Diagnosis of Viral Infections

Question 1

What are the possible test types for viral infections?

Answer 1

• Electron Microscopy
• Virus isolation (cell culture)
• Antigen detection
• Antibody detection; serology
• Nucleic acid amplification tests (NAATs e.g. PCR)
• Sequencing for genotype and detection of antiviral resistance

Question 2

How are bacteria and fungi visualised in a lab?

Answer 2

Bacteria and Fungi can be seen using light microscopy x400 – x1000

Question 3

Which pathogens are visible by a naked eye?

Answer 3

Protozoa, helminths can also be seen by naked eye

Question 4

How are viruses visualised?

Answer 4

Viruses need electron microscopy x20,000

Question 5

Describe when Electron microscopy is used fro viral infections?

Answer 5

• Viruses visualised with EM
• Mostly replaced by molecular techniques
• Possibly still useful for faeces and vesicle specimens
• Useful in characterising emerging pathogens

Question 6

Outline how a viral specimen is prepared fro EM

Answer 6

1. Specimens dried on a grid
2. Stained with heavy metal e.g. uranyl acetate
3. Concentrated with application of antibody i.e. immuno-EM to concentrate virus
4. Beams of electrons are used to produce images

Question 7

Why do EM have a higher resolution than light microscopes?

Answer 7

Wavelength of electron beam is much shorter than light, resulting in much higher resolution than light microscopy

Question 8

Outline the advantages o fusing EM

Answer 8

• Rapid
• Detects viruses that cannot be grown in culture
• Can visualise many different viruses

Question 9

What are the limitations associated with using EM for viral detection

Answer 9

• Low sensitivity need 106 virions/millilitre. May be enough in vesicle secretion/stool
• Requires maintenance
• Requires skilled operators
• Cannot differentiate between viruses of same family

Question 10

Describe what rotavirus looks like under EM

Answer 10

> getting very rare due to vaccination
Used to be a common cause of gastroenteritis in toddlers
Called ‘rota’ virus as it looks like a wheel

Question 11

What is the structure of adenovirus?

Answer 11

Characteristic appearance - icosahedral shape

Can cause coughs, colds, gastroenteritis

Question 12

Describe the structure of coronavirus

Answer 12

Crown like structure

Easily spotted on EM

Question 13

What is the structure of the norovirus

Answer 13

Member of calicivirus family
Commonest virus these days causing gastroenteritis

not easily distinguishable

Question 14

Describe the shape of astrovirus

Answer 14

Star like structure - similar shape to norovirus

Question 15

Where is herpes virus commonly collected from for visualisation?

Answer 15

Herpes virus from vesicular fluid

Question 16

What is the limitation of vesicular herpes?

Answer 16

2 types of herpes viruses that cause vesicles; vesicular illness

Herpes simplex
- cold sores, blisters, genital warts

Varicella zoster virus
- chicken pox, shingles

Question 17

How do we differentiate between VZV and Herpes simplex?

Answer 17

EM cannot differentiate these different viruses so depends on clinical context, site of vesicle and symptoms

Question 18

Name the members of the pox virus

Answer 18

Smallpox
Monkeypox
Orf
Cowpox

Question 19

Why are cell cultures used to identify viruses?

Answer 19

Viruses require host cells to replicate and may cause a Cytopathic Effect (CPE) of cells when a patient sample containing a virus incubated with a cell layer

Question 20

How are cell cultures prepared for viral detetcion?

Answer 20

Use different cell lines in test tubes or plates. Selection of cell types important.
Slow, but occasionally useful in antiviral sensitivity testing

Question 21

How is a viruses cytopathic effect used for detetcion?

Answer 21

Identify virus using antigen detection techniques or neutralisation of growth

Cell culture plus antiviral – look for inhibition of cytopathic effect