Virology: Introduction to Swine Flu and Avian Flu
May 5, 2009 – 1:31 pm
Viruses are submicroscopic, obligate intracellular parasites. Clearly, it is not a problem to differentiate viruses from higher macroscopic organisms. Even within a broad definition of microbiology encompassing prokaryotic organisms and microscopic eukaryotes such as algae, protozoa, and fungi, in most cases it will suffice. A few groups of prokaryotic organisms, however, have specialized intracellular parasitic life cycles and confound the above definition.
These are the Rickettsiae and Chlamydiae—obligate intracellular parasitic bacteria
which have evolved to be so cell-associated that they can exist outside the cells of their hosts for only a short period of time before losing viability. Therefore, it is necessary to add further clauses to the definition of what constitutes a virus.
Virus particles are produced from the assembly of preformed components,
whereas other agents grow from an increase in the integrated sum of their components
and reproduce by division.
Virus particles (virions) themselves do not grow or undergo division.
Viruses lack the genetic information that encodes apparatus necessary for the
generation of metabolic energy or for protein synthesis (ribosomes).
No known virus has the biochemical or genetic potential to generate the energy
necessary to drive all biological processes (e.g., macromolecular synthesis).They are
therefore absolutely dependent on the host cell for this function. It is often asked
whether viruses are alive or not. One view is that inside the host cell viruses are
alive, whereas outside it they are merely complex assemblages of metabolically inert
chemicals. That is not to say that chemical changes do not occur in extracellular
virus particles, as will be explained elsewhere, but these are in no sense the ‘growth’
of a living organism.
Living Host Systems
In 1881, Louis Pasteur began to study rabies in animals. Over several years, he
developed methods of producing attenuated virus preparations by progressively
drying the spinal cords of rabbits experimentally infected with rabies which,
when inoculated into other animals, would protect from challenge with virulent
rabies virus. In 1885, he inoculated a child, Joseph Meister, with this, the first artificially
produced virus vaccine (as the ancient practice of variolation and Jenner’s
use of cowpox virus for vaccination relied on naturally occurring viruses).Whole
plants have been used to study the effects of plant viruses after infection ever since
tobacco mosaic virus was first discovered by Iwanowski. Usually such studies
involve rubbing preparations containing virus particles into the leaves or stem of
the plant.
During the Spanish–American War of the late nineteenth century and the subsequent
building of the Panama Canal, the number of American deaths due to yellow fever was colossal. The disease also appeared to be spreading slowly northward into the continental United States. In 1990, through experimental transmission to mice,Walter Reed demonstrated that yellow fever was caused by a virus spread by mosquitoes. This discovery eventually enabled Max Theiler in 1937 to propagate the virus in chick embryos and to produce an attenuated vaccine—the 17D strain—which is still in use today.The success of this approach led many other investigators from the 1930s to the 1950s to develop animal systems to identify and propagate pathogenic viruses.
Eukaryotic cells can be grown in vitro (tissue culture) and viruses can be propagated
in these cultures, but these techniques are expensive and technically quite
demanding. Some viruses will replicate in the living tissues of developing embryonated
hens eggs, such as influenza virus. Egg-adapted strains of influenza virus
replicate well in eggs and very high virus titres can be obtained. Embryonated
hens eggs were first used to propagate viruses in the early decades of the twentieth
century. This method has proved to be highly effective for the isolation and
culture of many viruses, particularly strains of influenza virus and various poxviruses
(e.g., vaccinia virus). Counting the ‘pocks’ on the chorioallantoic membrane of eggs
produced by the replication of vaccinia virus was the first quantitative assay for any
virus. Animal host systems still have their uses in virology:
To produce viruses that cannot be effectively studied in vitro (e.g., hepatitis B
virus)
To study the pathogenesis of virus infections (e.g., coxsackieviruses)
To test vaccine safety (e.g., oral poliovirus vaccine)
Nevertheless, they are increasingly being discarded for the following reasons:
Breeding and maintenance of animals infected with pathogenic viruses is
expensive.
Whole animals are complex systems in which it is sometimes difficult to discern
events.
Results obtained are not always reproducible due to host variation.
Unnecessary or wasteful use of experimental animals is morally repugnant.
They are rapidly being overtaken by ‘modern science’—cell culture and molecular
biology.
source: Cann AJ. 2005. Principles of Molecular Virology 4th Edition. New York: Elsevier Academic Press.
