Chapter 7.3 Effect of Viruses on Host Cells

What are cytopathic effects (CPEs)?

Virus-induced damage to cells that alters their microscopic appearance. These are abnormal features in virus-infected cells that are not normally present in healthy cells.

What are the three main types of cytopathic effects?

1) Morphological changes (shape/size alterations, cell separation)

2) Intracellular damage (inclusion bodies, organelle disruption)

3) Large-scale effects (syncytia formation)

What are inclusion bodies?

Compacted masses of viruses or damaged cell organelles found in the nucleus or cytoplasm of infected cells. They represent areas of viral replication or cellular damage.

What are syncytia?

The fusion of multiple host cells into single large cells containing multiple nuclei. Results from viral proteins causing membrane fusion (singular: syncytium).

How does poliovirus affect host cells?

Cells are killed completely - they shrink, detach from the surface, and lyse.

How does adenovirus affect host cells?

Cells round up and partially detach from the surface, and they tend to clump together.

How does respiratory syncytial virus affect host cells?

Adjacent cells fuse together, forming large multinucleated cells called syncytia.

How does cytomegalovirus affect host cells?

It causes formation of inclusion bodies in either the cytoplasm or nucleus of the host cell.

What is a latent viral infection?

A carrier relationship where the virus persists in the host long after symptoms disappear, remaining inside cells in an inactive/dormant state.

What is a provirus?

Viral DNA that has been incorporated into the DNA of the host cell chromosome, similar to a prophage in bacteriophages.

What is the difference between simple latent and chronic latent infections?

Simple latent infections may never reactivate or only reactivate once (like chickenpox/shingles). Chronic latent infections regularly reactivate repeatedly throughout the host's life (like herpes simplex virus).

Why are chronic latent infections also called persistent infections?

Because they can potentially reappear at any time throughout the host's life, creating recurring episodes of signs and symptoms.

Give an example of a simple latent infection.

Chickenpox (Varicella Zoster virus) - contracted in childhood, goes dormant, may reactivate later as shingles.

Give an example of a chronic latent infection.

Herpes simplex virus that causes recurring oral and genital lesions.

What percentage of human cancers are estimated to be caused by viruses?

Up to 20% of all human cancers may be caused by viruses.

What are oncogenic viruses?

Viruses that have been shown experimentally to directly induce cancer in healthy human cells. "Onco" means mass (tumors) and "genic" means to create or produce.

How do oncogenic viruses cause cancer?

They produce viral proteins that disrupt normal cellular mechanisms regulating cell growth and division, leading to loss of growth control and unchecked cell growth.

What is transformation in oncology?

The term used to denote the changes within a normal cell that lead it to become a cancerous cell.

Which types of viruses can potentially act as oncogenic viruses?

Any DNA virus or retrovirus that can make DNA and incorporate its provirus into the host cell's chromosomes.

Why is cancer considered a multifactorial cellular event?

Because it may require multiple different events to coincide for transformation to occur, making it difficult to treat and even harder to cure.

Why is cultivating viruses more challenging than cultivating bacteria?

Because viruses are obligate intracellular parasites and cannot multiply without a susceptible host cell.

What are the two main approaches to viral cultivation?

In vivo (cultivation in living organisms) and in vitro (cultivation in laboratory cell/tissue cultures).

What are the main purposes of viral cultivation?

1) Isolate and identify viruses in clinical specimens, 2) Prepare viruses for vaccines, 3) Conduct detailed research on viral structure, multiplication cycles, genetics, and effects on host cells.

What animals are commonly used for viral cultivation?

White mice, rats, hamsters, guinea pigs, and rabbits.

What are common injection sites for viral exposure in animal models?

Brain, muscle, body cavity, skin, and footpads.

Why are animal models now the exception rather than the norm?

Due to high costs of animal care and public disapproval of animal testing practices.

What are the advantages of using fertilized bird eggs for viral cultivation?

1) Protected by hard eggshell, 2) Intact and self-supporting unit with sterile environment, 3) Contains complete nutrition, 4) Provides multiple tissue types that support various viruses.

What are the disadvantages of using bird embryos?

Viral effects may not be easily visible to the naked eye, making detection difficult.

What observable signs indicate viral infection in bird embryos?

Death of the embryo, defects in embryonic development, and presence of pocks (discrete, opaque spots indicating localized damage).

What is the main advantage of tissue culture methods?

They allow study of human-specific viruses without using living humans, and most viruses studied worldwide are grown this way.

What are primary cell cultures?

Freshly isolated animal tissue placed in growth medium that undergoes mitotic division to produce a monolayer while retaining characteristics of the original tissue.

What are the limitations of primary cell cultures?

They survive only 1-2 weeks or roughly 5-20 rounds of cell division, making long-term viral effect studies impossible.

What are continuous cell cultures?

Cell cultures with altered chromosome numbers that grow rapidly, show morphological changes, and can be continuously subcultured indefinitely with fresh nutrient media.

What are the advantages of continuous cell cultures?

They can last indefinitely with proper care, making them ideal for long-term viral research studies.

What are the disadvantages of continuous cell cultures?

Changes may affect cell shape or composition, so they may not be suitable for studying every virus.

What are plaques in viral cultivation?

Clear, well-defined patches in the cell sheet that develop when viruses released from infected cells spread to surrounding cells and infect them.

How do plaques form and spread?

Infection spreads gradually and symmetrically from the original point of infection as more host cells die and lyse, releasing viruses to infect neighboring cells.

What is the primary indicator of successful viral cultivation?

Degeneration and lysis of infected cells, leading to plaque formation in the cell monolayer.

How are plaques visualized in laboratory settings?

Using specialized dyes that color the media without affecting the cells, making plaques appear as clear or colored areas against the cell background.

What does "obligate intracellular parasite" mean in relation to viruses?

It means viruses cannot multiply or complete their life cycle without being inside a susceptible host cell.

What is a monolayer in cell culture?

A single layer of evenly distributed cells that covers a culture dish surface.

What is the difference between in vivo and in vitro?

In vivo means "in living organisms" (using animals/embryos), while in vitro means "in laboratory" (using cell/tissue cultures outside living organisms).

Why do virologists take great care to maintain tissue cultures?

Because most viral research depends on healthy, viable cultures, and maintaining them requires expertise and careful attention to growth conditions.

What makes a virus have a "broad host range"?

The ability to infect multiple different species or types of organisms (like being able to infect various mammals).

What is the significance of cytopathic effects in viral diagnosis?

CPE patterns help identify specific viruses because different viruses cause characteristic changes in infected cells.