleic acid, determine their stability in a range of physical conditions and to categorise their biological properties such as serological relationships, host range, mode of transmission and pathogenesis.
The usefulness of this taxonomic system is that it allows the biological properties of newly- identified viruses to be predicted with some confidence once a few characteristics are known such as viral morphology (the form and structure of organisms) as revealed by electron microscopy and whether the nucleic acid is DNA or RNA.
For example, every herpesvirus that has been adequately studied has been found to produce lifelong latent infections of the host with intermittent periods of viral shedding. This epidemiologically important property could be predicted for any newly isolated virus once it was shown morphologically to be a herpesvirus.
Viruses are classified into two broad groups depending on whether their nucleic acid is RNA or DNA. Within these two major taxonomic groups are some 61 families, 21 of which contain members which infect man and other animals.
The individual families are defined on the basis of the morphology of the virus particle and detailed information on the structure of the viral nucleic acid and proteins and the precise replication strategy used.
Within the families, genera (category ranking below a family and above a species and generally consisting of a group of species exhibiting similar characteristics) may be defined which include the individual viruses, which are still often known by their common names, often incorporating both the name of the host species and some other obvious property such as disease signs, geographical location or method of transmission.
Predictions on a virus's properties based on taxonomy
Some general statements on likely genetic stability can be made on the basis of taxonomy but, as with all general statements or rules, there are exceptions.
1. Chemical Makeup.
Viruses which have a DNA genome are generally more genetically stable than those with a RNA genome. Also, viruses with a segmented (a clearly differentiated subdivision of an organism) genome (the set of genes characteristic of each species), rather than a genome consisting of a single strand of nucleic acid, can have an additional mechanism of genetic variation which is genetic recombination (the processes of crossing over or independent assortment).
In genetic recombination between viruses, whole segments of viral nucleic acid can be exchanged and progeny viruses can have an assortment of genes derived from the two parent viruses. This is believed to occur if two closely related but distinct viruses infect the same host animal simultaneously. Such a phenomenon is seen regularly in Orthomyxoviridae such as the human influenza viruses and this, together with antigenic drift (the process that occurs when a substance is introduced into a body that stimulates the production of an antibody), is responsible for the continual emergence of new strains of influenza virus.
The family Caliciviridae, however, consists of viruses with a RNA genome which is not segmented. Major, sudden genetic changes are therefore less likely, although slow genetic drift could be expected.
This is supported by the fact that there are 12 recognised serotypes of the calicivirus swine vesicular exanthema virus. It is likely that each of these 12 serotypes has evolved over time from a common parent calicivirus by an accumulation of many minor changes.
Other animal caliciviruses include the San Miguel sea lion viruses, feline calicivirus and caliciviruses of calves, swine, dogs, fowl and chimpanzee. Many of these latter viruses are found in intestinal contents of normal animals and there is no conclusive evidence that they are responsible for any significant diseases.
2. How the virus is spread.
This appears to be characteristic for a viral family although, again, there may be significant exceptions within a family.
3. Physical stability.
Viruses which have a lipid outer envelope are less stable generally than those which do not. The human AIDS virus, in common with other members of the Retroviridae family, survives only a matter of hours in dry desiccating conditions, whereas the virus of bovine foot and mouth disease, in common with most members of the family Picornaviridae, is extremely stable, lasting months in the environment.
4. Ability to cause disease and disease type.
This is a less rigid or binding characteristic, although there are some good examples. Those transmitted by insect bites may lead to severe viral damage to organs such as the liver. In almost all cases, infection of the central nervous system occurs and these viral infections are characterised by encephalitis (inflammation of the brain).
By contrast, the caliciviruses are associated with oral and upper respiratory infections, often with the production of vesicles or "blisters" around the mouth and on the skin. While this is characteristic of most of the known caliciviral diseases, it is quite different from symptoms of rabbit calicivirus disease.
Summary
Viral taxonomy based on a range of physical and biological characteristics of the viruses often provides a grouping of the same viruses according to important epidemiological and pathogenic features.
This approach is therefore useful when dealing with newly-discovered viruses, but it is probably safer to use it as a guide for efficient use of research resources, rather than as a foolproof way of predicting exactly how these viruses will behave in animal populations.
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