Viral diagnosis Flashcards
Name the three broad approaches to diagnosis of viral infections
- Detection of the virus or its components
- Detection of characteristic clinical signs, gross pathology, or histopathology
- Detection of antibodies to the virus in the affected animal
Name two key aspects that should be considered when taking samples for the diagnosis of viral disease
- the type of sample to be collected
- the timing of sample collection
Describe the sample collection method for detecting a virus (or its components) in acute disease
- type of sample depends on the disease: e.g. faeces, respiratory secretions, blood
- timing of sample: when the animal is sick
Describe two potential issues with trying to detect the virus (or its components) in a persistent viral disease
- Virus may be present all of the time, or intermittently
- “innocent bystander effect”: virus may not be responsible for the clinical signs
List four methods of detecting a virus, or it’s components
- Viral isolation (culture)
- Nucleic acid detection (PCR or reverse transcriptase PCR)
- Antigen detection tests (e.g. serology, ELISA)
- Electron microscopy
What is the time to result of viral isolation?
Viral isolation (culture) can take 1-3 weeks to complete (longest waiting period)
What is the relative cost and availability of viral isolation
Viral isolation (culture) is relatively, the most expensive technique, because it can only be performed by major labs (requires cell culture equipment and cell lines
Describe the relative sensitivity of viral isolation
Moderate
What are the limitations of viral isolation
- Not widely available
- Improper handling/transport may result in false negative
What is the time to result for detection of nucleic acids?
1-2 days (moderate)
Describe the cost and availability of detection of viral nucleic acids
Moderate cost, able to be done in most virology labs (molecular equipment required - PCR)
What are the limitations of detection of viral nucleic acids
Only a limited range of viruses can be tested (requires primers for PCR)
Describe the sensitivity of detection of viral nucleic acid
Very high - PCR can amplify even a few virus particles
What is the time taken for antigen detection (serology)?
Fast: minutes - < 1 day
Describe the cost and availability of antigen detection tests
Lowest cost, can be performed by most labs, some in clinic. Minor equipment needed
Describe the sensitivity of antigen detection tests
Low
What are the limitations of antigen detection tests
Not available for all viruses
What is the time to result for electron microscopy for viral detection
1-2 days (moderate)
Describe the cost and availability of electron microscopy for viral detection
Moderate-high cost; Only done in specialised labs with expensive EM equipment
What is the relative sensitivity of electron microscopy for virus detection?
Very low
What are the limitations of electron microscopy for viral detection?
Few viruses produce sufficient concentration of particles in lesions
What are the limitations of detecting viruses through characteristic pathology?
Most viruses do not produce lesions that are sufficiently characteristic to provide anything more certain than a tentative diagnosis
When might we use pathology to diagnose possible viral infections?
Usually best with other testing, however this may be cost prohibitive, or other viral detection tests may not be availaible
Describe collection of samples for use in viral diagnosis by pathology
Sample: depends on disease - organs most severely impacted by the virus
Timing: is best if done during the presence of clinical signs, but may also be possible during recovery or at necropsy
Describe the sample collection for detection of antibodies
Sample: blood
Timing: 1-2 weeks after disease period
What are the limitations of detection of antibodies for viral diagnosis
Presence of antibodies may have other interpretations
False negatives may occur if sample is taken too early
Requires >=4x increase in titer between acute and convalescent samples to be definitive (requires 2-3 weeks)
Name four types of serological tests that might be used in detection of antibodies
- ELISA
- hemagglutination assays
- immunofluorescence
- immunochromatography
What are the benefits of detection of antibodies for viral diagnosis?
Relatively cheap and quick
A detection test for antibodies returns a positive result. What are the possible interpretations of this outcome?
- Viral infection causing diseases
- Previous (resolved) infection
- Presence of maternal antibodies in young animals
- Vaccination response
- Persistent but non-causative viral infection (e.g herpesvirus, retroviruses)
- False positive
A detection test for antibodies returns a negative result. What are the possible interpretations of this outcome?
- Tentative diagnosis incorrect (virus not present)
- Sample taken too early (before AB development)
- False negative
Acute viral infections: Describe the timecourse of
a) duration of clinical signs
b) detectability of the virus
c) detectability of antibodies
d) Give an example
a) Period of clinical signs associated with increasing/maximal levels of the virus
b) Around the duration of clinical signs +/- either side of clinical signs
c) 1-3 weeks AFTER clinicals signs; may decline or persist long term
d) Most viruses; e.g. bovine ephemeral fever
List the three types of persistant infection
- latent infection
- chronic infection
- slowly progressive
Latent infections: Describe the timecourse of
a) duration of clinical signs
b) detectability of the virus
c) detectability of antibodies
d) Give an example
a) Intermittant periods of reoccurance and latency following initial infection
b) usually around the period of reoccurence (recrudence);
c) 1-3 weeks following initial infection, usually lifelong
d) herpesviruses
Chronic infections: Describe the timecourse of
a) duration of clinical signs
b) detectability of the virus
c) detectability of antibodies
a) Period of clinical signs associated with increasing/maximal levels of the virus, lasting months-years
b) Around the duration of clinical signs +/- either side of clinical signs
c) 1-3 weeks AFTER clinicals signs; may decline or persist long term
Slowly progressive infections: Describe the timecourse of
a) duration of clinical signs
b) detectability of the virus
c) detectability of antibodies
d) Give an example
a) after a long duration of infection; may result in death
b) from infection, throughout life
c) 1-3 weeks after infection, throughout life
d) retroviruses
Compare and contrast the three main methods of viral diagnosis
Compare and contrast the four approaches to detection of viruses or virus particles
