Cytopathic Effect

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What is Cytopathic Effect? 

When cells become abnormal, we can observe cell abnormalities or structural changes with the help of a high/low power microscope or some stains. These abnormal cells are cytopathic cells that are caused by viral infections. It occurs when the infecting cells causes death or lysis of the host cell making it unable to reproduce. Therefore, we can say that cytopathogenic effect or cytopathic effect definition as the condition where a virus causes morphological changes in the host cell. 

The initial sign of cytopathic effect or viral infection is the rounding of the cells. We can observe inclusion bodies within the cytoplasm and nucleus of the host cell. We can identify these in the blood smear of the patients under microscope depending on the power it requires. 

Types of Cytopathic Effect of Viruses

  1. Total Destruction

It is the most severe type of Cytopathic Effect where total destruction of the host cell monolayer takes place. Observing this process is easier as when cells are seeded on a glass surface, we can see a confluent monolayer of the host cell forming on it. Thereafter, the viral infection is introduced and then all cells in the monolayer start shrinking rapidly to create a dense structure and this process is known as pyknosis; this is detached from the glass within three days. An example of total destruction CPE is enteroviruses.

  1. Subtotal Destruction 

It is less severe than the total destruction type of CPE. In the exact manner to the total destruction, the CPE is observed as a confluent monolayer on the host cell when cells are put on a glass surface and introduces a viral infection. This kind of CPE characteristically shows detachment of some, not all the cells, in the monolayer. Examples: Subtotal destruction can be seen in togaviruses, picornaviruses and some paramyxoviruses. 

  1. Focal Degeneration

Here, a direct transfer of the virus takes place from one cell to another. The host cell monolayer gets a localized attack that affects the entire tissue. Initial stages are affected that caused the focal degeneration to spread at localized viral centers that are called foci. It takes place because of the cell to cell transfer of the viruses, unlike diffusion through extracellular medium. Here, the host cells change their structure and turn into rounded, enlarged and refracting cells. As a result, the host cells detach themselves from the surface and spreading of the virus occurs. The spreading takes place concentrically, and the cells lifting off are actually rounded cells which are surrounded by healthy tissue. Examples: Focal degeneration occurs in Herpesviruses and poxviruses.

  1. Swelling and Clumpy

It is the kind of CPE where host cells significantly swell. Once enlarged, the host cells clump together to form clusters. As a result, they become so large that they need to detach. Example: Adenoviruses are examples of swelling and clumping CPE.

  1. Foamy Degeneration

Also known as vacuolization, foamy degeneration takes place due to the formation of numerous and large cytoplasmic vacuoles. It can be only observed with the help of fixation and staining of the host cells that are involved. It is found in retroviruses, flaviviruses and paramyxoviruses.

  1. Syncytium CPE

Also known as cell fusion and polykaryon formation, this CPE is involved with the plasma membranes of four or more than four host cells which fuse together and produce an enlarged cell with at least four nuclei. Although other large size cell fusions are visible without staining, this kind of CPE is different and is detected only after fixation of the host cell and then staining. Examples of this kind of cytopathic effect can be seen in herpes viruses that produce cell fusion and also other forms of CPE and also paramyxoviruses.

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  1. Inclusion Bodies

These are insoluble structures present within cytoplasm or cell nuclei. The inclusion bodies can be seen only with the help of staining as they can be observed as the areas that have altered staining in the host cells. These represent all the areas where nucleic acid or viral protein are synthesised and virions are assembled. Also, sometimes, these present areas that are without active virus but indicate areas of viral scarring. However, these bodies differ from viral strain. Inclusion bodies can be single/multiple, single/large, round or irregularly shaped. These can be intranuclear or intracytoplasmic. eosinophilic/basophilic.

So, we can say that the types of cytopathic effect depending upon their behaviour and effects on the host cells are the different examples of cytopathic effect of viruses, which are:

  • Change in shape ranging from flat to round

  • Loss of surface (adherence)

  • Shrinking of nucleus

  • Inclusion bodies in cytoplasm or nuclei

  • Formation of multinucleated syncytia 

  • Disruption of host cell chromatin

  • Cell lysis or death

  • Transformation into immortal cells

  • Loss of senescence or ability to divide

FAQ (Frequently Asked Questions)

1. How is the Cytopathic Effect used in Diagnostics?

Cytopathic effects are often used to diagnose certain viral infections as these can be observed without staining/with staining or with unfixed cells under low power optical microscope/ light microscope.

In humans and animals, the effects of a few viruses can be diagnosed and the type of infections; thus, it is helpful for virologists.

2. What are the different Changes in the Host Cells by Cytopathogenic Effects?

  • Changes in the structure of the host cells due to viral infections

  • Cell lysis or death of the cell.

  • Infection reduces the cell’s ability to reproduce leading to the death of the cell.

  • Formation of inclusion bodies or syncytia which help in diagnosing viral infections.

3. Is Cytocidal Infection due to Cytopathic Effect?

Cytocidal infection is due to the death of host cells that are caused due to lysis and viral replication. Viruses attach themselves to the cell surface and either inject their genome, elsse get engulfed by the cell.