Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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. RURAL INDUSTRIES R&D CORPORATION project summary 1 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 RURAL INDUSTRIES R&D CORPORATION project summary 2 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 RURAL INDUSTRIES R&D CORPORATION project summary 3