Principles of Virology, Volume 1. Jane Flint

Principles of Virology, Volume 1 - Jane Flint


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      The time required for the development of cytopathology varies considerably among animal viruses. For example, depending on the size of the inoculum, enteroviruses and herpes simplex virus can cause cytopathic effects in 1 to 2 days and destroy the cell monolayer in 3. In contrast, cytomegalovirus, rubella virus, and some adenoviruses may not produce such effects for several weeks.

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      Some viruses multiply in cells without causing obvious cytopathic effects. For example, many members of the families Arenaviridae, Paramyxoviridae, and Retroviridae do not cause obvious damage to cultured cells. Infection by such viruses must therefore be assessed using alternative methods, as described in “Assay of Viruses” below.

      In the early 1900s, when viruses were first isolated, freezers and cell cultures were not available, and it was necessary to maintain virus stocks by continuous passage from animal to animal. This practice not only was inconvenient but also, as we shall see, led to the selection of viral mutants (Volume II, Chapter 7). For example, monkey-to-monkey intracerebral passage of poliovirus selected a mutant that could no longer infect chimpanzees by the oral route, the natural means of infection.

      Although cell culture has supplanted animals for propagating most viruses, experimental infection of laboratory animals has always been, and will continue to be, obligatory for studying the processes by which viruses cause disease. The study in monkeys of poliomyelitis, the paralytic disease caused by poliovirus, led to an understanding of the basis of this disease and was instrumental in the development of a successful vaccine. Similarly, the development of vaccines against hepatitis B virus would not have been possible without experimental studies with chimpanzees. Understanding how the immune system or any complex organ reacts to a virus cannot be achieved without research on living animals. The development of viral vaccines, antiviral drugs, and diagnostic tests for veterinary medicine has also benefited from research on diseases in laboratory animals. Despite their utility, it must be appreciated that all animal models are surrogates for the events that occur during viral infections of humans.

      There are two main types of assay for detecting viruses: biological and physical. Because viruses were first recognized by their infectivity, the earliest assays focused on this most sensitive and informative property. However, biological assays such as the plaque assay and end-point titration methods do not detect noninfectious particles. In contrast, all particles are accounted for with physical assays such as electron microscopy or by immunological methods. Knowledge of the number of noninfectious particles is useful for assessing the quality of a virus preparation.

      One of the most important procedures in virology is measuring the virus titer, the concentration of infectious virus particles in a sample. This parameter is determined by inoculating serial dilutions of virus into host cell cultures, chicken embryos, or laboratory animals and monitoring for evidence of virus multiplication. The response may be quantitative (as in assays for plaques, fluorescent foci, infectious centers, or abnormal growth and morphology) or all-or-none, in which the presence or absence of infection is measured (as in an end-point dilution assay). Please note that “titer” is not a verb.

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       Plaque Assay


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