The replication of viruses in living host cell can be detected in various ways. However, cytopathic effect, hemadsorption and interference are the most common techniques or approaches of doing this.
- CYTOPATHIC EFFECT
Cytopathic effect (CPE) is defined as those biochemical and/or morphological changes that occur in viral infected cells, and which indicate viral infectivity and replication in a particular host cell. They are degenerative changes (which could be microscopic or macroscopic in nature) that occur in host cells infected by virus; and they generally indicate viral replication. Cytopathic effect is also an early indication that a particular host cell is infected. And they may even start appearing before the infected cells start to die or fragment into different forms. CPE is more of an infectivity assay that allows virologists to decipher the death of a host cell due to viral infection and replication. Cytopathic effect assumes different forms including total cell lysis, cell fusion, cell detachment, production of inclusion bodies and cell rounding.
Though not all virus show CPE during their infectivity and replication (because they do not necessarily lead to the death of the infected host cell), CPE is an important aspect of detecting viral replication in infected host cells. For viruses that do not show CPE during their replication in host cells, other methods of detecting viral replication are used. CPE can be observed in viral infected cells visually with the naked eyes and macroscopically using light microscopy; and the production of CPE is a common assaying approach in cell/tissue culture technique of viral cultivation. Electron microscopy and other advanced viral detection techniques such as neutralization and immunoflourescence can be used to confirm the production of CPE in viral infected host cells. It is noteworthy that CPE can also be produced by other obligate intracellular parasites that are not viruses. Typical examples include bacteria in the genus Rickettsia and Chlamydia – which both exhibit some characteristics of viruses.
Hemadsorption is defined as the adsorption of red blood cells (erythrocytes) to the surface of virus-infected host cells. It is an important detection technique which is used to assay for or identify the synthesis of viral proteins in infected host cells. Hemadsorption is typically used to identify viruses whose replication in infected host cells cannot be easily detected by other means of viral deception. And this include some enveloped viruses in the family Orthomyxoviridae, Togaviridae and Paramyxoviridae – that are known to bud from the cell surface of the host cells they infect. In the technique of hemadsorption, red blood cells (RBCs) are added to virus infected cells in a cell/tissue culture medium during their incubation. The RBCs adhere or attach specifically to the virus infected cells; and this can be observed microscopically using the microscope.
The RBCs adhere to the virus infected cells because the viral replication of some virus (such as orthomyxoviruses and paramyxoviruses as aforesaid) leads to the outright destruction of the plasma or cell membrane of the infected cells in culture plates; and this allows erythrocytes to firmly attach to them in vitro, and thus allow their detection by the hemadsorption technique. Hemadsorption is an important detection technique for the recognition of virus infection in cultures especially for those viruses whose growth or replication produces little or no evidence of CPE. The cell attachment proteins (i.e. viral proteins) synthesized by viruses as aforementioned bind specifically to some host molecules such as sialic acid which are abundantly found on several host cells including the RBCs; and this confirms the basis for the hemadsorption technique used for detecting viral replication in infected host cells.
Interference is defined as the prevention of the replication of one virus by another. In interference, the attachment of a given virus to a host cell could be prevented by exposing the virus to another virus that destroys it or alter their ability to attach to their specific host cell. Interference is another viral detection technique aside hemadsorption and cytopathic effect that is used to assay viral replication in infected host cells. Typical examples of viruses that can be detected by viral interference test include rubella virus – which has the ability to prevent another virus from infecting the same cells it normally replicates in. Viral replication interference mechanisms can also be mediated by interferons and some antiviral drugs as well. Generally, in viral interference test, a given concentration or amount of a virus (usually the primary infecting or avirulent virus) is mixed or added to the culture fluid of another virus capable of exhibiting interference (e.g. rubella virus); and the culture is incubated. Rubella virus in this case inhibits the replication of the former virus, and this allows for their detection in virus infected samples.
- Neutralization test is an antigen-antibody based test which is used to assay viral infectivity by determining whether a virus has been neutralized by a specific antibody. Neutralization test can be performed with any virus especially with those that do not form CPE or adhere to RBCs.
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