Diagnosis of Viral Infections Flashcards
What are the possible test types for viral infections?
- 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
How are bacteria and fungi visualised in a lab?
Bacteria and Fungi can be seen using light microscopy x400 – x1000
Which pathogens are visible by a naked eye?
Protozoa, helminths can also be seen by naked eye
How are viruses visualised?
Viruses need electron microscopy x20,000
Describe when Electron microscopy is used fro viral infections?
- Viruses visualised with EM
- Mostly replaced by molecular techniques
- Possibly still useful for faeces and vesicle specimens
- Useful in characterising emerging pathogens
Outline how a viral specimen is prepared fro EM
- Specimens dried on a grid
- Stained with heavy metal e.g. uranyl acetate
- Concentrated with application of antibody i.e. immuno-EM to concentrate virus
- Beams of electrons are used to produce images
Why do EM have a higher resolution than light microscopes?
Wavelength of electron beam is much shorter than light, resulting in much higher resolution than light microscopy
Outline the advantages o fusing EM
- Rapid
- Detects viruses that cannot be grown in culture
- Can visualise many different viruses
What are the limitations associated with using EM for viral detection
- 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
Describe what rotavirus looks like under EM
> 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
What is the structure of adenovirus?
Characteristic appearance - icosahedral shape
Can cause coughs, colds, gastroenteritis
Describe the structure of coronavirus
Crown like structure
Easily spotted on EM
What is the structure of the norovirus
Member of calicivirus family
Commonest virus these days causing gastroenteritis
not easily distinguishable
Describe the shape of astrovirus
Star like structure - similar shape to norovirus
Where is herpes virus commonly collected from for visualisation?
Herpes virus from vesicular fluid
What is the limitation of vesicular herpes?
2 types of herpes viruses that cause vesicles; vesicular illness
Herpes simplex
- cold sores, blisters, genital warts
Varicella zoster virus
- chicken pox, shingles
How do we differentiate between VZV and Herpes simplex?
EM cannot differentiate these different viruses so depends on clinical context, site of vesicle and symptoms
Name the members of the pox virus
Smallpox
Monkeypox
Orf
Cowpox
Why are cell cultures used to identify viruses?
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
How are cell cultures prepared for viral detetcion?
Use different cell lines in test tubes or plates. Selection of cell types important.
Slow, but occasionally useful in antiviral sensitivity testing
How is a viruses cytopathic effect used for detetcion?
Identify virus using antigen detection techniques or neutralisation of growth
Cell culture plus antiviral – look for inhibition of cytopathic effect
