Current Status of Poultry Recombinant Virus Vector Vaccine Development

Inactivated and live attenuated vaccines are the mainstays of preventing viral poultry diseases. However, the development of recombinant DNA technology in recent years has enabled the generation of recombinant virus vector vaccines, which have the advantages of preventing multiple diseases simultaneously and simplifying the vaccination schedule. More importantly, some can induce a protective immune response in the presence of maternal antibodies and offer long-term immune protection. These advantages compensate for the shortcomings of traditional vaccines. This review describes the construction and characterization of primarily poultry vaccine vectors, including fowl poxvirus (FPV), fowl adenovirus (FAdV), Newcastle disease virus (NDV), Marek's disease virus (MDV), and herpesvirus of turkey (HVT). In addition, the pathogens targeted and the immunoprotective effect of different poultry recombinant virus vector vaccines are also presented. Finally, this review discusses the challenges in developing vector vaccines and proposes strategies for improving immune efficacy.

Effects of in ovo vaccination time on broiler performance parameters under field conditions

Hatchery performance is often evaluated based on descriptors such as hatchability, 7-d mortality, and cost. In addition to these descriptors, it is useful to include in this analysis aspects of chick quality through post-hatch performance. Realizing the bird's complete genetic potential necessitates meeting various criteria, with effective support for the chick's immune system being among the pivotal factors. To be effective, in ovo vaccination systems must deliver the vaccines to specific sites in the egg, a circumstance that directly depends on when the injection is made. We examined production data to evaluate the impact of in ovo vaccination time on performance parameters of male Ross308AP chicks. A comprehensive survey was conducted examining records from 3,722 broiler flocks produced and raised by the same company under standard nutrition and management conditions. The selected data specifically pertained to flocks that underwent slaughter between 41 and 45 d. In our analysis, 4 different linear models were built, one for each response variable: mean weight (MW), body weight gain (BWG), corrected feeding conversion rate (cFCR), and total mortality (TM). The linear models used in the analyses included as main predictor the timing of in ovo vaccination (440, 444, 448, 452, 456, 458, and 460 h of incubation), and as additional predictors: age of the breeding flock (26-35, 36-55 and 56-66 wks old), slaughter age, identity of the hatchery, and the season at which the data was collected. Our results showed that the timing of in ovo vaccination significantly affected BWG and cFCR, with procedures performed at 460 h of incubation showing the best outcomes. Breeding flock age affected all response variables, with older breeding flocks delivering increased MW, BWG and TM, and middle-aged flocks increased cFCR. Increasing slaughter age reduced BWG while MW, cFCR and TM were all increased. These data emphasize the benefits of performing in ovo vaccination as close as possible to 460 h of incubation to extract the best BWG and cFCR from Ross308AP male broiler.

Advances in Poultry Vaccines: Leveraging Biotechnology for Improving Vaccine Development, Stability, and Delivery

With the rapidly increasing demand for poultry products and the current challenges facing the poultry industry, the application of biotechnology to enhance poultry production has gained growing significance. Biotechnology encompasses all forms of technology that can be harnessed to improve poultry health and production efficiency. Notably, biotechnology-based approaches have fueled rapid advances in biological research, including (a) genetic manipulation in poultry breeding to improve the growth and egg production traits and disease resistance, (b) rapid identification of infectious agents using DNA-based approaches, (c) inclusion of natural and synthetic feed additives to poultry diets to enhance their nutritional value and maximize feed utilization by birds, and (d) production of biological products such as vaccines and various types of immunostimulants to increase the defensive activity of the immune system against pathogenic infection. Indeed, managing both existing and newly emerging infectious diseases presents a challenge for poultry production. However, recent strides in vaccine technology are demonstrating significant promise for disease prevention and control. This review focuses on the evolving applications of biotechnology aimed at enhancing vaccine immunogenicity, efficacy, stability, and delivery.

Vaccination Technology in Poultry: Principles of Vaccine Administration

Poultry vaccines are very important tools for disease prevention and may be administered collectively by drinking water or spray or individually by injection or oculonasal drop, whereas inactivated vaccines are administered by injection only. Poultry vaccines are increasingly delivered at the hatchery to day-old chicks or in ovo, because mass vaccination is much more efficiently implemented and controlled at the hatchery than on the farm. Mass administration on the farm by drinking water or spray requires strict compliance with guidelines regarding water quality, preparation of vaccines, and application, so as to cover the whole flock. Vaccination at the hatchery uses integrated machines to deliver vaccines to day-old chicks or, increasingly, in ovo at transfer from setters to hatchers. Regardless of the route and technology, a high quality of monitoring is critically important to maintain strict compliance and best practices from the vaccine vial to the bird, to guarantee efficient administration and intake of the vaccine by the whole flock and to secure the integrity of the vaccine itself. Major recent technical innovations in poultry vaccination covering both biology and technology open a very exciting era.

Effects of the in ovo injection of an Escherichia coli vaccine on the hatchability and quality characteristics of commercial layer hatchlings

In the commercial egg industry, avian pathogenic Escherichia coli (APEC) can lead to significant economic loss. The Poulvac E. coli vaccine (PECV) is a commercially available attenuated live vaccine commonly applied via spray or drinking water to protect against losses associated with colibacillosis. The PECV has not been tested in layer hatching eggs using in ovo injection. Therefore, the purpose of this experiment was to determine the effects of injecting 50 μL of different doses of the PECV into Hy-Line W-36-layer hatching eggs on the hatchability and quality characteristics of hatchlings. At 18 d of incubation (DOI), treatments included 1 noninjected and 1 diluent-injected control. Furthermore, PECV treatments included a full dose (4.4 × 108E. coli CFU) or serial dilutions of the full dose to produce 4.4 × 106, 4.4 × 104, or 4.4 × 102 CFU doses of E. coli. In ovo injections targeted the amnion. Percent hatchability of live embryonated eggs (HI), percent residue eggs, hatchling mortality, and female chick whole and yolk-free BW, relative yolk sac weight, and body length were among the variables examined. Treatment significantly (P < 0.0001) affected HI, with HI being highest in the control groups (97.3% in the noninjected and 94.2% in the diluent-injected), and with HI values being 89.0, 88.9, 84.4, and 71.2% in the 4.4 × 102, 4.4 × 104, 4.4 × 106, and 4.4 × 108 CFU E. coli dose treatments, respectively. The percentage of live embryos that did not complete hatch but that pipped internally (P = 0.024) or externally (P < 0.0001) were significantly affected by treatment, with percentages being highest in the 4.4 × 108 CFU treatment. Female chick body length was significantly (P < 0.0001) affected by treatment and was longer in both control groups and in the 1 × 102 CFU E. coli treatment in comparison to all other treatments. Yolk-free female chick BW was significantly (P = 0.034) affected by treatment and was lower in the 4.4 × 106 CFU and 4.4 × 108 CFU treatments when compared to the diluent-injected control group. An increase in the E. coli concentration administered in the amnion of embryonated layer hatching eggs at 18 DOI decreased hatch success and female chick yolk-free BW and body length.

Potential Implication of in ovo Feeding of Phytogenics in Poultry Production

Hatchery's goals include maximizing revenue by achieving high hatchability with day-old birds of excellent quality. The advancement of technology has benefited the poultry sector since breeding and genetics technology have increased the rates of meat maturation in developing birds in a short period of time. Excessive use of in-feed antibiotics has been shown in studies to increase the chance of resistance to human infections. Bacterial resistance and antibiotic residues in animal products raised concerns about using antibiotics as growth promoters, eventually leading to a prohibition on using in-feed antibiotics in most industrialized nations. In ovo technology is a novel method for delivering bioactive chemicals to developing avian embryos. In ovo feeding technologies may provide additional nutrients to the embryos before hatching. The introduction of bioactive compounds has the potential to assist in decreasing and eventually eliminating the problems associated with traditional antibiotic delivery in chicken production. Phytobiotics were advocated as an alternative by researchers and dietitians. So far, several studies have been conducted on the use of phytogenic feed additives in poultry and swine feeding. They have primarily demonstrated that phytobiotics possess antibacterial, antioxidant, anti-inflammatory, and growth-stimulating properties. The antioxidant effect of phytobiotics can improve the stability of animal feed and increase the quality and storage duration of animal products. In general, the existing documentation indicates that phytobiotics improve poultry performance. To effectively and efficiently use the in ovo technique in poultry production and advance research in this area, it is important to have a thorough understanding of its potential as a means of nutrient delivery during the critical stage of incubation, its effects on hatching events and posthatch performance, and the challenges associated with its use. Overall, this review suggests that in ovo feeding of phytobiotics has the potential to improve the antioxidant status and performance of chickens.

Vaccines against Major Poultry Viral Diseases: Strategies to Improve the Breadth and Protective Efficacy

The poultry industry is the largest source of meat and eggs for human consumption worldwide. However, viral outbreaks in farmed stock are a common occurrence and a major source of concern for the industry. Mortality and morbidity resulting from an outbreak can cause significant economic losses with subsequent detrimental impacts on the global food supply chain. Mass vaccination is one of the main strategies for controlling and preventing viral infection in poultry. The development of broadly protective vaccines against avian viral diseases will alleviate selection pressure on field virus strains and simplify vaccination regimens for commercial farms with overall savings in husbandry costs. With the increasing number of emerging and re-emerging viral infectious diseases in the poultry industry, there is an urgent need to understand the strategies for broadening the protective efficacy of the vaccines against distinct viral strains. The current review provides an overview of viral vaccines and vaccination regimens available for common avian viral infections, and strategies for developing safer and more efficacious viral vaccines for poultry.

Electromagnetic Force-Driven Needle-Free in Ovo Injection Device

Needle-free injections are mainly used for administering human or mammalian vaccines or drugs. However, poultry vaccines, in ovo injections to embryos, subcutaneous injections to chickens, and intramuscular injections are administered using needle injections. This article presents a new needle-free in ovo injection device method that uses push-pull solenoids to eject liquid jets, mainly for embryonic eggs of chickens. Furthermore, our study investigated the suitable jet pressures for using this method and the post-injection hatching rates in 18-day-old embryonic eggs. Using this method, we could deliver the liquid to the allantoic and amniotic cavities or the muscle tissue through the egg membrane of the air chamber using a jet pressure of ~6–7 MPa or ~8 MPa. After injecting 0.25 mL of 0.9% saline into 18-day-old Lohmann breed layer embryonic eggs and specific pathogen-free (SPF) embryonic eggs at a jet pressure of ~7 MPa, we observed hatching rates of 98.3% and 85.7%, respectively. This study’s electromagnetic needle-free in ovo injection device can apply vaccine or nutrient solution injection for embryo eggs and serve as a reference for future studies on needle-free in ovo injection automation systems, jet pressure control, and injection pretreatment processes.

A Cell Line Adapted Infectious Laryngotracheitis Virus Strain (BΔORFC) for in ovo and Hatchery Spray Vaccination Alone or in Combination with a Recombinant HVT-LT Vaccine

To produce more-stable, live attenuated vaccines for infectious laryngotracheitis virus (ILTV), deletion of genes related to virulence has been extensively pursued. Although its function remains unknown, the open reading frame C (ORF C) is among the genes potentially associated with viral virulence that is nonessential for replication in vitro. Earlier results indicated that the ILT virus with deletion of the ORF C gene (BΔORFC) was suitable and safe for eye drop administration but was not sufficiently attenuated for in ovo administration. The objective of this study was to evaluate the safety and protection efficacy of a cell line-adapted, gene-deleted strain (BΔORFC) of ILTV when administered in ovo and/or spray (SP) by itself, or in combination with the recombinant HVT-LT (rHVT-LT) vaccine. Results indicated that vaccination with the BΔORFC strain, either by itself or in combination with an rHVT-LT vaccine, did not affect hatchability, and only marginal signs of respiratory distress were recorded for groups of chickens that received the BΔORFC strain via SP. The replication and seroconversion induced by the BΔORFC strain after in ovo and SP administration was very limited, whereas the replication of the rHVT-LT vaccine was delayed when combined with the BΔORFC strain in ovo. Compared to rHVT-LT or BΔORFC when administered alone, dual vaccination with rHVT-LT + BΔORFC was more effective in mitigating clinical signs of the disease and reducing challenge virus load in the trachea. To our knowledge, this study provides the first proof of concept that ILTV strains can be sufficiently attenuated for early vaccination in ovo or at hatch; also, this study documented the benefits of using a dual (recombinant and live attenuated) hatchery vaccination strategy for ILTV.

Review of Poultry Recombinant Vector Vaccines

The control of poultry diseases has relied heavily on the use of many live and inactivated vaccines. However, over the last 30 yr, recombinant DNA technology has been used to generate many novel poultry vaccines. Fowlpox virus and turkey herpesvirus are the two main vectors currently used to construct recombinant vaccines for poultry. With the use of these two vectors, more than 15 recombinant viral vector vaccines against Newcastle disease, infectious laryngotracheitis, infectious bursal disease, avian influenza, and Mycoplasma gallisepticum have been developed and are commercially available. This review focuses on current knowledge about the safety and efficacy of recombinant viral vectored vaccines and the mechanisms by which they facilitate the control of multiple diseases. Additionally, the development of new recombinant vaccines with novel vectors will be briefly discussed.

Activation of Cytotoxic Lymphocytes and Presence of Regulatory T Cells in the Trachea of Non-Vaccinated and Vaccinated Chickens as a Recall to an Infectious Laryngotracheitis Virus (ILTV) Challenge

While the protective efficacy of the infectious laryngotracheitis virus (ILTV) vaccines is well established, little is known about which components of the immune response are associated with effective resistance and vaccine protection. Early studies have pointed to the importance of the T cell-mediated immune responses. This study aimed to evaluate the activation of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells and to quantify the presence of regulatory T cells (Tregs) in the larynx-trachea of chickens vaccinated with chicken embryo origin (CEO), tissue culture origin (TCO) and recombinant Herpesvirus of Turkey-laryngotracheitis (rHVT-LT) vaccines after challenge. Our results indicated that CEO vaccine protection was characterized by early CTLs and activated CTLs enhanced responses. TCO and rHVT-LT protection were associated with a moderate increase in resting and activated CTLs followed by an enhanced NK cell response. Tregs increase was only detected in the non-vaccinated challenged group, probably to support healing of the severe trachea epithelial damage. Taken together, our results revealed main differences in the cellular immune responses elicited by CEO, TCO, and rHVT-LT vaccination in the upper respiratory tract after challenge, and that activated CTLs rather than NK cells play a main role in vaccine protection.

Success rates of inoculation of the various compartments of embryonated chicken eggs at different incubation days

Inoculation of embryonated chicken eggs has been widely used during the past decades; however, inoculation success rates have not been investigated systematically. In this study named success rates were assessed in brown eggs incubated between 5 and 19 days, which were inoculated with 0.2 ml methylene blue per egg. Inoculations were performed in a simple and fully standardized way. Five embryonic compartments were targeted blindly (amniotic cavity, embryo, allantoic cavity, albumen and yolk) with needles of four different lengths; albumen and yolk were targeted with eggs in upside down position. Three compartments were inoculated within sight (air chamber, chorioallantoic membrane and blood vessel). Twenty embryos were used per incubation day, intended deposition site and needle length. Success rates were assessed by visual inspection after breaking the eggs. The inoculations targeting albumen, yolk, amniotic cavity and embryo yielded low scores. Magnetic resonance imaging was performed to elucidate the reason(s) for these low success rates: needles used were of appropriate length, but embryo and amniotic cavity had variable positions in the eggs, while albumen and yolk rapidly changed position after turning the eggs upside down. The latter led to adjustment of the inoculation method for albumen and yolk. Failures to inoculate compartments within sight were immediately visible; therefore, these eggs could be discarded. Except for the amniotic cavity, full scores (20/20) were obtained for all compartments although not always on every day of incubation. In conclusion, the present study may serve as a guide to more accurately inoculate the various chicken embryo compartments. RESEARCH HIGHLIGHTS Blind inoculation of embryonated egg compartments was successful, except for the amniotic cavity. MRI showed rapid position change of albumen and yolk after turning eggs upside down. In ovo vaccination against Marek's disease might be improved by using 38 mm needles.

Protection Efficacy of a Recombinant Herpesvirus of Turkey Vaccine Against Infectious Laryngotracheitis Virus Administered In Ovo to Broilers at Three Standardized Doses

Infectious laryngotracheitis (ILT) is a highly contagious respiratory disease of chickens that produces significant economic losses to the poultry industry. The disease is caused by Gallid alpha herpesvirus-1 (GaHV-1), commonly known as the infectious laryngotracheitis virus (ILTV). Vaccination remains necessary for the control of the disease. Due to the inherent virulence of live attenuated vaccines, in particular that of the chicken embryo origin (CEO) vaccines, the use of ILT viral vector recombinant vaccines has significantly expanded worldwide as a safer vaccination strategy. However, the protective efficacy of recombinant ILT vaccines can be compromised by the use of fractional doses and improper handling and administration of the vaccine. The objective of this study was twofold: 1) to evaluate the protection efficacy induced by a commercial recombinant HVT-LT (rHVT-LT) vaccine when administered in ovo to broilers at three standardized doses (6000 plaque-forming units [PFU], 3000 PFU, and 1000 PFU), and 2) to assess the potential of rHVT-LT-vaccinated chickens to spread virus to contact chickens after challenge. Independently of the vaccine dose, vaccinated chickens showed reduction in clinical signs, maintained body weight gain after challenge, and lessened the challenge virus replication in the trachea at a rate of 52%-65%. However, in spite of this reduction, transmission of challenge virus from rHVT-LT-vaccinated (6000/Ch, 3000/Ch) to contact-naive chickens was evident. This study is the first to support that rHVT-LT vaccination did not prevent spread of challenge virus to contact birds.

Effects of stage of broiler embryo development on coccidiosis vaccine injection accuracy, and subsequent oocyst localization and hatchling quality

Control of coccidiosis in broiler chickens continues to pose challenges to commercial poultry producers, especially in an era of increased consumer demand for antibiotic-free broiler production. As a result, coccidiosis vaccines are now commonly used in rotation programs to achieve effective coccidiosis control. Inovocox EM1 vaccine (EM1) is a coccidiosis vaccine that allows for earlier immune acquisition through oocyst cycling, which reduces the effects of wild-type coccidia. The EM1 vaccine is administered to embryonated broiler hatching eggs between 18 and 19 D of incubation (doi). In the U.S., commercial broiler hatcheries vaccinate embryonated eggs at either 18.5 or 19 doi. However, it is unclear whether a difference in embryo age at the time of in ovo injection can impact the actual site of vaccine delivery. In addition, it is unclear where oocysts eventually become localized within the embryo following the in ovo injection of EM1. Therefore, the objective of this study was to determine the effects of stage of embryonic development on the actual deposition site of the EM1 vaccine oocysts when they are in ovo injected and to subsequently investigate the movement and eventual location of EM1 oocysts after in ovo injection. Because all eggs were injected at the same time, a 12-h difference in set time was a means to derive 18.5 and 19.0 incubation age of injection (IAN) treatments. The experimental design was a 3 injection treatment (noninjected, diluent-injected, and vaccine-injected) × 2 IAN factorial. There was a significant main effect of IAN on site of vaccine oocysts delivery, and subsequent hatching chick quality. Qualitative histological evaluation revealed the oral uptake of vaccine oocysts through the amnion, with their subsequent presence in the gizzard and intestinal lumen by 24 to 36 h postinjection. In conclusion, physiological development influenced the site of injection, and oocysts imbibed along with the amniotic fluid in late stage broiler embryos are subsequently transported to the gastrointestinal tract.

In ovo delivery of various biological supplements, vaccines and drugs in poultry: current knowledge

The technique of delivering various nutrients, supplements, immunostimulants, vaccines, and drugs via the in ovo route is gaining wide attention among researchers worldwide for boosting production performance, immunity and safeguarding the health of poultry. It involves direct administration of the nutrients and biologics into poultry eggs during the incubation period and before the chicks hatch out. In ovo delivery of nutrients has been found to be more effective than post-hatch administration in poultry production. The supplementation of feed additives, nutrients, hormones, probiotics, prebiotics, or their combination via in ovo techniques has shown diverse advantages for poultry products, such as improved growth performance and feed conversion efficiency, optimum development of the gastrointestinal tract, enhancing carcass yield, decreased embryo mortality, and enhanced immunity of poultry. In ovo delivery of vaccination has yielded a better response against various poultry pathogens than vaccination after hatch. So, this review has aimed to provide an insight on in ovo technology and its potential applications in poultry production to deliver different nutrients, supplements, beneficial microbes, vaccines, and drugs directly into the developing embryo to achieve an improvement in post-hatch growth, immunity, and health of poultry. © 2019 Society of Chemical Industry.

In ovo applications in poultry: A review,

The various methods employed for the in ovo administration of different materials for promoting the health and productivity of poultry are discussed in this review article. The amnion has proven to be an effective site for injection and the timing of in ovo injection has commonly occurred at transfer. However, the volumes and dosages or concentrations of the materials administered vary depending on bird type, egg size, timing and site of injection, incubation system and regimen, and the type of material. Both manual and automated injections have been shown to be effective. Nevertheless, commercial application mandates automation. Materials described in the literature over the past 20 years or more for in ovo use in avian species include vaccines, drugs, hormones, competitive exclusion cultures and prebiotics, and supplemental nutrients. Vaccines approved for in ovo delivery include those for Marek's disease, infectious bursal disease, fowl pox, Newcastle disease, and coccidiosis. Some of the materials listed above have been shown to be viable candidates for enhancing immunity and for promoting embryonic and posthatch development. Several reports have indicated that probiotics may be effectively used to fight intestinal bacterial infections, and folic aid, as well as egg white protein and various amino acids, including L-arginine, L-lysine, L-histidine, HMB, and threonine alone or in combination, have been shown to benefit embryonic development or posthatch performance. Furthermore, CpG oligodeoxynucleotides, vitamins C and E, and thyme and savory have the potential to enhance immunity, carbohydrates can be used to increase tissue glycogen stores, and creatine can be used to promote muscle growth. Trace minerals and vitamin D3 have shown potential to improve bone strength, and potassium chloride may be an effective alternative electrolyte in vaccine diluent. The in ovo application of these and other materials will continue to expand and provide further benefits to the poultry industry.

Vaccines as alternatives to antibiotics for food producing animals. Part 2: new approaches and potential solutions

Vaccines and other alternative products are central to the future success of animal agriculture because they can help minimize the need for antibiotics by preventing and controlling infectious diseases in animal populations. To assess scientific advancements related to alternatives to antibiotics and provide actionable strategies to support their development, the United States Department of Agriculture, with support from the World Organisation for Animal Health, organized the second International Symposium on Alternatives to Antibiotics. It focused on six key areas: vaccines; microbial-derived products; non-nutritive phytochemicals; immune-related products; chemicals, enzymes, and innovative drugs; and regulatory pathways to enable the development and licensure of alternatives to antibiotics. This article, the second part in a two-part series, highlights new approaches and potential solutions for the development of vaccines as alternatives to antibiotics in food producing animals; opportunities, challenges and needs for the development of such vaccines are discussed in the first part of this series. As discussed in part 1 of this manuscript, many current vaccines fall short of ideal vaccines in one or more respects. Promising breakthroughs to overcome these limitations include new biotechnology techniques, new oral vaccine approaches, novel adjuvants, new delivery strategies based on bacterial spores, and live recombinant vectors; they also include new vaccination strategies in-ovo, and strategies that simultaneously protect against multiple pathogens. However, translating this research into commercial vaccines that effectively reduce the need for antibiotics will require close collaboration among stakeholders, for instance through public–private partnerships. Targeted research and development investments and concerted efforts by all affected are needed to realize the potential of vaccines to improve animal health, safeguard agricultural productivity, and reduce antibiotic consumption and resulting resistance risks.

Evaluation of a thermostable Newcastle disease virus strain TS09-C as an in-ovo vaccine for chickens

In-ovo vaccination is an attractive immunization approach for poultry industry. However, most of the Newcastle disease virus (NDV) vaccine strains used after hatch are unsafe, as in-ovo vaccines, due to their high pathogenicity for chicken embryos. In this study, we evaluated the safety and immunogenicity of a thermostable NDV strain TS09-C, derived from V4 strain, as in-ovo vaccine. Chickens in-ovo vaccinated with the parental V4 strain displayed greatly reduced hatchability and severe histopathological lesions in both trachea and intestine tissues, while the hatchability was not affected by in-ovo vaccination withTS09-C strain. The safe dose that infected all chicken embryos without obviously histopathological lesions was 10^3.0 EID₅₀ per bird. In-ovo vaccination of chickens with TS09-C virus conferred complete protection against virulent NDV challenge. Results suggest that the thermostable NDV strain TS09-C is a safe and immunogenic in-ovo vaccine candidate that can be delivered quickly and uniformly, and induce earlier immune response.

Safety and vaccine efficacy of a glycoprotein G deficient strain of infectious laryngotracheitis virus delivered in ovo

Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes respiratory disease in chickens and is commonly controlled by vaccination with conventionally attenuated vaccines. Glycoprotein G (gG) is a virulence factor in ILTV and a gG deficient strain of ILTV (ΔgG-ILTV) has shown potential for use as a vaccine. In the poultry industry vaccination via drinking water is common, but technology is now available to allow quicker and more accurate in ovo vaccination of embryos at 18 days of incubation. In this study ΔgG-ILTV was delivered to chicken embryos at three different doses (10(2), 10(3) and 10(4) plaque forming units per egg) using manual in ovo vaccination. At 20 days after hatching, birds were challenged intra-tracheally with wild type ILTV and protection was measured. In ovo vaccination was shown to be safe, as there were no developmental differences between birds from hatching up to 20 days of age, as measured by weight gain. The highest dose of vaccine was the most efficacious, resulting in a weight gain not significantly different from unvaccinated/unchallenged birds seven days after challenge. In contrast, birds vaccinated with the lowest dose showed weight gains not significantly different from unvaccinated/challenged birds. Gross pathology and histopathology of the trachea reflected these observations, with birds vaccinated with the highest dose having less severe lesions. However, qPCR results suggested the vaccine did not prevent the challenge virus replicating in the trachea. This study is the first to assess in ovo delivery of a live attenuated ILTV vaccine and shows that in ovo vaccination with ΔgG-ILTV can be both safe and efficacious.