Viral diagnosis

Question 1

Name the three broad approaches to diagnosis of viral infections

Answer 1

1. Detection of the virus or its components
2. Detection of characteristic clinical signs, gross pathology, or histopathology
3. Detection of antibodies to the virus in the affected animal

Question 2

Name two key aspects that should be considered when taking samples for the diagnosis of viral disease

Answer 2

1. the type of sample to be collected
2. the timing of sample collection

Question 3

Describe the sample collection method for detecting a virus (or its components) in acute disease

Answer 3

1. type of sample depends on the disease: e.g. faeces, respiratory secretions, blood
2. timing of sample: when the animal is sick

Question 4

Describe two potential issues with trying to detect the virus (or its components) in a persistent viral disease

Answer 4

1. Virus may be present all of the time, or intermittently
2. “innocent bystander effect”: virus may not be responsible for the clinical signs

Question 5

List four methods of detecting a virus, or it’s components

Answer 5

1. Viral isolation (culture)
2. Nucleic acid detection (PCR or reverse transcriptase PCR)
3. Antigen detection tests (e.g. serology, ELISA)
4. Electron microscopy

Question 6

What is the time to result of viral isolation?

Answer 6

Viral isolation (culture) can take 1-3 weeks to complete (longest waiting period)

Question 7

What is the relative cost and availability of viral isolation

Answer 7

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

Question 8

Describe the relative sensitivity of viral isolation

Answer 8

Moderate

Question 9

What are the limitations of viral isolation

Answer 9

• Not widely available
• Improper handling/transport may result in false negative

Question 10

What is the time to result for detection of nucleic acids?

Answer 10

1-2 days (moderate)

Question 11

Describe the cost and availability of detection of viral nucleic acids

Answer 11

Moderate cost, able to be done in most virology labs (molecular equipment required - PCR)

Question 12

What are the limitations of detection of viral nucleic acids

Answer 12

Only a limited range of viruses can be tested (requires primers for PCR)

Question 13

Describe the sensitivity of detection of viral nucleic acid

Answer 13

Very high - PCR can amplify even a few virus particles

Question 14

What is the time taken for antigen detection (serology)?

Answer 14

Fast: minutes - < 1 day

Question 15

Describe the cost and availability of antigen detection tests

Answer 15

Lowest cost, can be performed by most labs, some in clinic. Minor equipment needed

Question 16

Describe the sensitivity of antigen detection tests

Answer 16

Low

Question 17

What are the limitations of antigen detection tests

Answer 17

Not available for all viruses

Question 18

What is the time to result for electron microscopy for viral detection

Answer 18

1-2 days (moderate)

Question 19

Describe the cost and availability of electron microscopy for viral detection

Answer 19

Moderate-high cost; Only done in specialised labs with expensive EM equipment

Question 20

What is the relative sensitivity of electron microscopy for virus detection?

Answer 20

Very low

Question 21

What are the limitations of electron microscopy for viral detection?

Answer 21

Few viruses produce sufficient concentration of particles in lesions

Question 22

What are the limitations of detecting viruses through characteristic pathology?

Answer 22

Most viruses do not produce lesions that are sufficiently characteristic to provide anything more certain than a tentative diagnosis

Question 23

When might we use pathology to diagnose possible viral infections?

Answer 23

Usually best with other testing, however this may be cost prohibitive, or other viral detection tests may not be availaible

Question 24

Describe collection of samples for use in viral diagnosis by pathology

Answer 24

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

Question 25

Describe the sample collection for detection of antibodies

Answer 25

Sample: blood

Timing: 1-2 weeks after disease period

Question 26

What are the limitations of detection of antibodies for viral diagnosis

Answer 26

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)

Question 27

Name four types of serological tests that might be used in detection of antibodies

Answer 27

• ELISA
• hemagglutination assays
• immunofluorescence
• immunochromatography

Question 28

What are the benefits of detection of antibodies for viral diagnosis?

Answer 28

Relatively cheap and quick

Question 29

A detection test for antibodies returns a positive result. What are the possible interpretations of this outcome?

Answer 29

• 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

Question 30

A detection test for antibodies returns a negative result. What are the possible interpretations of this outcome?

Answer 30

• Tentative diagnosis incorrect (virus not present)
• Sample taken too early (before AB development)
• False negative

Question 31

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

Answer 31

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

Question 32

List the three types of persistant infection

Answer 32

1. latent infection
2. chronic infection
3. slowly progressive

Question 33

Latent infections: Describe the timecourse of

a) duration of clinical signs
b) detectability of the virus
c) detectability of antibodies
d) Give an example

Answer 33

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

Question 34

Chronic infections: Describe the timecourse of

a) duration of clinical signs
b) detectability of the virus
c) detectability of antibodies

Answer 34

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

Question 35

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

Answer 35

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

Question 36

Compare and contrast the three main methods of viral diagnosis

Answer 36

Question 37

Compare and contrast the four approaches to detection of viruses or virus particles

Answer 37