Cultivation of animal viruses

CULTIVATION OF ANIMAL VIRUSES

Microbiologists culture viruses for the purpose of isolation and production them in quantity for study and for the production of vaccines. Usually, there are three main ways of cultivating animal viruses: in living beings, in embryonated chicken (or duck) egg , and in cell cultures.

Living Animals

Using animals for the cultivation of viruses in the cell process of choice, for the following reasons: it is expensive, it is labor-intensive, and it may harbor other factors, such as contamination bacteria, into a study. But some animal viruses, such as hepatitis. The virus could also be cultured only in living animals. Investigation of host immune response to a viral infection also demand the use of living animals. In the laboratory, mice, guinea pigs, and nonhuman primates are formally used. Animal inoculation is also a good diagnostic tool to determine whether a virus will cause the infection; the animal will show typical disease symptoms, and sections of infected tissue can be examined microscopically for evidence of infection.

Embryonated Chicken Eggs

One of the usual economical and convenient process for cultivating a wide variety of animal viruses is the utilization of the fertilized, embryonated chicken eggs. The discovery was made in 1931. Embryonated chicken eggs could be inoculated aseptically within the virus, through a needle and syringe, through a hole drilled in the shell. The mouth is sealed with paraffin and the eggs are incubated at the 36°C for 2 to 3 days to give the viruses to multiply.

Chick embryos harbor many kinds of cells and tissues in which various viruses will replicate. By utilization of embryos of different ages and using a different process of inoculation, it is able to cultivate the type of virus desired. Different tissues within the egg are inoculated, seeing on the type of virus used. For instance, the chorioallantoic membrane will support the growth of the herpes virus, smallpox virus (variola), Rous sarcoma virus, and eastern equine encephalitis virus; the amniotic cavity will permit growth of influenza and mumps viruses. The influenza virus also has a capability for the allantoic cavity. As a matter of truth, the embryonated chicken egg is used formally for the production of influenza vaccines, as well as for vaccines against smallpox, yellow fever, and other diseases. Moreover, if a virus, such as the vaccinia virus, multiplies on one of the embryonic membranes, it produces a very different type of lesion called a pock. The yolk sac arid the embryo itself can also be used to cultivate viruses (rabies virus and yellow fever virus in the yolk sac; eastern equine encephalitis virus in the embryo).

Tissue Cultures

The most globally used process for cultivation of the animal viruses is cell culture, or tissue culture, a single layer (monolayer) of cell growing in the liquid medium in a flat-bottomed vessel. Once a cell culture has been obtained, it is possible to use it as an in the video host for a virus (in vitro means “in glass”). Viruses enter the cells, usually causing some type of visible change in the growth of the monolayer tissue cells near the site of initial infection. This localized difference called the cytopathic effect (CPE), is a damage of the tissue culture cells caused by the virus. The CPE may have unique appearances, depending on the particular virus and kind of cell in the culture, shows a typical CPE in cell culture.

One type of CPE is of inclusion body, which is an abnormal intracellular structure. Inclusion bodies are needed because their presence can help identify the virus capability of an infection. For example, small particles called Guarnieri bodies can be seen in the cytoplasm of the cells infected with the smallpox virus. These are aggregates of virions that have grown in the cytoplasm. Negri bodies occur in the cytoplasm of nerve cells from the brain of animals with rabies, and their presence is needed for the diagnosis of this disease. Intracytoplasmic inclusion bodies also were seen in several other diseases such as sheep pox, fowl pox, and molluscum contagiosum. Intranuclear inclusions are seen in cells infected with viruses of chicken pox herpes.

In cell culture, groups of lysed cells (plaques) have been utilized in the assay of virus concentration the type of plaques is proportional to the number of infectious virus particles following in a sample. Each animal virion harbor the formation of a single plaque on a monolayer of susceptible animal cells, same as a bacteriophage forms a plaque on a lawn of susceptible bacterial host cells.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.