Evaluation of mass vaccination against avian infectious laryngotracheitis virus by PCR testing of settled chicken house dust and potential extension of dust-based molecular monitoring to other chicken pathogens and live vaccines

Abstract

<p>With the rapid growth of the poultry industry, modern intensive poultry farms house very large numbers of chickens at high densities. This facilitates infectious disease transmission and has necessitated the implementation of strict biosecurity protocols and the development of vaccines as key disease prevention and control measures. For practical and economic reasons, the majority of the vaccines in meat chickens are administered by mass vaccination methods which are prone to maladministration and can affect the success of vaccination. In flocks with suboptimal vaccination coverage, infectious diseases can occur with very significant consequences and need frequent monitoring. Currently, there is very little routine evaluation of pathogen status or vaccination success in the modern intensive meat chicken production system due to reliance on tests, often invasive, based on sampling individual chickens and the prohibitive costs associated with this. The development of effective economic population-level sampling and detection systems would allow routine monitoring of live vaccination success and/or flock pathogen status. Poultry house dust has been identified as promising population-level sample material for qPCR evaluation of certain pathogens or vaccine organisms in the chicken industry. However considerable additional work is required to expand the range of pathogens or live vaccine candidates for which population-level testing based on dust may be useful.</p> <p>In response to these gaps, this PhD project was developed with three broad objectives, namely 1) Extensive field testing and validation of population-level qPCR testing of dust for infectious laryngotracheitis virus (ILTV) in meat chickens to assess outcomes of drinking water vaccination" 2) Investigating the potential of dust-based PCR testing methods for monitoring other important poultry vaccines and pathogens" and 3) Evaluating the consequences of poor initial vaccine coverage with live attenuated ILT vaccines on protection against subsequent challenge with virulent ILTV.</p> <p>To achieve the first objective, the studies reported in Chapters 3 and 4 involved the assessment of drinking water vaccination outcome in meat chickens following vaccination with live ILT vaccine by comparing the population-level dust sample with swabs from the trachea and choanal cleft of birds using qPCR test (chapter 3) and the field application of dust sample-based approach for routine ILTV monitoring and as a tool to evaluate outcomes of interventions during ILT outbreak eradication. Regarding the second objective, the potential of dust-based PCR testing to monitor other poultry live vaccines and pathogens was evaluated for Newcastle disease virus (NDV), infectious bursal disease virus (IBDV), infectious bronchitis virus (IBV), fowl adenovirus (FAdV), fowl pox virus (FPV), avian encephalomyelitis virus (AEV), <i>Mycoplasma gallisepticum</i> (MG) and <i>Mycoplasma synoviae</i> (MS) and reported in chapters 7 and 8. The third objective was addressed in chapters 5 and 6. The consequences of poor initial ILT vaccination coverage on the level of protection against challenge wih virulent ILTV in meat chickens were evaluated following different initial vaccination coverage using two live ILT vaccines (Serva and A20) that are widely used in the Australian meat chicken industry.</p> <p>The main findings of the studies in this thesis are: Objective 1) PCR-based testing of settled poultry house dust is a useful tool to assess ILT vaccination outcomes and to monitor chicken flocks for circulating virus. The absence of ILTV genome copies (GC) in dust samples at 7 days post-vaccination (DPV) indicates suboptimal initial vaccine take. Detection of ILTV GC in 0 DPV dust samples spoils the use of the 7 DPV dust samples for assessing vaccination outcomes, rather indicates the presence of circulating virus or carryover DNA. PCR testing of ILTV in dust samples is an easier and non-invasive method to monitor chicken flocks for circulating ILTV. Outcomes of intervention measures during ILT outbreaks can be assessed by PCR testing of few dust samples and is helpful to make decisions and assist in outbreak eradication. Typing of circulating ILTV from dust samples is possible and important to identify the strain of the circulating virus and understand its epidemiology. Objective 2) Nucleic acids of NDV, IBV, IBDV, FAdV, AEV, MG, and MS are detectible by PCR in settled chicken house dust from commercial settings with a potential to be used for assessing live vaccination outcomes and monitoring flocks for pathogens. Concentrations of NDV GC in dust samples reflect the proportion of birds infected with the vaccine virus following spray vaccination of meat chickens at the hatchery. IBV GC concentration in dust remains persistently high despite a significant reduction in IBV-positive birds. Objective 3) Partial ILT initial vaccination coverage of 20% with live vaccines can provide substantial protection against subsequent virulent ILTV challenge. Initial vaccination of 20% of birds via eye-drop is associated with substantial transmission of the vaccine virus to in-contact birds and protection against challenge. Lower initial vaccine coverages of 7-10% have the risk of absence of or limited transmission to in-contact birds and lack of protection.</p> <p>The outcomes of this thesis have further confirmed the use of settled poultry house dust as a population-level sample to assess ILT mass vaccination using live vaccines and its wider application in the field for ILT monitoring. The studies also showed nucleic acids of other important poultry live vaccines can be detected in dust that dust, giving insights for further research to investigate the practical utility.</p>