Protection Induced in Broiler Chickens following Drinking-Water Delivery of Live Infectious Laryngotracheitis Vaccines against Subsequent Challenge with Recombinant Field Virus

In Ovo Vaccination with Recombinant Herpes Virus of the Turkey-Laryngotracheitis Vaccine Adjuvanted with CpG-Oligonucleotide Provides Protection against a Viral Challenge in Broiler Chickens

Infectious laryngotracheitis (ILT) is an economically important disease in chickens. We previously showed that an in ovo adjuvantation of recombinant herpesvirus of the turkey-Laryngotracheitis (rHVT-LT) vaccine with CpG-oligonucleotides (ODN) can boost vaccine-induced responses in one-day-old broiler chickens. Here, we evaluated the protective efficacy of in ovo administered rHVT-LT + CpG-ODN vaccination against a wild-type ILT virus (ILTV) challenge at 28 days of age and assessed splenic immune gene expression as well as cellular responses. A chicken-embryo-origin (CEO)-ILT vaccine administered in water at 14 days of age was also used as a comparative control for the protection assessment. The results showed that the rHVT-LT + CpG-ODN or the CEO vaccinations provided significant protection against the ILTV challenge and that the level of protection induced by both the vaccines was statistically similar. The protected birds had a significantly upregulated expression of interferon (IFN)γ or interleukin (IL)-12 cytokine genes. Furthermore, the chickens vaccinated with the rHVT-LT + CpG-ODN or CEO vaccine had a significantly higher frequency of γδ T cells and activated CD4+ or CD8+ T cells, compared to the unvaccinated-ILTV challenge control. Collectively, our findings suggest that CpG-ODN can be used as an effective adjuvant for rHVT-LT in ovo vaccination to induce protective immunity against ILT in broiler chickens.

Poultry Vaccine Technology Platforms

The different technology platforms used to make poultry vaccines are reviewed. Vaccines based on classical technologies are either live attenuated or inactivated vaccines. Genetic engineering is applied to design by deletion, mutation, insertion, or chimerization, genetically modified target microorganisms that are used either as live or inactivated vaccines. Other vaccine platforms are based on one or a few genes of the target pathogen agent coding for proteins that can induce a protective immune response ("protective genes"). These genes can be expressed in vitro to produce subunit vaccines. Alternatively, vectors carrying these genes in their genome or nucleic acid-based vaccines will induce protection by in vivo expression of these genes in the vaccinated host. Properties of these different types of vaccines, including advantages and limitations, are reviewed, focusing mainly on vaccines targeting viral diseases and on technologies that succeeded in market authorization.

Protection Efficacy of the Infectious Laryngotracheitis (ILT) Serva CEO Vaccine Strain in Broiler Chickens Under Different Vaccination Coverage Conditions

Mass vaccination against infectious laryngotracheitis virus (ILTV) in drinking water can result in variable initial vaccine take. Partial initial vaccine coverage of 20% with an Australian ILT vaccine (A20) previously resulted in significant protection against virulent ILTV challenge. This follow-up study used the international Serva ILT vaccine strain in a factorial design testing four levels of vaccination coverage (0%, 10%, 20%, or 100% of chicks eye-drop vaccinated with the live vaccine at 7 days of age) and three levels of ILTV challenge (no challenge or challenge at 7 or 21 days postvaccination [DPV]). The increase in ILTV load in choanal cleft swabs detected by qPCR after challenge was significantly reduced by 20% and 100% but not by 10% vaccination coverage. Vaccination reduced weight gain in unchallenged birds. Daily weight gain of birds was not affected by ILTV challenge at 7 DPV in any group, but following challenge at 21 DPV, it was significantly reduced in unvaccinated and 10% vaccinated groups relative to 20% and 100% vaccinated groups. Vaccination of 20% of the chickens provided substantial but incomplete protection (protective index range 44%-70%) against the severity of clinical signs and mortality following challenge while 10% vaccination coverage provided limited or no protection. Clinical signs were more severe and appeared earlier following challenge at 21 DPV than at 7 DPV. Within the vaccination treatments, eye-drop-vaccinated birds were better protected than their in-contact cohorts. In conclusion, partial vaccination of 20%, but not 10% of chickens, induced substantial protection against subsequent challenge. However, the attendant risks of reduced protection against early challenge and the possible reversion to virulence of vaccine virus when transmitted to unvaccinated chickens make it essential that 100% initial vaccine take be the goal of mass vaccination programs.

Uptake and spread of infectious laryngotracheitis vaccine virus within meat chicken flocks following drinking water vaccination

Vaccination against infectious laryngotracheitis virus (ILTV) in commercial broiler flocks in the field, which is only undertaken in the face of a local outbreak, requires mass administration techniques, usually via drinking water. This is often fraught with difficulties such as variable vaccination "reactions" and sometimes, vaccination failure. Laboratory testing of the outbreak strains however invariably shows the vaccines in use to be protective. To investigate this paradox, the dynamics of an ILT vaccine virus was examined within broiler flocks during a natural outbreak. In an initial flock, 70 birds were individually identified and had tracheal swabs collected sequentially at intervals from 1 to 26 days after vaccination and submitted for ILTV detection using qPCR. This evaluation was extended by collection of tracheal swabs from 40 to 45 random birds at 4, 7-8, 12-13 and 25-26 days post vaccination (pv) across a further 7 flocks. The results showed a very variable early uptake of vaccine virus from the drinking water (between 3% and 52% of tested birds with detectable virus in trachea at 4 days pv) and revealed that actual vaccination of the flocks relied on bird to bird transmission of the vaccine virus. In flocks with very low (<10%) initial bird uptake, successful exposure of vaccine virus to the majority of the flock can be delayed, leaving a large proportion of birds as susceptible at the likely time of possible exposure to wild virus. This may explain the cases of apparent failure of vaccination in the field. The variable bird to bird spread can be associated with reversion to virulence, this may explain the rolling vaccine reactions often observed. The variation in initial vaccine uptake may be affected by some factors involved with the administration technique and this requires further study in a larger sample size.

Determination of the minimum protective dose of a glycoprotein-G-deficient infectious laryngotracheitis virus vaccine delivered via eye-drop to week-old chickens

Infectious laryngotracheitis (ILT) is an upper respiratory tract disease of chickens that is caused by infectious laryngotracheitis virus (ILTV), an alphaherpesvirus. This disease causes significant economic loses in poultry industries worldwide. Despite widespread use of commercial live attenuated vaccines, many poultry industries continue to experience outbreaks of disease caused by ILTV. Efforts to improve the control of this disease have resulted in the generation of new vaccine candidates, including ILTV mutants deficient in virulence factors. A glycoprotein G deletion mutant vaccine strain of ILTV (ΔgG ILTV), recently licenced as Vaxsafe ILT (Bioproperties Pty Ltd), has been extensively characterised in vitro and in vivo, but the minimum effective dose required to protect inoculated animals has not been determined. This study performed a vaccination and challenge experiment to determine the minimum dose of ΔgG ILTV that, when delivered by eye-drop to seven-day-old specific pathogen-free chickens, would protect the birds from a robust challenge with a virulent field strain of virus (class 9 ILTV). A dose of 10(3.8) plaque forming units was the lowest dose capable of providing a high level of protection against challenge, as measured by clinical signs of disease, tracheal pathology and virus replication after challenge. This study has shown that the ΔgG ILTV vaccine strain is capable of inducing a high level of protection against a virulent field virus at a commercially feasible dose. These results lay the foundations upon which a commercial vaccine can be developed, thereby offering the potential to provide producers with another important tool to help control ILTV.

Replication-independent reduction in the number and diversity of recombinant progeny viruses in chickens vaccinated with an attenuated infectious laryngotracheitis vaccine

Recombination is closely linked with virus replication and is an important mechanism that contributes to genome diversification and evolution in alphaherpesviruses. Infectious laryngotracheitis (ILTV; Gallid alphaherpesvirus 1) is an alphaherpesvirus that causes respiratory disease in poultry. In the past, natural (field) recombination events between different strains of ILTV generated virulent recombinant viruses that have caused severe disease and economic loss in poultry industries. In this study, chickens were vaccinated with attenuated ILTV vaccines to examine the effect of vaccination on viral recombination and diversity following subsequent co-inoculation with two field strains of ILTV. Two of the vaccines (SA2 and A20) prevented ILTV replication in the trachea after challenge, but the level of viral replication after co-infection in birds that received the Serva ILTV vaccine strain did not differ from that of the mock-vaccinated (control) birds. Even though the levels of viral replication were similar in the two groups, the number of recombinant progeny viruses and the level of viral diversity were significantly lower in the Serva-vaccinated birds than in mock-vaccinated birds. In both the mock-vaccinated and Serva-vaccinated groups, a high proportion of recombinant viruses were detected in naïve in-contact chickens that were housed with the co-inoculated birds. Our results indicate that vaccination can limit the number and diversity of recombinant progeny viruses in a manner that is independent of the level of virus replication. It is possible that immune responses induced by vaccination can select for virus genotypes that replicate well under the pressure of the host immune response.