Viral Propagation

Propagation is required for the isolation and identification of viruses involved in disease and for the production of vaccines and stocks for research purposes. As stated above, viable host cells are required for viral reproduction. Therefore tissue culturing is commonly used to facilitate virus propagation. Other less commonly used methods of virus propagation include inoculation of chick embryos and infection of live animals.

Tissue/Cell Culture
Although tissue culture is still used in circumstances such as isolating viruses from animals with chronic infections, cell culture is now the commonly used method. Cell cultures can be primary, semi-continuous or continuous. Primary cell cultures are derived directly from tissues and will contain numerous cell types. Semi-continuous cell lines retain their original tissue based characteristics but are closer to individual cell culturing than primary cell culturing. Continuous cell lines are derived from either normal or neoplastic tissue and the cell line can be continued indefinitely. These cell lines are referred to as heteroploid lines.

The host cell damage caused by viruses is detectable under light microscopy. These changes to the host cell are referred to as "cytopathic" effects (CPE) and include changes in shape, cell detachment, formation of inclusion bodies (aggregates of stainable substances, usually proteins) and cell death. However some viruses do not produce CPE and therefore cannot necessarily be detected by light microscopy, even with differential staining.

In cell cultures exhibiting CPE inclusion bodies maybe located in the cytoplasm or the nucleus and can be stained for examination. Inclusion bodies stained with eosin are called acidophilic and basophiliic when stained with haematoxylin. Inclusion bodies within the cytoplasm may be found in cells affected in poxviruses, reoviruses, rabies virus and paramyxoviruses. Inclusion bodies within the nucleus of the host cell can be caused by adenoviruses, herpesviruses and parvoviruses. Infections of some viruses are able to cause both types of inclusion bodies such as canine distemper virus.

Non-cytoplasmic viruses require an alternative method of detection. As mentioned previously enveloped viruses often insert glycoproteins into the host cell membrane prior to budding. In some cases these glycoproteins allow infected cells to produce syncytia (cell fusion) or promote haemadsorption (haemagglutinins - binding of erythrocytes to the surface of infected cells). The formation of syncitium in cell cultures can occur in cells infected with lentiviruses, paramyxoviruses and some herpesviruses.

Serological testing also represents another virus identification technique. Viruses are identified by employing a virus specific fluorescein-labelled antibody.

Embryonated Eggs
This is no longer a common virus identification method, although it is still the most common method used to isolate influenza A viruses and for many avian viruses. Pathogen-free flocks are used to select eggs to prevent maternally-derived anti-viral antibodies interfering with propagation.

Experimental Animals
The experimental infection of animals is no longer commonly used due to ethical reasoning and many alternative methids including those above have been developed to replace experimenting with live animals. Animal experimentation is mentioned here though because for some virus families the use of the other identification methods above remain inadequate. Where animal testing is performed, suckling mice are used for the detection of arthropod-borne viruses and for rabies virus. Also the production of polyclonal antisera requires the inoculation of laboratory animals with the virus.