Pathogens- Viruses

Question 1

What is the general structure of a virus?

Answer 1

• Contains genetic material, either DNA or RNA, and they can be single-stranded or double-stranded
• Contains a protein coat (e.g a capsid). This can be helical (spiral) or icosahedral (20-sides) or other complex shapes
• Has a lipid membrane envelope which is derived from the host cell it infects

Question 2

What is the structure of the HIV virus?

Answer 2

Is a retrovirus, is enveloped with an ssRNA genome

• Has a nucleocapsid, a capsid
• Has matrix proteins which are structural proteins linking the viral envelope with the virus core

Question 3

How are viruses grouped into types?

Answer 3

Grouped roughly into 4 types
-Nonenveloped with DNA
-Enveloped with DNA
-Nonenveloped with RNA
Enveloped with RNA
(differ still if they are ss or ds DNA/RNA)

Question 4

What is a bacteriophage?

Answer 4

A type of virus that infects bacteria

-Bacteriophages spread pathogenicity genes of bacteria which increases bacterial virulence

Question 5

Can bacteriophages be harnessed?

Answer 5

Bacteriophages can be used in ‘phage therapy’ of bacterial infections
-not used in UK or Europe (has only been experimental uses in rare cases) as seen as unsafe

Question 6

What is viral pathogenesis?

Answer 6

Is the process by which a viral infection leads to disease
-abnormal situation as there is no value to the virus. Most viral infections have no symptoms, as it is not in the interest of the virus to kill the host

Question 7

What are the outcomes of viral infection?

Answer 7

Acute infection - rapid onset of disease, brief symptoms and resolution within days
Chronic infection - symptoms develop slwoly and gradually, and is slow to resolve, taking weeks or months

Question 8

What are the factors and process of viral pathogenesis?

Answer 8

Cellular pathogensis
Entry into host
Course of infection
Cell/tissue tropism
Cell/tissue damage
Host immune response
Virus clearance or persistance

Question 9

What is cellular pathogensis and how do cells respond to this?

Answer 9

Cellular pathogenesis is the effect of viral infection on host cells

• cells respond in 3 ways:
  1. No apparent change
  2. Death
  3. Transformation

Question 10

How do viruses cause direct cell damage?

Answer 10

Direct cell damage:

• diversion of cell energy
• shutoff of cell macromolecular synthesis
• competition for cellular transcription factors or ribosomes etc

Question 11

How do viruses cause indirect cell damage?

Answer 11

Indirect cell damage:

• intergration of viral genome
• induction of mutations into host genome
• inflammation

Question 12

How do viruses enter the body?

Answer 12

• Skin- via a breach in the skin like a cut, or viruses emply vectors like mosquitos to breach the skin for them
• Respiratory tract-however has sophisticated immune defence mechanisms which viruses must overcome
• Gastrointestinal tract- however a very hostile enviroment due to things like gastric acid and bile salts, virus must be well adapted
• Genitourinary tract (urinary system)- less hostile than those above but not as often exposed for entry
• Conjunctiva and mucous membranes- common as they are unprotected and exposed

Question 13

What is the process of viral replication?

Answer 13

1. Recognition, adsorption, penetration of virus
2. Release of DNA or RNA
3. Either: synthesis or DNA and RNA, Synthesis of RNA, or reverse synthesis of DNA
4. Synthesis of viral subunits
5. Protease action on polypeptides
6. Assembly of Viruses
7. Lysis of cell and release of mature viruses
8. Infection of new cells

Question 14

What is primary replication of a virus?

Answer 14

Replication at the site of entry to the host

-often determines if the infection will be localized or systemic

Question 15

What is systemic spread?

Answer 15

Where the virus spreads via the blood stream and the CNS (instead of cell to cell contact)

Question 16

What is secondary replication of a virus?

Answer 16

Replication at susceptible organs/tissues following systemic spread of the virus

Question 17

What is cell tropism?

Answer 17

Cells that support the growth of a particular virus or bacterium

Question 18

What factors determine cell tropism?

Answer 18

Cell recpetors for viruses
Cell transcription factors that can recognise viral promoters
Enviroment- whether it is harsh or not
Ability of the cell to support viral replication

Question 19

What are the main types of immunity that respond to viruses?

Answer 19

Generally, cellular immunity plays the major role in clearing virus infections whereas humoral immunity prevents against reinfection

Question 20

What are the 2 types of chronic persistant infections?

Answer 20

True latency

Persistence

Question 21

What is true latency chronic persistent infection?

Answer 21

The virus remains completely latent following primary infection. Its viral genome is integrated into cellular genome, or exists as episomes

Question 22

What is persistent chronic persistent infection?

Answer 22

The virus replicates continuously in the body at a very low level

Question 23

What are some mechanisms of viral persistence?

Answer 23

Antigenic variation
Mimicary- virus produces same/similar antigens to immune system
Down regulation of MHC class 1 expression
Infection of an immunoprivileged site
Direct infection of immune cells (HIV)

Question 24

What is an immunoprivileged sites?

Answer 24

An area of the body that is able to tolerate the introduction of antigens without eliciting an immune response e.g sensory ganglia in the CNS

Question 25

What is hepatitis B and how is it treated?

Answer 25

An infection of the liver caused by a virus

• treated using pegelated interferon ( peg-IFN) and nucleotide analogs (NA)
• used in different combinations to treat the disease

Question 26

What is the biology of influenza viruses (Orthomyxoviruses)?

Answer 26

3 distinct types: A, B, C. With type A causing most infections
Contain single stranded RNA- consisting of 10 genes on 8 seperate RNA segments

Question 27

What are glycoprotein spikes?

Answer 27

They are protein spikes on the surface of some viruses such as influenza
-responsible for the binding of the virus to a susceptible host cells

Question 28

What are the different types of glycoprotein spikes and what are their respective functions?

Answer 28

Hemagglutinin- 15 different subtypes, binds to host cells

Neuraminidase- 9 different subtypes, hydrolyses mucus and assists viral budding and release

Question 29

Why do glycoproteins frequently undergo genetic changes?

Answer 29

To decrease the effectivness of the host immune response as the glycoproteins often targeted to stop viruses binding to host cells or infecting other cells

Question 30

What is antigenic drift?

Answer 30

Constant, slow mutations to change amino acid compostiton to create new antigens that are less targeted by host response/drugs

Question 31

What is antigenic shift?

Answer 31

A bigger jump and change than antigenic drift. One of the genes or strands is substituted with a gene or strand from another influenza virus
-creates a new structure that is harder to recognize and destroy

Question 32

What is influenza A and how does it cause illness?

Answer 32

Highly contagious respiratory disease-causes the seasonal pandemics

• binds to ciliated cells of respiratory mucosa
• causes rapid shedding of cells, stripping respiratory epithelium, leading to severe inflammation
• Weakened host defenses- therefore at risk of secondary bacterial infections like pneumonia

Question 33

What is the diagnosis, treatment and prevention of influenza A ?

Answer 33

Diagnosis- immunofluorescence tests to detect antigens in pharyngeal specimen
Treatment- control symptoms using drugs like oseltamivir, zanamivir and amantadine
Prevention- annual vaccine recommended

Question 34

What are the major targets for antiviral drugs?

Answer 34

Virus adsorption
Virus penetration
Uncoating
Expression of viral genome, transcription/translation
Genome replication
Protein synthesis
Assembly
Release of virus