Intro to virology

Question 1

What are the 4 different ways viruses have been classified through history?

Answer 1

Classification by host e.g. human, plant

Classification according to target organ e.g. respiratory (flu, Sars-Cov-2), enteric (norovirus), hepetitis

Classification according to vector e.g. arboviruses, avian (birds)

Classification by molecular biology e.g. genetic sequencing & biophysical structure

Question 2

What does taxonomy of a virus describe

Answer 2

• Virus order
• Virus family
• Subfamily
• Type species
• Morphology - what they look like? e.g. shape
• Genetic material (DNA or RNA) —> HIV, Sars-Cov-2, Flu = RNA or EBV (causes glandular fever), Adenovirus, Hepatitis B = DNA
• Envelope - a ‘cloak’ that some viruses possess that has a role in viral transmission.

Question 3

What is the main classification system for Human viruses?

Answer 3

Baltimore classification

E.g.
dsDNA- double-stranded DNA viruses
ssDNA- single-stranded DNA virus

Question 4

How do we differentiate between bacterial & viral causes of infection? Why is this not easy?

Answer 4

Both may cause v. similar symptoms
- e.g. fever, irritability, CNS (e.g. photophobia), respiratory (e.g. cough), gastrointestinal (e.g diarrhoea) symptoms.

Some symptoms are pathognomonic - classical for a particular pathogen.
1. e.g. respiratory symptoms - productive cough vs. non-productive.
- Productive cough = usually bacterial cause
- Non-productive = usually viral cause
2. Symptoms persist longer in bacterial infection than the expected 10 days virus tends to last.
3. Fever is higher in bacterial infection than in viral infection.
4. Rely on diagnostic tests to differentiate between the two: CRP; FBC; BC, PCR, CSF cell count etc
- CRP high - suggests bacterial.
- FBC - high Neutrophils suggests bacterial; high lymphocytes & monocytes suggests viral.
- Blood cultures - can tell you which bacteria it is.
- PCR - detects both bacterial or viral infection.
- CSF cell count - lymphocytes = viruses or TB; neutrophils = bacteria.

Question 5

Why do we diagnose viral infections?

Answer 5

1. Medical - patient management
2. Epidemiological- public health e.g. Influenza, Sars-cov-2, mumps, measles…
3. Intra-hospital Infection prevention & control
4. Academic - research

Question 6

What are the methods for detecting virus infections?

Answer 6

1. Detection of viral antigens
2. Detection of nucleic acids (PCR)
3. Electron microscopy
4. Virus culture
5. Histopathology staining
6. Serology testing: presence of virus-specific antibodies (IgM & IgG). e.g. for Hep B, HIV

Question 7

Detection of viral pathogens is highly dependable on 3 important variables: what are they?

Answer 7

1. Obtaining an adequate specimen from the appropriate site.
2. Timing of specimen collection relative to onset of symptoms.
3. Timely processing of the sample.

Question 8

Why is timing important for viral infections?

Answer 8

• For many viral infections, viral shedding begins shortly before the symptoms occur.
• Peaks rapidly after onset of symptoms, & declines steadily as the infection resolves e.g. flu (exception chronic viral infections, e.g. HIV, Hep C).
• samples should be collected as soon as possible after the onset of symptoms.
• Rapid diagnosis important for treatment start (e.g. influenza)
• Serological response sometimes lengthy (IgM and IgG).

Question 9

How to decide how urgent a viral case is?

Answer 9

• The nature of suspected infection e.g. HIV vs. cold…
• The ease of spread (how contagious)
• The ways in which the spread of the infection can be prevented or controlled
• The patient circumstances (age, sex, occupation).
• Urgent cases should be reported by phone w/in 24 hours.