Control programmes Flashcards
What are the three ways of controlling an outbreak?
Case numbers (per time or total), eradication, impact
What should you aim for Re to be below?
1
How many people must be vaccinated to reach herd immunity?
1 - (1/R0)
What can happen if vaccination is even only partially infective?
Less susceptible people being exposed, latent period increased so less people infectious, infectious period may be reduced
What three factors does the rate of new infection depend on?
The number susceptible, the number infectious, the number of susceptible becoming exposed
How can you minimise the number of infectious?
Vaccinate, diagnose early, isolate, cull, treat, quarantine
Hoe can you reduce the number of susceptible becoming exposed?
Reduce contact rates, biosecurity, imperfect vaccine reduces secretion
What were four methods of controlling FMD?
Diagnose early, stop mixing, cull, trace contacts
What happens to the number needed to be culled if you cull immediately?
Half
Which kind of vaccination is traditional in an outbreak?
Ring
How big must the vaccination ring be?
9km
Which type of farms are most susceptible to outbreaks?
Large cattle or mixed farms
How long should immunity be following ring vaccination?
At least 6 months
What is the problem with “vaccination to live”?
May miss long term carriers
How can you diagnose in acute disease?
Virus isolation, antigen test, nucleic acid
How can you diagnose in later disease?
Antibody test (serology)
What is viraemia like in later disease?
Already reduced, but could be present in other animals
How do you send skin scrapings?
Dry
What do viral tubes contain?
Balanced salts, antibiotics, antimycotics
What should you not do to viral samples?
Freeze
How stable is serum?
Relatively
How specific is PM for the virus?
Can only identify the type, not the specific virus
What do you need for virus isolation and identification?
A replication competent virus and tissue to grow on
What’s the problem with virus isolation and identification?
Get contamination, slow and expensive (2-3 weeks)
Can you see oncogenic virus transformation in vitro?
Yes
What happens to non-cytopathic BVDV in vitro?
Interferes with replication of indicator virus Newcastle disease virus
What does Fowl plague cause in chicken eggs?
Death of embryo
What does infectious bronchitis virus cause in chicken eggs?
Dwarfing of embryo
What do fowlpox and herpesvirus (laryngotracheitis) cause in chicken eggs?
Pock lesions
Which viruses was EM previously used for?
Orf and rotavirus
What is the problem with EM?
Need very high titre
What are the advantages of antigen or nucleic acid tests?
Easier, faster, detect non-viable virus, can’t be contaminated easily, can be used after virus isolation
What does precipitation on an Agar immunodiffusion test show?
Confirms antibody presence
How do you test for a DNA virus?
PCR
How do you test for an RNA virus?
Reverse transcriptase test first (RT)-PCR
What is the advantage of real-time PCR?
Looks at what is produced after every cycle so is quicker and can be cleaner
What does Next Generation Sequencing do?
Reads fragments of nucleic acids and compares to known sequences
Why does a negative result not exclude the virus?
Could be neutralising Ab or bad method
When might a positive result not indicative disease?
May be from subclinical shedding and something else is causing the disease
How does a neutralisation test work?
Mix virus with test serum and if cells/eggs aren’t infected them antibodies must be present
How do you work out a titre?
Do assay with two-fold serial dilutions and the titre is the reciprocal of the dilution that gave the last positive result
How do you identify a recent/active infection in an endemic area?
Take samples 2-4 weeks apart and titre will increase over 2 dilutions
How do you diagnose an exotic disease?
Antibody positivity in just one sample
When can you get a false positive for an endemic disease?
Cross reacts with endemic viruses giving low antibody titre
What does antibody positive show in a PI animal?
Infected carrier
What does antibody negative show in a PI animal?
Slow induction of Ab response so re-test in 6 months
What should antibody status be after vaccination?
Positive
How can you tell if infected or vaccinated?
DIVA ELISA
How does DIVA work?
WT infection = gE present
Why might some animals never become antibody positive?
Tolerant or immunosuppressed
What kind of viruses may show antibody negative?
In-utero BVDV infection or FIV cats
