Diagnosis of Viral Infection Flashcards
Why is it not always possible to diagnose an infection clinically?
- Most viral infections present with very non-specific symptoms such as fever and coughing
- This is why it often requires a lab diagnostic test
- To aid the diagnosis, history, examination and special investigations are needed.
Why is rapid diagnosis of viral infections important?
To reduce need for unnecessary tests and inappropriate use of antibiotics.
- Could leave to antibiotic resistance
- Key to prevent spread of infection due to viral infections being highly infectious.
What does significance of test results depend on?
- Depend heavily on prevalence in population e.g. HIV.
- Sometimes false positives and false negatives and it all depends on how common the infection is in the population.
- The specificity of a test changes with the prevalence of a disease in a population
Why is it important to know the natural history of the pathogen in the type of patient being tested?
This will affect test selection and interpretation
Give an example of a test where a consent must be obtained
HIV test
What are the 3 different ways of using viral tests?
- Diagnostic: checking if the pt has the virus or not
- Monitoring: see if the pt needs treatment or not
- Screening: used when the patient is perfectly fine but what to check if they have any underlying viruses such as in pregnant women
Possible test types
- Electron microscopy
- Virus Isolation (cell culture)
- Antigen detection
- Antibody detection by serology
- Nucleic acid amplification tests (NAATs and PCR)
- Sequencing for genotype and detection of antiviral resistance
Why do you need to use a electron microscopy to see a virus?
Viruses are a lot smaller compared to other pathogens which can be seen with a light microscopy or even sometimes with the naked eye. Need a magnification of x20000
What techniques have replaced electron microscopy?
Molecular techniques
- Possibly still in useful for faeces and vesicle specimens
- Also useful in characterising emerging pathogens
Describe the specimen preparation
- Specimens are dried on a metal grid.
- The speciman is then stained with heavy metal such as uranyl acetate.
- The specimen can then be concentrated by adding antibodies which will bind to the virus and bring them all together.
- Then use a immune electron microscopy to concentrate the virus.
- The beams of electrons are used to produce an image.
Why is an electron microscopy better than a light microscopy?
The wavelength of electron beams is much shorter than light resulting in much higher resolution than light microscopy and hence why using electron microscopy is better at detecting viruses than light microscopy.
Advantages of electron microscopy
- Rapid
- Detects viruses that cannot be grown in culture
- Can visualise many different viruses
Disadvantages of electron microscopy
- Low sensitivity therefore, you need lots of the virus to be there
- Requires manitenance
- Requires skilled operators
- Cannot differentiate between viruses of the same virus family
Viruses that are easy to detect via electron microscopy
- Rotavirus
- Adenovirus
- Coronavirus
Rotavirus under electron microscopy
- Causes gastroenteritis
- Can be seen in faeces
- Has a wheel like structure
Adenovirus under electron microscopy
- Causes gastroenteritis
- Has an icosahedral shape
Coronavirus under electron microscopy
- Causes gastroenteritis
- Has a crown structure around a dumbbell like structure
Viruses that are harder to detect via electron microscopy
- Astrovirus
- Norovirus
Astrovirus under a microscopy
- Star like structure
- Star of David
- Causes gastroenteritis
Norovirus under a microscopy
- Belongs to the calicivirus family
- Most common cause of gastroenteritis
Herpes simplex
- Causes vesicles or blisters
- It has a ‘fried egg appearance’: when the virulent is inside an envelope. The envelope is formed from the host cell.
- If the patient has concentrated vesicles just around the lips then it is most likely a cold sore.
- Hard to differentiate between this and varicella zoster virus under the microscope so depends on the clinical context: site of vesicle and symptoms
Varicella zoster virus
- Causes vesicles or blisters
- If a patient has vesicles/white rash all over the body and fever, then it is most likely chicken pox
- Electron microscopes cannot differentiate these different viruses so depends on the clinical context: site of vesicle and symptoms.
Poxvirus
- Often described as looking like a ball of wool
- There are different types of the poxvirus - smallpox, monkeypox, Orf and cowpox.
- Using an electron microscope will not be able to determinate the difference between the poxviruses therefore it again depends on clinical context
Why are viruses unable to grow on an agar plate?
Virus need host cells for them to be able to replicate and carry on living
What is a cytopathic effect (CPE)?
The ability of a virus to cause morphological changes in the host cells. The CPE changes can be linked to which virus causes these changes.
Give an example of how an CPE can be seen
- When a virus is added a CPE effect can be seen, the cells might change shape.
How can you find out which virus is causing the CPE?
- Use a cell culture which supports the growth of only one virus
- Cell cultures can also be used to see if an antiviral drug works or not by looking for inhibition of CPE
- Do a neutralisation effect which is when you use antibodies against a specific virus antigen and if they stop the CPE then that virus is present.