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PROJECT SUMMARY
Controlling virulent ILTV field
recombinants by vaccination
Infectious laryngotracheitis virus (ILTV) causes acute respiratory
disease (ILT) in chickens and results in significant economic
losses worldwide.
In 2011 is was demonstrated that multiple natural recombination
events between ILTV vaccines led to the creation of virulent
recombinants that spread and caused severe disease in
Australian poultry flocks.
The Purpose of the current study was to determine the efficacy
of existing vaccines against these dominant and more virulent
field strains of ILTV and to prevent future outbreaks of disease
caused by ILTV recombinants (e.g. class 9). Reducing the level of
virus replication within flocks is important for the control of ILT,
as this is likely to reduce the viral load in the environment and
potentially decrease the risk of spread beyond the infected flock
to new flocks or farms.
Key Findings
• Class 9 ILTV was more virulent, had more rapid growth, a lower infectious dose and was more easily transmitted from
bird-to-bird than the class 2 virus.
• Currently available attenuated vaccines and the ΔgG vaccine candidate can be used to help control class 9 ILTV when
delivered by drinking water.
• However it is important to note that neither the protection induced in the directly-vaccinated birds, nor that induced
in birds that were contact-exposed to vaccinated birds, was complete.
• Taken together these results suggest that the persistence of class 9 ILTV infection and disease in commercial poultry
flocks in Australia is likely to result from the incomplete protection of birds from class 9 ILTV afforded by the existing
vaccines when administered via drinking water, combined with the growth and transmission advantage of class 9
compared to the class 2 viruses.
Implications
All vaccines used in this study induced some level of protection against challenge by class 9 viruses and resulted in reduced
virus replication or the proportion of infected birds in directly vaccinated and contact exposed birds. These findings strongly
suggest that combining vaccination programs with other disease control measures, such as biosecurity measures, are necessary
for improved ILT control.
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Issues with mass vaccination against ILTV
Drinking water vaccination is a preferred method for mass
delivery of vaccine to large meat chicken flocks due to the
ease and cost effectiveness of application. However, under
field conditions, some birds may not consume drinking water
containing the vaccine, or may receive a suboptimal dose of
the vaccine because of reduced consumption.
All vaccines reduced the amount of detectable virus,
but protection was not complete
These birds, therefore, remain either unvaccinated or
incompletely vaccinated. In order to simulate the lack of
uniform vaccination that can occur in commercial flocks
vaccinated by drinking water, the present study included five
unvaccinated, age-matched birds that were placed in-contact
with vaccinated birds in each of the groups immediately after
vaccination.
This study examined vaccine efficacy against class 9 challenge
in directly vaccinated birds, as well as contact exposed birds,
that were not directly vaccinated, but were added to groups
after the removal of vaccine in the drinking water.
Their subsequent protection against challenge with virulent
virus was assessed. This challenge virus was administered
via intra-tracheal inoculation and via eye-drop in order to
simulate the respiratory and ocular routes of infection that
occur under field conditions.
Development of an ILTV challenge model
This experiment identified the suitable dose of a current
field strain of ILTV (class 9) to be used in the subsequent
vaccination and challenge studies that was capable of reliably
inducing clinical signs in infected birds, but without inducing
unnecessarily severe consequences for animal welfare.
Three different doses, delivered by eye-drop and by intratracheal inoculation of the class 9 virus were compared to
the same concentrations of a previously dominant, nonrecominant field strain of ILTV (class 2).
Differences in virulence between the replication and
transmission dynamics of class 2 and class 9 ILTV.
Specifically, class 9 ILTV was more virulent, had more
rapid growth, a lower infectious dose and was more easily
transmitted from bird-to-bird than the class 2 virus.
Once the challenge dose of class 9 virus was identified, the
level of protection induced by single oral doses of the current
ILT vaccines (SA2, A20, Serva) and a candidate vaccine
(ΔgG) administered in drinking water could be examined.
Results indicated that currently available attenuated vaccines
and the ΔgG candidate vaccine can be used to help control
class 9 ILTV when delivered under conditions resembling
field conditions (via drinking water). All vaccines reduced the
amount of detectable virus in the trachea at both four and
seven days after challenge compared to unvaccinated birds.
All vaccines also reduced the proportion of birds that were
positive for the presence of ILTV DNA in conjunctival and/
or tracheal swabs seven days after challenge.
Furthermore, the SA2, A20 and Serva ILTV vaccines reduced
the amount of virus detected in the conjunctiva seven days
after challenge. However, it is important to note that neither
the protection induced in the directly-vaccinated birds, nor
that induced in birds that were contact-exposed to vaccinated
birds, was complete.
Development of an assay to detect cellular immunity is
ongoing
The results of these and other studies have shown that the
titres or presence of antibody are insufficient measures of
protection, and this is reflected in part by the finding that the
ΔgG vaccine candidate did not induce any antibody in birds
after vaccination, despite the finding that ΔgG vaccinated
birds showed levels of protection generally comparable to the
other vaccine strains that did induce antibody production.
This research required the development of a suitable purified
ILTV antigen, in vitro stimulation assays and a chicken
interferon-gamma ELISA.
Work is ongoing to develop an ILTV specific interferongamma stimulation assay to determine whether this assay can
be used to better assess flock responses to vaccination and
better estimate the level of protection against challenge.
ILTV infected chicken tracheal tissue
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Publications
The bulk of this research was published in the two articles below, and are open access (meaning there is no restriction on
accessing the full publication).
1. Lee, S.-W., Hartley, C. A., Coppo, M. et al (2015). Growth Kinetics and Transmission Potential of Existing and
Emerging Field Strains of Infectious Laryngotracheitis Virus. PLoS One, 10(3): e0120282.
Weblink: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0120282
2. Korsa, M. G., Browning, G. F., Coppo, M. J. et al. (2015). Protection Induced in Broiler Chickens following DrinkingWater Delivery of Live Infectious Laryngotracheitis Vaccines against Subsequent Challenge with Recombinant
Field Virus. PLoS One, 10(9): e0137719.
Weblink: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0137719
Recommendations
ILTV vaccination via drinking water combined with strict biosecurity measures are currently the best industry applicable
methods to prevent ILTV infection in meat chickens.
• Continued monitoring of ILTV field strains is recommended to understand the change in ILTV viruses over time
and how these changes impact on vaccine efficacy. Existing methods for genotype analysis such as PCR-RFLP and
particularly full genome sequencing, are the best available methods to define the extent of changes to field viruses over
time and the relationship between current and previous filed strains and vaccine strains of ILTV
• Continued development of new vaccines and vaccine delivery methods is recommended, with the aim of providing
protection from clinical disease as well as prevention of virus shedding after vaccination
• The development of an assay to detect cellular immunity to ILTV is technically difficult but has the potential to
improve the ability to determine flock immunity to ILTV. Continued development of this assay is recommended
(and is currently ongoing) so that it can be assessed as tool to aid in the development of improved field vaccination
programs for ILT.
More Information
JM Devlin*; SW Lee; AH Noormohammadi; PF Markham; GF Browning; CA Hartley
Asia Pacific Centre for Animal Health, The University of Melbourne
*corresponding author: [email protected]
RIRDC Project number: PRJ-008792
Pub Number: 16/022
ISBN: 978-1-74254-868-5
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