Viruses

Question 1

What are viruses? What is their genome? What determines their shape?

Answer 1

Obligate intracellular parasites
Genome is RNA or DNA, single stranded or double stranded, circular or linear or segmented
Their shape is determined by the capsid (protein coat)
Some viruses are naked, some viruses have a lipid bilayer

Question 2

What are capsomeres?

Answer 2

Individual subunits of the capsid

Question 3

What is a nucleocapsid?

Answer 3

Nucleic acid + capsid

Question 4

What is a virion?

Answer 4

An entire viral particle

Question 5

What type of virus is more susceptible to environmental factors: naked or lipid?

Answer 5

Lipid viruses

Question 6

What are the different routes of transmission of viruses?

Answer 6

Oral transmission (contaminated food and drink, saliva)
Droplet transmission (inhalation, sneezing)
Direct inoculation (injections, trauma, insect bites)
Direct skin contact
Trans-placental
Sexual transmission

Question 7

Describe the specificity of viruses

Answer 7

Viruses are very host and receptor specific. If a virus causes infection in bacteria, it won’t infect humans. If a virus causes infection in particular tissue, it won’t infect other tissue because it is receptor specific (e.g., influenza does not cause skin infection).
A person with a mutated receptor gene might be immune to infection of a virus specific for that receptor (e.g., CCR5 mutation and HIV infection)

Question 8

Describe the replication cycle of a virus

Answer 8

Binding to the receptor of the host cell
Entering host cell
Release of viral genome
Use host machinery to synthesize viral proteins
Viral assembly
Viral release (budding, in which host cell is not killed, or lysis, in which host cell is killed)

Question 9

What are known outcomes to viral infection (viral pathogenesis)?

Answer 9

Cell death (due to vital functions being taken over by the virus and the cell bursts upon virus release, e.g., all naked viruses)
Multinucleated giant cell formation (fusion of plasma membranes of adjacent cells during the adsorption phase, e.g., herpes virus)
Malignant transformation
Inclusion Bodies
No change

Question 10

What is malignant transformation (viral pathogenesis)? What is CPE?

Answer 10

Unrestricted growth and division as caused by certain retroviruses
Cytopathic effects (CPE) - most viruses cause this. It is the rounding of cells due to cytoskeletal changes and brown colouring due to over production of pigments

Question 11

Why is it important to know the pathogen outcomes?

Answer 11

For diagnosis

Question 12

What are inclusion bodies?

Answer 12

Localized areas in cells where virus replication takes place (we see this a lot in rabies)

Question 13

What is the significance of viruses that cause no change?

Answer 13

They present the biggest problems
If a virus causes no change in the cell morphology, then it it impossible to figure out if a patient has this virus (e.g., HIV latency)
The viral genome is incorporated into the host genome and remains dormant

Question 14

What are the different routes of viral transmission?

Answer 14

Person to person (direct contact, airborne, fecal contamination, across the placenta)
Animal to human (bite, insect vector)
Fomites (pencils, cups)
Food/water borne

Question 15

Where does viral replication take place?

Answer 15

In a specific host cell (it is always intracellular)

Question 16

How do viruses spread throughout the body?

Answer 16

They can cause localized infections (infections in discrete areas. For example, rhinovirus causes infections in the upper respiratory tract, HBV causes infection in the liver)
They can cause systemic infections, where the initial infection is localized but then it penetrates into the circulatory or nervous system

Question 17

Explain how polio causes systemic infection

Answer 17

Polio is transmitted through the oral/fecal route. It penetrates from the small intestine into the lymph nodes and then the circulatory system, brain and motor neurons and causes paralysis

Question 18

What, quite often, causes the symptoms of viral infections?

Answer 18

Symptoms are often causes by the immune system. Destruction of host cells can result in cell debris, which can result in inflammation and other symptoms. For example, Hep B-induced damage to the liver is primarily caused by cytotoxic T cells killing the liver cells. With measles, cytotoxic T cells kill the surface capillary cells, causing a rash

Question 19

What are the different ways in which a virus can persist (not be eliminated by host defences)?

Answer 19

Proviral formation (the viral particle is not active; the genome is incorporated into our DNA)
Immune tolerance or ineffective antibodies
Virus localized in sheltered areas like the brain
Antigenic shift (our immune system is constantly playing catchup)
Cell-to-cell spread without contacting the immune system cells
Immunocompromised host

Question 20

What are the different types of persistent infections?

Answer 20

Chronic carrier

Latent infection

Question 21

What is a chronic carrier infection?

Answer 21

The patient is infected throughout the infection cycle (e.g., if someone is infected with Hep B, they will be infected in al stages and they will always be able to spread the virus to other people

Question 22

What is a latent infection?

Answer 22

There is dormant phase (e.g., someone who has had chicken pox will carry the genome but will not be able to spread chicken pox to other people)

Question 23

What are the different ways in which viruses can interfere with the host defences?

Answer 23

Production of receptors for the immune system mediators (Vaccina virus’s coat proteins bind to and inactivate IL-1)
Interference with antigen presentation (Adenovirus-12 binds to MHC-1 and prevents its migration to the cell surface)
Interference with complement pathway (Herpes virus binds to and inactivates C3)
Interference with the interferons (HIV RNA block phosphorylation of an initiation factor required in the blocking step of the interferon pathway)

Question 24

What is a major issue with combating viral infections?

Answer 24

Viruses use the host cell replicative machinery
Antiviral agents can’t target virus without affecting the host cell
Most of these compounds adversely affect the host (they don’t just target infected cells)
Individuals being treated must be closely monitored for side effects (e.g., bon marrow suppression, renal toxicity)

Question 25

How do we prevent viral infections?

Answer 25

Ant-viral vaccines have been hugely successful in preventing infections
Behavioural changes are also important for limiting the chance of infection (e.g., STIs)