Evaluation of vaccination against infectious laryngotracheitis (ILT) with recombinant herpesvirus of turkey (rHVT-LT) and chicken embryo origin (CEO) vaccines applied alone or in combination

Spray vaccination with a Newcastle disease virus (NDV)-vectored infectious laryngotracheitis (ILT) vaccine protects commercial chickens from ILT in the presence of maternally-derived antibodies

Infectious laryngotracheitis (ILT) poses a significant threat to the poultry industry, and vaccines play an important role in protection. However, due to the increasing scale of poultry production, there is an urgent need to develop vaccines that are suitable for convenient immunization methods such as spraying. Previous studies have shown that Newcastle disease virus (NDV)-ILT vaccines administered via intranasal and intraocular routes to commercial chickens carrying maternally-derived antibodies (MDAs) are still protective against ILT. In this study, a recombinant NDV (rNDV) was generated to express infectious laryngotracheitis virus (ILTV) glycoprotein B (gB), named rLS-gB, based on a full-length cDNA clone of the LaSota strain. The protective effect of different doses of rLS-gB administered by spray vaccination to commercial chickens at 1 d of age (doa) was evaluated. The chickens were exposed to 160-μm aerosol particles for 10 min for spray vaccination, and no adverse reactions were observed after vaccination. Despite the presence of anti-NDV MDAs and anti-ILTV MDAs in chickens, the ILTV- and NDV-specific antibody titres were significantly greater in the vaccinated groups than in the unvaccinated group. After challenge with a virulent ILTV strain, no clinical signs were observed in the 107 EID50/ml group compared to the other groups. Furthermore, vaccination with 107 EID50/ml rLS-gB significantly reduced the ILTV viral load and ameliorated gross and microscopic lesions in the trachea of chickens. Overall, these results suggested that rLS-gB is a safe and efficient candidate spray vaccine for ILT and is especially suitable for scaled chicken farms.

Co-administration of chicken IL-2 alleviates clinical signs and replication of the ILTV chicken embryo origin vaccine by pre-activating natural killer cells and cytotoxic T lymphocytes

IMPORTANCE

Chickens immunized with the infectious laryngotracheitis chicken embryo origin (CEO) vaccine (Medivac, PT Medion Farma Jaya) experience adverse reactions, hindering its safety and effective use in poultry flocks. To improve the effect of the vaccine, we sought to find a strategy to alleviate the respiratory reactions associated with the vaccine. Here, we confirmed that co-administering the CEO vaccine with chIL-2 by oral delivery led to significant alleviation of the vaccine reactions in chickens after immunization. Furthermore, we found that the co-administration of chIL-2 with the CEO vaccine reduced the clinical signs of the CEO vaccine while enhancing natural killer cells and cytotoxic T lymphocyte response to decrease viral loads in their tissues, particularly in the trachea and conjunctiva. Importantly, we demonstrated that the chIL-2 treatment can ameliorate the replication of the CEO vaccine without compromising its effectiveness. This study provides new insights into further applications of chIL-2 and a promising strategy for alleviating the adverse reaction of vaccines.

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 Recombinant HVT-ND-LT and the Live Attenuated Tissue Culture Origin Vaccines Against Infectious Laryngotracheitis Virus When Administered Individually or in Combination

Infectious laryngotracheitis (ILT) is a respiratory disease that causes significant economic losses to the poultry industry. Control of the disease is achieved by vaccination and implementation of biosecurity measures. The use of bivalent and trivalent recombinant herpesvirus of turkey (rHVT) vaccines expressing infectious laryngotracheitis virus (ILTV) genes has increased worldwide. In the United States, vaccination programs of long-lived birds (broiler breeders and commercial layers) against ILT include immunizations with either HVT recombinant vector vaccines, in ovo or at hatch, or live attenuated vaccines administered via drinking water (chicken embryo origin [CEO]) or eye drop (tissue culture origin [TCO]). The efficacy of bivalent rHVT-LT at hatch followed by drinking water or eye-drop CEO vaccination has been shown to provide more robust protection than rHVT-LT alone. The objective of this study was to evaluate the protection efficacy of a commercial trivalent rHVT-ND-LT when administered at 1 day of age followed by TCO vaccination via eye drop at 10 wk of age. Groups vaccinated with only rHVT-ND-LT or TCO, the combination of rHVT-ND-LT + TCO, and one nonvaccinated group of chickens were challenged with a virulent ILTV strain at 15 wk of age. After challenge, mortalities were prevented only in the group of chickens vaccinated with the rHVT-ND-LT + TCO. Clinical signs of the disease and challenge virus replication in the trachea were significantly reduced for both the rHVT-ND-LT + TCO- and TCO-vaccinated groups of chickens. To assess challenge virus transmission, contact-naive chickens were introduced to all vaccinated groups immediately after challenge. At 8 days postintroduction, infection of contact-naive chickens was evidenced in those introduced to the rHVT-ND-LT and TCO group but prevented in the rHVT-ND-LT + TCO group. Overall, these results indicated that compared to rHVT-ND-LT or TCO when administered alone, the rHVT-ND-LT + TCO vaccination strategy improved protection against disease and reduced shedding of the challenge virus.

Characterization of a Recombinant Thermostable Newcastle Disease Virus (NDV) Expressing Glycoprotein gB of Infectious Laryngotracheitis Virus (ILTV) Protects Chickens against ILTV Challenge

Infectious laryngotracheitis (ILT) and Newcastle disease (ND) are two important avian diseases that have caused huge economic losses to the poultry industry worldwide. Newcastle disease virus (NDV) has been used as a vector in the development of vaccines and gene delivery. In the present study, we generated a thermostable recombinant NDV (rNDV) expressing the glycoprotein gB (gB) of infectious laryngotracheitis virus (ITLV) based on the full-length cDNA clone of the thermostable TS09-C strain. This thermostable rNDV, named rTS-gB, displayed similar thermostability, growth kinetics, and pathogenicity compared with the parental TS09-C virus. The immunization data showed that rTS-gB induced effective ILTV- and NDV-specific antibody responses and conferred immunization protection against ILTV challenge in chickens. The efficacy of rTS-gB in alleviating clinical signs was similar to that of the commercial attenuated ILTV K317 strain. Furthermore, rTS-gB could significantly reduce viral shedding in cloacal and tracheal samples. Our study suggested that the rNDV strain rTS-gB is a thermostable, safe, and highly efficient vaccine candidate against ILT and ND.

Effect of CpG-Oligonucleotide in Enhancing Recombinant Herpes Virus of Turkey-Laryngotracheitis Vaccine-Induced Immune Responses in One-Day-Old Broiler Chickens

Infectious laryngotracheitis (ILT) is an economically important disease of chickens. While the recombinant vaccines can reduce clinical disease severity, the associated drawbacks are poor immunogenicity and delayed onset of immunity. Here, we used CpG-oligonucleotides (ODN) as an in ovo adjuvant in boosting recombinant herpesvirus of turkey-laryngotracheitis (rHVT-LT) vaccine-induced responses in one-day-old broiler chickens. Two CpG-ODN doses (5 and 10 μg/egg) with no adverse effect on the vaccine-virus replication or chick hatchability were selected for immune-response evaluation. Results showed that while CpG-ODN adjuvantation induced an increased transcription of splenic IFNγ and IL-1β, and lung IFNγ genes, the IL-1β gene expression in the lung was significantly downregulated compared to the control. Additionally, the transcription of toll-like receptor (TLR)21 in the spleen and lung and inducible nitric oxide synthase (iNOS) in the spleen of all vaccinated groups was significantly reduced. Furthermore, splenic cellular immunophenotyping showed that the CpG-ODN-10μg adjuvanted vaccination induced a significantly higher number of macrophages, TCRγδ+, and CD4+ T cells as well as a higher frequency of activated T cells (CD4+CD44+) when compared to the control. Collectively, the findings suggested that CpG-ODN can boost rHVT-LT-induced immune responses in day-old chicks, which may help in anti-ILT defense during their later stages of life.

Diagnosis of Infectious Laryngotracheitis Outbreaks on Layer Hen and Broiler Breeder Farms in Vojvodina, Serbia

Infectious laryngotracheitis (ILT) is a respiratory disease of poultry characterized by high morbidity and variable mortality. ILT is caused by Gallid alpha herpesvirus-1 (GaHV-1), which is transmitted horizontally and most susceptible are chickens older than 4 weeks. After almost two decades since last appearance of this disease in Vojvodina, an outbreak occurred from April 2020 to August 2021 on five laying hen farms and one broiler breeder flock farm. Clinical signs were mild to severe respiratory symptoms, unilateral or bilateral head swelling, serous nasal discharge, conjunctivitis and increased tearing. There was a decrease in feed consumption (2.1-40.0%) and egg production (2.7-42.0%), weight loss and mortality increased (0.8-31.5%). Pathomorphological changes were localized in the upper respiratory tract. Total of 200 carcasses were examined; 40 pooled samples were analyzed by PCR, and 40 by bacteriological analysis. ILT virus was confirmed in tracheal tissue samples. Infected flocks were not vaccinated against this disease. Five flocks had coinfection with Avibacterium paragallinarum. Three-to-four weeks after the first reported case in the flock, clinical symptoms had ceased. Future control and prevention strategies will involve the procurement of flocks vaccinated by recombinant vaccine or the registration of live attenuated vaccines and their use during the rearing period.

Serological Evidence of Infectious Laryngotracheitis Infection and Associated Risk Factors in Chickens in Northwestern Ethiopia

Infectious laryngotracheitis (ILT) is a disease of high economic consequence to the poultry sector. Gallid herpesvirus 1 (GaHV-1), a.k.a infectious laryngotracheitis virus (ILTV), under the genus Iltovirus, and the family Herpesviridae, is the agent responsible for the disease. Despite the clinical signs on the field suggestive of ILT, it has long been considered nonexistent and a disease of no concern in Ethiopia. A cross-sectional study was conducted from November 2020 to June 2021 in three selected zones of the Amhara region (Central Gondar, South Gondar, and West Gojjam zones), Ethiopia, with the objective of estimating the seroprevalence of ILTV in chickens and identifying and quantifying associated risk factors. A total of 768 serum samples were collected using multistage cluster sampling and assayed for anti-ILTV antibodies using indirect ELISA. A questionnaire survey was used to identify the potential risk factors. Of the 768 samples, 454 (59.1%, 95% CI: 0.56–0.63) tested positive for anti-ILTV antibodies. Mixed-effect logistic regression analysis of potential risk factors showed that local breeds of chicken were less likely to be seropositive than exotic breeds (OR: 0.38, 95% CI: 0.24–0.61). In addition, factors such as using local feed source (OR: 6.53, 95% CI: 1.77–24.04), rearing chickens extensively (OR: 1.97, 95% CI: 0.78–5.02), mixing of different batches of chicken (OR: 14.51, 95% CI: 3.35–62.77), careless disposal of litter (OR: 1.62, 95% CI: 0.49–4.37), lack of house disinfection (OR: 11.05, 95% CI: 4.09–47.95), lack of farm protective footwear and clothing (OR: 20.85, 95% CI: 5.40–80.45), and careless disposal of dead chicken bodies had all been associated with increased seropositivity to ILTV. Therefore, implementation of biosecurity measures is highly recommended to control and prevent the spread of ILTV. Furthermore, molecular confirmation and characterization of the virus from ILT suggestive cases should be considered to justify the use of ILT vaccines.

Effect of Immune Stress on Growth Performance and Immune Functions of Livestock: Mechanisms and Prevention

Simple Summary

Immune stress is an important stressor in domestic animals that leads to decreased feed intake, slow growth, and reduced disease resistance of pigs and poultry. Especially in high-density animal feeding conditions, the risk factor of immune stress is extremely high, as they are easily harmed by pathogens, and frequent vaccinations are required to enhance the immunity function of the animals. This review mainly describes the causes, mechanisms of immune stress and its prevention and treatment measures. This provides a theoretical basis for further research and development of safe and efficient prevention and control measures for immune stress in animals.

Abstract

Immune stress markedly affects the immune function and growth performance of livestock, including poultry, resulting in financial loss to farmers. It can lead to decreased feed intake, reduced growth, and intestinal disorders. Studies have shown that pathogen-induced immune stress is mostly related to TLR4-related inflammatory signal pathway activation, excessive inflammatory cytokine release, oxidative stress, hormonal disorders, cell apoptosis, and intestinal microbial disorders. This paper reviews the occurrence of immune stress in livestock, its impact on immune function and growth performance, and strategies for immune stress prevention.

Evaluation of Recombinant Herpesvirus of Turkey Laryngotracheitis (rHVT-LT) Vaccine against Genotype VI Canadian Wild-Type Infectious Laryngotracheitis Virus (ILTV) Infection

In Alberta, infectious laryngotracheitis virus (ILTV) infection is endemic in backyard poultry flocks; however, outbreaks are only sporadically observed in commercial flocks. In addition to ILTV vaccine revertant strains, wild-type strains are among the most common causes of infectious laryngotracheitis (ILT). Given the surge in live attenuated vaccine-related outbreaks, the goal of this study was to assess the efficacy of a recombinant herpesvirus of turkey (rHVT-LT) vaccine against a genotype VI Canadian wild-type ILTV infection. One-day-old specific pathogen-free (SPF) White Leghorn chickens were vaccinated with the rHVT-LT vaccine or mock vaccinated. At three weeks of age, half of the vaccinated and the mock-vaccinated animals were challenged. Throughout the experiment, weights were recorded, and feather tips, cloacal and oropharyngeal swabs were collected for ILTV genome quantification. Blood was collected to isolate peripheral blood mononuclear cells (PBMC) and quantify CD4+ and CD8+ T cells. At 14 dpi, the chickens were euthanized, and respiratory tissues were collected to quantify genome loads and histological examination. Results showed that the vaccine failed to decrease the clinical signs at 6 days post-infection. However, it was able to significantly reduce ILTV shedding through the oropharyngeal route. Overall, rHVT-LT produced a partial protection against genotype VI ILTV infection.

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.

Assessing the infiltration of immune cells in the upper trachea mucosa after infectious laryngotracheitis virus (ILTV) vaccination and challenge

The types of immune cells that populate the trachea after ILTV vaccination and infection have not been assessed. The objective of this study was to quantify CD4⁺, CD8α⁺, CD8β⁺, TCRγδ⁺, and MRC1LB⁺ cells that infiltrate the trachea after vaccination with chicken embryo origin (CEO), tissue culture origin (TCO), and recombinant herpesvirus of turkey-laryngotracheitis (rHVT-LT) vaccines, and after challenge of vaccinated and non-vaccinated chickens with a virulent ILTV strain. Eye-drop vaccination with CEO, or TCO, or in ovo vaccination with rHVT-LT did not alter the number of CD4⁺, CD8α⁺, CD8β⁺, TCRγδ⁺, and MRC1LB⁺ cells in the trachea. After challenge, the CEO vaccinated group of chickens showed swift clearance of the challenge virus, the mucosa epithelium of the trachea remained intact, and a limited number of CD4⁺, CD8α⁺, and CD8β⁺ cells were detected in the upper trachea mucosa. The TCO and rHVT-LT vaccinated groups of chickens showed narrow viral clearance with moderate disruption of the trachea epithelial integrity, and a significant increase in CD4⁺, CD8α⁺, CD8β⁺, and TCRγδ⁺ cells infiltrated the upper trachea mucosa. Non-vaccinated challenged chickens showed high levels of viral replication, the epithelial organization of the upper trachea mucosa was heavily disrupted, and the predominant infiltrates were CD4⁺, TCRγδ⁺, and MRC1LB⁺ cells. Hence, the very robust protection provided by CEO vaccination was characterized by minimal immune cell infiltration to the trachea mucosa. In contrast, partial protection induced by the TCO and rHVT-LT vaccines requires a prolonged period of T cell expansion to overcome the established infection in the trachea mucosa.

Control, Suppression, and Monitoring of Infectious Laryngotracheitis in a Multiage Commercial Layer Pullet Farm in Canada

A multiage commercial layer pullet operation with a history of chicken embryo-origin (CEO) modified live infectious laryngotracheitis (ILT) virus vaccination suffered severe ILT outbreaks in 2017. The initial sequencing revealed that the circulating virus was of vaccine origin. Changes to the timing and dosage of CEO ILT vaccine failed to control the outbreak. The clinical resolution of the outbreak occurred with the transition to a turkey herpesvirus vector vaccine given in-hatchery, followed by a tissue culture-origin vaccine given on the farm. The circulating ILT viruses were monitored periodically by next-generation sequencing. This site became free of ILT virus within 1 yr after implementing the new vaccination program.

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.

Infectious laryngotracheitis: Etiology, epidemiology, pathobiology, and advances in diagnosis and control - a comprehensive review

Infectious laryngotracheitis (ILT) is a highly contagious upper respiratory tract disease of chicken caused by a Gallid herpesvirus 1 (GaHV-1) belonging to the genus Iltovirus, and subfamily Alphaherpesvirinae within Herpesviridae family. The disease is characterized by conjunctivitis, sinusitis, oculo-nasal discharge, respiratory distress, bloody mucus, swollen orbital sinuses, high morbidity, considerable mortality and decreased egg production. It is well established in highly dense poultry producing areas of the world due to characteristic latency and carrier status of the virus. Co-infections with other respiratory pathogens and environmental factors adversely affect the respiratory system and prolong the course of the disease. Latently infected chickens are the primary source of ILT virus (ILTV) outbreaks irrespective of vaccination. Apart from conventional diagnostic methods including isolation and identification of ILTV, serological detection, advanced biotechnological tools such as PCR, quantitative real-time PCR, next generation sequencing, and others are being used in accurate diagnosis and epidemiological studies of ILTV. Vaccination is followed with the use of conventional vaccines including modified live attenuated ILTV vaccines, and advanced recombinant vector vaccines expressing different ILTV glycoproteins, but still these candidates frequently fail to reduce challenge virus shedding. Some herbal components have proved to be beneficial in reducing the severity of the clinical disease. The present review discusses ILT with respect to its current status, virus characteristics, epidemiology, transmission, pathobiology, and advances in diagnosis, vaccination and control strategies to counter this important disease of poultry.

Comparison of Marek's disease virus challenge strains and bird types for vaccine licensing

Marek's disease virus (MDV) is an important poultry pathogen which is controlled through widespread vaccination with avirulent and attenuated strains, but continued evolution of field viruses to higher virulence has required ongoing improvement of available vaccine strains, and these vaccine strains also offer an attractive platform for designing recombinant vector vaccines with cross-protection against MDV and additional pathogens.  Recent reports of failures in vaccine licensing trials of positive controls to reach appropriately high levels of MD incidence prompted us to evaluate possible combinations of outbred specific pathogen-free (SPF) layer lines and alternative virulent challenge strains which could provide more consistent models for serotype-3 vectored vaccine development.  Choice of layer line and virulent MDV challenge strain each contributed to the ability of a challenge model to reach 80 percent virulence in unvaccinated positive control groups in the majority of trials without overwhelming serotype-3 vectored vaccine protection in vaccinated groups.  Conversely, reducing challenge virus dose by a factor of four, or vaccine dose by half, had no consistent effect across these models.  Although MDV strain 617A had the most potential as an alternative to strains that are currently approved for licensing trials, no combination of layer line and challenge virus consistently met the goals for a successful challenge model in all study replicates, indicating that high variability is an inherent difficulty in MDV challenge studies, at least when outbred birds are used.