Download Factors influencing the outcome of infectious bronchitis vaccination

Factors influencing the outcome of infectious
bronchitis vaccination and challenge experiments
J.J. (Sjaak) de Wit and Jane K.A. Cook
Avian Pathology 2014, 43:6, pp 485-497
Contents
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Review of the factors influencing the outcome of infectious bronchitis
vaccination and challenge experiments (respiratory and renal
systems)
Definition of protection
+ and – of the available techniques for measuring protection
Suggestions are made towards standardisation of a recognised
protocol and areas where more work is needed are indicated
Definition of protection
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In simple terms, it might be defined as the ability of an IBV vaccine to
prevent infection of the chicken following challenge with a field strain
of IBV.
‘prevention of infection’.
– freedom from clinical signs: in general mild under experimental conditions, hard to
quantify, subjective, double blind. Check for co-infections!
– freedom from IBV infection of the host?
• freedom from replicating virus (VI)
• freedom from viral RNA (RT-PCR)
• absence of infected cells (staining techniques)
– freedom of spread: no transmission to other birds (H120, R0 from 20 to 0,7
)
– lack of functional damage to the tissue,
• ciliostasis test
• lack of increased susceptibility to secondary infections, which might be most
relevant for the field situation
de Wit et al 1998
Factors/methods for performing protection studies
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Group size: confidence level and power
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Type of bird: SPF or MDA+ (quantify)
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Genetics: haplotype B15 less susceptable?
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Age at vaccination: also MDA
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Route of vaccination: oculonasal or ‘field’?
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Vaccine dose: EID50, TOC, TCID50?? o.n. vs spray/water, ‘commercial dose’
Factors/methods for performing protection studies
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Interval between vaccination and challenge: OOI, level of immunity (potency), DOI.
21-28 days mostly used
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Route of challenge: o.n., i.t., aerosol. O.n. = gold standard
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Dose of virus used for challenge: 103-105 EID50
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Time of assessing protection post challenge: peak of replication/damage, might be
different for different organs and level of protection. VI/RT-PCR in respiratory tract: 3-5 days,
kidneys later. Ciliostasis around 5 days.
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Assesment of protection against an experimental challenge
Assesment of protection against an experimental challenge
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Virus re-isolation
Use of molecular methods to detect viral RNA
– RT-PCR
– In situ hybrydization
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Functional tests
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Ciliostasis test
Histology
Immunostaining (IF, IHC)
Serology
– Relevance of extremely sensitive tests? Quantification?
– Comparative sensitivity of ‘same test’ in different laboratories?
Kidney
• more complicated
– sampling process: contamination by airsac
– possible presence of viraemia: infectious virus or its RNA in blood stream,
which can also result in virus or genome positive results without replication of IBV
in the kidney tissue.
– Both complications can be avoided by using techniques that show actual
replication in the kidney. IF, IHC, ISH, histology
Time of assesing protection post challenge
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Might be different from that of the respiratory tract
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Lambrechts et al: peak at 9 dpc (B1648)
GD
De Wit and Cook, AvP 2014
Conclusion
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not easy to compare the results of the many challenge studies
– For licensing vaccines: national or international guidelines.
– Where possible, follow these guidelines (which one??) in other studies, and this
should be possible as far as most of the parameters are concerned (for the
respiratory tract)
– Most disagreement both between groups of workers and with the regulatory
guidelines, is about the method by which protection should be assessed:
• more work is required in order to try and establish the optimum protocol to
use.
• Including standardization of these tests, what is relevant
Conclusion
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Guidelines for licencing of vaccines are not suitable for specific
questions of the field (e.g. false layers, ‘proventriculitis’)
Existing guidelines are not always appropriate to measure the level
of protection against renal damage.
– need for a standardised protocol to cover this aspect of IBV infections in young
birds, particularly with regard to the method(s) used to determine the ability of
different IBV vaccines to protect against nephritis associated with IBV.
recommendations
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Difficult to recommend a standard protocol:
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For licensing vaccines: local guidelines laid down by the appropriate regulatory authorities
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Studies performed for research or epidemiology: factors concerned with the reason for
performing the study will govern the design of the experiment. SPF or MDA+: antibody titres
should be determined and defined since MDA levels will vary considerably.
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In many situations it will be desirable to use day-old chicks as being the most susceptible to
IBV. On the other hand, some vaccines would only be applied at an older age, so this would
need to be taken into consideration when designing the study.
recommendations
– the oculonasal route for both vaccination and challenge.
– the interval between vaccination and challenge ‘should’ be standardised to
around 3-4 weeks post vaccination (not for OOI, DOI)
– Measure protection at peak of replication/damage:
• protection of the respiratory tract is being determined between 5 and 7 days
post challenge (ciliostasis, 7 is late for VI).
• protection of the kidney around 8 -9 days might be best
recommendations
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The most contentious aspect remains the method by which
protection is assessed.
– Both VI and ciliostasis from the upper respiratory tract are widely used: studies to
select the most meaningful one?
– RT-PCR: studies to determine if it provides a reliable alternative. Including
standardization, extremely sensitive is meaningful? Quantify!
– Kidney: assessment of local replication might be more relevant than VI+ or
PCR+, and therefore the use of histological/histochemical methods might give
more meaningful results.
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Until such a defined protocol is established, it is unlikely that an
agreed protocol for performing studies aimed at licensing IBV
vaccines in different countries can be agreed.
Thank you for your attention