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.
The
use of whole plants as host organisms does not give rise to the same moral
objections as the use of living animals and continues to play an important part
in the study of plant viruses, although such systems are sometimes slow to
deliver results and expensive to maintain.
In
recent years, an entirely new technology has been employed to study the effects
of viruses on host organisms. This involves the creation of transgenic animals
and plants by inserting all or part of the virus genome into the DNA of the
experimental organism, resulting in expression of virus mRNA and proteins in
somatic cells (and sometimes in the cells of the germ line).Thus, the
pathogenic effects of virus proteins, individually and in various combinations,
can be studied in living hosts. ‘SCID-hu’ mice have been constructed from immunodeficient
lines of animals transplanted with human tissue. These mice form an intriguing
model to study the pathogenesis of human immunodeficiency virus (HIV) as there
is no real alternative to study the properties of this important virus in vivo.
While these techniques often raise the same moral objections as ‘old-fashioned’ experimental infection of animals by viruses, they are immensely
powerful new tools for the study of virus pathogenicity. A growing number of
plant and animal viruses genes have been analysed in this way, but the results
have not always been as expected, and in many cases it has proved difficult to
equate the observations obtained with those gathered from experimental
infections. Nevertheless, this method will undoubtedly become much more widely
used as more of the technical difficulties associated with the construction of
transgenic organisms are solved.
