How Viruses Induce Cytopathic Effects in Host Cells
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
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.
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.
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.
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.
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.
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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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
FAQs on Cytopathic Effect: Types and Mechanisms
1. What is the cytopathic effect (CPE) in virology?
In virology, the cytopathic effect, or CPE, refers to the observable structural and functional changes in host cells that are caused by a viral infection. These changes are a direct result of the virus hijacking the cell's machinery for its own replication. Observing CPE is a primary method for identifying viral presence in laboratory cell cultures.
2. What are the main types of cytopathic effects caused by viruses?
The cytopathic effects (CPE) vary depending on the virus and the host cell. Common types include:
- Cell Lysis: The host cell bursts and is destroyed upon the release of new virus particles.
- Rounding and Detachment: Infected cells lose their normal shape, become rounded, and detach from the culture surface.
- Syncytia Formation: The membranes of infected cells fuse with neighbouring cells, creating large, multinucleated giant cells.
- Inclusion Bodies: Accumulations of viral components or proteins appear as distinct masses inside the cell's nucleus or cytoplasm.
3. How is the cytopathic effect used to diagnose viral infections?
Virologists use CPE as a key diagnostic tool. A patient's sample (e.g., throat swab) is introduced to a susceptible cell culture. If a virus is present, it will infect the cells and produce a characteristic CPE over time. By observing the type and progression of the CPE under a microscope, scientists can often make a preliminary identification of the infecting virus or confirm its presence.
4. What is the difference between a cytopathic effect and a cytolytic effect?
A cytopathic effect is a broad term for any visible damage to a host cell caused by a virus. A cytolytic effect is a specific type of CPE where the virus's replication cycle culminates in the complete destruction and lysis (bursting) of the host cell. Therefore, all cytolytic effects are cytopathic, but not all cytopathic effects are cytolytic; some viruses cause structural changes without immediately killing the cell.
5. What are inclusion bodies and why are they a significant cytopathic effect?
Inclusion bodies are abnormal structures that appear within the nucleus or cytoplasm of a virus-infected cell. They are essentially 'viral factories' where viral proteins and nucleic acids are being assembled. Their presence, size, location (nuclear or cytoplasmic), and staining properties are highly characteristic of certain viruses and serve as important clues for identifying the infectious agent during microscopic examination.
6. Can a virus infect a cell without causing a visible cytopathic effect?
Yes, some viruses can establish a persistent or latent infection without causing any immediate visible CPE. In these non-cytopathic infections, the virus may replicate slowly, release particles without lysing the cell, or remain dormant within the host cell's genome. The absence of CPE does not always mean a culture is free from viral infection.
7. How does the formation of syncytia demonstrate a cytopathic effect?
Syncytia formation is a dramatic cytopathic effect where a virus forces an infected cell to fuse with its neighbours. Viruses like HIV, measles, and herpesvirus produce special fusion proteins on the surface of the infected cell, causing it to merge with adjacent cells. This results in a large, non-functional cell mass with multiple nuclei, which is a clear and destructive sign of viral activity.
8. Why is monitoring the cytopathic effect important for developing new antiviral drugs?
Monitoring CPE is crucial in the field of pharmacology for screening potential antiviral drugs. To test a drug's effectiveness, scientists treat virus-infected cell cultures with the compound. If the drug works, it will interrupt the viral replication cycle and thereby prevent or significantly reduce the development of the cytopathic effect. This provides a clear, visual endpoint to measure the drug's efficacy.
