Viruses and Disease

Question 1

What is the result of bacteria being so small?

Answer 1

Bacteria are at the limits of light microscopy.

Question 2

What Is the structure of viruses?

Answer 2

Simple structure biochemically.
All contain RNA or DNA, never both - very small genome, limited by size of the capsid (protein coat).
Protein coat is made up of repeated subunits of a virally encoded protein, built like a lego house into a simple geometric structure.
Some viruses have their protein coat surrounded by a lipid envelope, which is derived from the host cell that the virus grew in.
Spike of viral protein may protrude from the protein coat or the envelope.

Question 3

what are Icosahedral Symmetry made up of and what are the 3 types of building block required. Icosahedral symmetry causes a variety of illnesses most commonly what and how are the different types varied and similar?

Answer 3

Adenovirus (a type of virus), icosahedral symmetry.
made up of repeated subunits, only 3 types of building block required, apex, edge and centre of face.
Adenoviruses can cause a variety of illnesses, most commonly conjunctivitis, pharyngitis and diarrhoea, less commonly pneumonia and disseminated infection in the immunocompromised. They exist as many different types, antigenically and genetically different with different clinical associations but the same structure and appearance in the electron microscope.

Question 4

Why do viruses only have to code for a single protein for Helical symmetry? how may helical capsids be supercoiled?

Answer 4

Made up of a single repeated unit, so virus only has to code for a single protein which will aggregate spontaneously around the viral nucleic acid.
Helical capsids may be supercoiled into an envelope.
Other capsids have more complex geometry.

Question 5

What is an Adenovirus a type of?

Answer 5

A type of virus.

Question 6

What illnesses does the Herpes Simplex Virus cause, why is it so difficult to differentiate it from other types of herpes viruses and why does this illustrate a limitation of microscopy?

Answer 6

HSV electron microscopy; this is the cause of oral herpes, including cold sores and also genital herpes and rare severe diseases such as herpes encephalitis.
Not that HSV looks indistinguishable from all other herpes group of viruses which cause many other illnesses, e.g. chickenpox (VZV) infectious mononucleosis (EBV) etc, this is another limitation of microscopy, it inly tells you the shape of a microbe.

Question 7

What does the Rabies Virus cause?

Answer 7

Rabies e.m., this causes a fatal encephalitis following a bite from and infected mammal.

Question 8

What does the Orf pathogen cause?

Answer 8

Orf e.m., this is a veterinary pathogen of sheep which sometimes causes skin lesions on sheep farmers.

Question 9

What is the process of Attachment, where the virus attaches to a cell and what is an example of this?

Answer 9

Example, influenza and cell of the respiratory tract. Cell has various normal proteins inserted into it’s membrane, performing normal physiological functions.
Viruses all interact with specific receptors in the target cell, analogous to lock and key or antibody and antigen.
HIV- CD4 molecule
The ligand of the virus attaches to the receptor protein on the surface of the cell.

Question 10

What is the process by which a virus enters a cell?

Answer 10

Non-enveloped viruses may enter by endocytosis.
Fusion of viral and cell envelope can be mediated by viral enzyme.
Fusion of viral and cell membranes.
or endocytosis - binding to receptor initiates internalisation of both the receptor and the virus.
Some antivirals inhibit entry by fusion.

Question 11

What is the process of Nucleic Acid and Protein synthesis within a host cell?

Answer 11

Some steps may be mediated by viral enzymes e.g. protease
Host ribosomes always used.
Some viral enzymes may be used e.g. RNA dependent RNA polymerase.
Nucleic acid is used as instructions to produce new viral proteins - host ribosomes are used and host polymerases may be used.
Viral nucleic acid is replicated into progeny genomes.

Question 12

What is the process of Assembly inside a host cell?

Answer 12

Crystals of assembling virus may be visible by light microscope as “inclusions”.
Nucleic acid and proteins packaged together.
Intra-cellular arrays of virus may be visible by light microscopy as inclusions. No antivirals target this step yet.

Question 13

What is the process of Release of viruses from host cells?

Answer 13

Release can be through a process essentially the reverse of the entry process of phagocytosis. A piece of host cell membrane ends up around the capsid.

Question 14

What is the process by which viruses are released from host cells by budding?

Answer 14

Mature progeny virus released with envelope derived from the host cell membrane.
Not all viruses are released like this.

Question 15

What is the process by which viruses are released from a host cell by lysis?

Answer 15

Some viruses accumulate until the virus lyses the cell.

These large aggregates of virus may be visible by light microscopy as inclusions.

Question 16

Are Viral inclusions (adenovirus) large enough to be seen through light microscopy?

Answer 16

Inclusions may be large enough to see by light microscopy, through individual viruses are not.

Question 17

What is the role of Antibiotics and what are 3 examples of targets for antibiotics?

Answer 17

Active against bacteria, not viruses.
Objective is selective toxicity.
Structures targeted include:
- Bacterial ribosomes (e.g. erythromycin)
- Growing bacterial cell wall (e.g. penicillin)
- Other bacterial structures/enzymes
Viruses have few of these targets.

Question 18

What are possible targets and other strategies for Antiviral Drugs?

Answer 18

Possible targets
- viral nucleic acid polymerases
- other viral enzymes involved in viral nucleic acid replication or in protein synthesis (Integrase, Protease)
- uncoating
- attachment/entry
- release
Other strategies
- immune adjuvants.
Ribosomes: erythromycin
Cell wall: penicillin

Question 19

What is the process of rational drug design?

Answer 19

The use of detailed molecular analysis of viral targets to design a molecule that might inhibit its function, rather than blind testing of random molecules for antiviral properties.
Not successful strategy in bacteria.

Question 20

Why is Drug discovery/ development pipeline a long process?

Answer 20

This is an expensive as well as long process with many drugs failing at each of the stages before reaching licensing.

Question 21

How is Antiviral resistance normally analysed and when is it normally used?

Answer 21

Can be analysed phenotypically
- Grown in vitro in presence of drug
More often analysed genotypically
- Phenotype inferred from genotype
Most often seen in setting where long periods of treatment are given, especially in immunocompromised patients.

Question 22

What is pathogenesis and what are the causes and some examples?

Answer 22

Cell death due to lysis or hijacking of cell machinery.
Cell death due to immune system
- especially cytotoxic T cells
Cell proliferation
Note that viruses can “infect” a person and that person can be infectious but without symptoms.
Pathogenesis means the mechanism by which the virus causes disease.
Examples of virally induced cancer include HPV and cervical cancer and Hep B and Hep C and hepatocellular carcinoma.
Examples of asymptomatic infection in rhinovirus and poliovirus and TTV.

Question 23

What are the reasons that when a virus is detected, sometimes no symptoms are present?

Answer 23

Convalescent
Late stages of incubation
Asymptomatic
Reactivation
A very few viruses have no proven relationship with disease
- Torque teno viruses (TTVs) are ubiquitous in >90% of adults worldwide but no human pathogenicity of TTV has been fully established.

Question 24

How to establish if a newly discovered virus is a pathogen?

Answer 24

Case-control study
- Take a large group of people with clinical signs of infection.
- Take a second large group of people, as similar as possible to the first group, except that they are well
- Compare prevalence of virus in the cases and the controls.
Same age mix
Same sex mix
Same geographic spread
Same socio-economic mix
groups must be large enough to detect a difference reliably (powered correctly).

Question 25

Why may Cell proliferation cause cancer?

Answer 25

Examples of infection causing malignancy in mankind:
- human papilloma viruses. Some types cause cervical cancer.
- Hepatitis B and C viruses. Primary hepatocellular carcinoma.
- Some lymphoma and leukemia cases. Epstein Barr virus, Human T Leukaemia/lymphoma viruses
- Kaposi’s sarcoma. Human Herpes virus type 8.
Gastric cancer and Helicobacter pylori (a bacterium).
Overall 16% of cancers worldwide are caused by infection.
Overall 20% of deaths from cancer are in those cancers caused by infection.
Immunosuppression increases the incidence of infection related cancers.

Question 26

What is the trend of Incidence HPV related cancers in UK?

Answer 26

Incidence of cervical cancer is falling due to screening and vaccination and non-cervical cancer in women due to vaccination.
Vaccination of males is to start to improve their protection against HPV related cancer.
Note that cervical cancer is likely to be overtaken by head and neck cancers as the most common HPV related cancer in the future.

Question 27

How does immunity and virus infection occur?

Answer 27

Cytotoxic T lymphocytes can recognise proteins on the cell surface as being foreign and will signal the infected cell to commit suicide in order to prevent the formation of further mature virus.
Neutralising antibodies (IgG, IgM) can prevent virus binding to cellular receptors.
Cell mediated immunity is generally more important than humoral immunity in providing recovery from viral infection, through antibodies have a clear role in long term adaptive immunity.

Question 28

What is Innate Immunity?

Answer 28

Double stranded RNA

• Potent inducer of interferon
• Ligand for Toll-Like Receptor 3

Question 29

What does the Public Health etc. (Scotland) act 2008 cover?

Answer 29

Defines 3 lists:

• Notifiable diseases: for example measles that need to be reported to public health, when suspected or confirmed.
• Notifiable organisms: whenever las detect it we pass that info on to Health Protection Scotland electronically. Some notifiable organisms do not cause notifiable diseases, for example, chickenpox is not notifiable disease but VZV is a notifiable organism.
• Notifiable health risk states: these are syndromes that may be caused by serious and transmissible infections, such as viral haemorrhagic fever where you may not yet know the identity of the organism. It also applies to non-infectious states, like if some ex Russian spy were to get ill with suspected poisoning.

Question 30

What is Persistence?

Answer 30

Virus may become quiescent - no active replication
- Latency
Such viruses may reactivate
- Herpes simplex virus
- Varicella-zoster virus
Virus may remain continually active for years. Chronic infection
- HIV
- Hepatitis C virus
These patients may remain asymptomatic but infectious for decades before getting life threatening complications.
Usually virus infections are controlled and eliminated by the body’s immune reactions. Some viruses can elude the immune system to establish infection for years, even for the lifetime of that individual.
Persistent infections can be intermittent, with periods of active viral replication punctuated by viral latency. Illustrate with discussion of cold sores and latency in dorsal root ganglia.

Question 31

How can you confirm virus infection in the laboratory?

Answer 31

Detect the antibody response against the virus.

Detect presence of the virus itself.

Question 32

What is the antibody response?

Answer 32

There is an incubation periods and then you can see clinical signs.

Question 33

What is Humoral immunity?

Answer 33

Recent infection can be differentiated from past infection in the following ways:
- detection of virus specific IgM antibodies
- detection of rising titre of IgG antibodies
- detection of very high titre of IgG antibodies
Paired (acute and convalescent) blood samples may be required.

Question 34

How do you detect a virus?

Answer 34

Currently in use:
- PCR (polymerase chain reaction)
- antigen detection (by various methods)
Being developed or used in certain niches
- newer technologies such as combined PCR and mass spectrometry and next generation sequencing technologies.
Older techniques now less used
- cell culture
- electron microscopy
Explain cell culture (rarely used, PCR preferred)
PCR detects the virus nucleic acid. Not that there are other methods similar to PCR for amplifying and detecting nucleic acids, they can collectively be known as NAATs (nucleic acid amplification technologies).
EM required since light microscopy unable to detect particles as small as individual virions.

Question 35

Antibody response and viral detection in acute infection.

Answer 35

Titre means amount.

The antibody response may be a better test later in the disease.

Question 36

What is the process of uncoating, whereby the virus is uncoated?

Answer 36

Uncoating can be due to viral ion pump.
Viral nucleic acid is released from the capsid.
One antiviral works by inhibiting the ion pump in the virus capsid responsible for uncoating.

Question 37

What are antimicrobials?

Answer 37

Antimicrobials is a more generic term which can be used to mean drugs that will be used to treat infections which may be bacterial or viral or fungal or protozoa.

Question 38

What is the main reason that most viruses carry a few of their own enzymes, how is uncoating achieved and what is the target for new anti-influenza drugs and why?

Answer 38

Fortunately many viruses carry a few of their own enzymes, especially for transcription of viral nucleic acids.
Other enzymes (e.g. HIV protease) are involved in the maturation of viral protein before assembly.
Uncoating may be achieved through ion pumps which can be selectively blocked.
Penetration of the mucus layer and release are the targets for new ant-influenza drugs since release may be hammered by the progeny sticking to the receptors on the cell they are being released from.