Diagnosis of viral infections Flashcards
What are the key concepts?
- cant always clinically diagnose a viral infection
- need history, examination, investigation and imaging to diagnose
- rapid investigations of viral infections reduce need for unnecessary tests and innappropriate antibiotics
- test results depends on prevalence of disease in population
- certain infections tests need consent e.g. HIV
What are the tests for virology?
- electron microscopy (not used anymore)
- virus isolation and cell culture
- antigen detection
- antibody detection by serology
- nucleic acid amplification tests (NAATs) eg PCR
- sequencing for genotype and detection of antiviral resistance
What can be seen by light microscopy?
- bacteria
- fungi
- helminths
- protozoa
Why cant a virus be seen on light microscopy?
too small
What is the magnification of an electron microscope?
(x20,000
magnification)
How does an electron microscope work?
- electron beams
- electron beam wavelength much shorter than light wavelength and so it has high resolution
- electrons scatter when they pass thin sections of specimen
- electrons that dont scatter, pass through the specimen and make an image
- denser regions scatter less electrons and appear darker
What are the advantages of electron microscopy?
- fast
- dont need to culture anything on the virus
- virus can be looked at on the same day
- can detect viruses that you cant grow in culture
- can visualise many different viruses
- dont need to know what you are looking for before hand
What are the disadvantages of electron microscopy?
- very low sensitivity = need sample of
10^6 viral particles/ml to visulise them - require lot of maintenance
- require lot of skilled operators
- cant differentiate between viruses of same family e.g. HSV1, HSV2 and varicella zoster
- need to do further tests like immunofluorescence or PCR to diagnose which herpes virus it is
What is a cytopathic effect?
effect virus has on host cell
How is viral isolation in cell culture conducted?
- incubate patients viral sample on a cell layer
- take cell culture every 2 days
- USE light microscopy to see CPE
- once visualised the cytopathic effect, use immunofluorescence to CONFIRM the virus
To culture herpes, what cell line is used?
human embryonic lung cell lines
What was viral isolation in cell culture used to discover?
- hMPV and Nipha virus in the last 20
years
What can viral isolation in cell culture be used for?
- sometimes used in anti viral sensitivity
testing
in some viruses like herpesvirus, it can cause a
cytopathic effect in 24 to 48 hours
in other viruses such as cytomegalovirus, it can take up to 21 days
Which viruses are vieed by antigen detection?
- respiratory syncytial virus and influenza from nasopharyngeal aspirates
- hepatitis B or dengue from blood serum or plasma samples
- herpes simplex and varicella zoster from fluid taken from skin vesicular swabs (from vesicular rashes)
- rotavirus and adenovirus from faeces (cause gastroenteritis)
What is good about antigen detection technique?
- detects actual virus not the immune response to it