Increased virulence of an infectious bursal disease live virus vaccine after passage in chicks

Field and laboratory findings following the large-scale use of intermediate type infectious bursal disease vaccines in Denmark

Following a period of clinical outbreaks of very virulent infectious bursal disease virus (vvIBDV) in Denmark, the histological bursal lesion score (HBLS) was used on a national scale to screen broiler flocks vaccinated with intermediate IBD vaccines for lesions indicative of IBDV challenge. High lesion scores were detected in a high percentage of healthy and well performing flocks despite the lack of other indications of the presence of vvIBDV. RT-PCR and subsequent sequencing showed the frequent presence of H253Q and H253N IBDV strains that were genetically close to the sequence of the intermediate vaccines with a relative risk ratio of 13.0 (P < 0.0001) in intermediate vaccine A or B vaccinated flocks compared to unvaccinated flocks. The relevance of these H253Q and H253N strains was tested under experimental conditions using a protocol derived from the European Pharmacopoeia for safety of live IBD vaccines. The results confirmed the higher pathogenicity for the bursa of these strains compared to intermediate vaccines as well as the negative effect on antibody response to a Newcastle disease (ND) vaccination performed at the peak of the bursa damage. The efficacy of the ND vaccination was still 100% showing that the H253N and H253Q IBDV strains would be considered as safe vaccine viruses. In conclusion, the use of the HBLS to screen commercial broiler flocks vaccinated with intermediate IBD vaccines for the presence of vvIBDV does not seem to be a reliable method due to the frequent occurrence of H253N and H253Q strains in those flocks. For screening of IBD vaccinated flocks for the presence of vvIBDV or other field strains, the RT-PCR with subsequent sequencing seems to be most suitable.

Protective efficacy of a novel recombinant Marek's disease virus vector vaccine against infectious bursal disease in chickens with or without maternal antibodies

Infectious bursal disease (IBD) causes significant clinical and economic losses to the poultry industry worldwide. Current vaccine programs using live attenuated and inactivated vaccines have numerous drawbacks. As an alternative solution to control IBD, a Marek's disease virus (MDV) vector vaccine (rMDV-VP2) expressing the VP2 gene of infectious bursal disease virus (IBDV) has been developed. In this study, the protective efficacy of rMDV-VP2 was evaluated in a dose-related experiment which showed that a single dose of 1000 PFU was sufficient to fully protect chickens against IBDV infection. Chickens inoculated with lower doses of rMDV-VP2 (250 or 500 PFU) conferred 80 and 90% protection against IBDV. Next, rMDV-VP2 vaccine provided 90% protection against IBDV in commercial layer chickens with maternal antibodies, which was higher than the protective efficacy using the B87 live vaccine of IBDV. Additionally, rMDV-VP2 conferred effective protection against very virulent MDV challenge in chickens (95% for chickens vaccinated with 250 or 500 PFU and 100% for chickens vaccinated with 1000 or 2000 PFU). These results demonstrated that rMDV-VP2 may be a novel bivalent vaccine against IBD and Marek's disease in chickens.

Recombinant Marek's disease virus type 1 provides full protection against very virulent Marek's and infectious bursal disease viruses in chickens

Marek’s disease virus (MDV) is a preferred vector in the construction of recombinant vaccines. However, bivalent vaccine based on MDV that confers full protection against both very virulent Marek’s and infectious bursal disease virus (IBDV) infections in chickens has not been produced. Here we developed a system utilizing overlapping fosmid DNAs transfection that rescues an MDV type 1 (MDV1) vaccine strain. Using this system, we inserted the IBDV VP2 gene at MDV1 genome sites UL41, US10 and US2. The VP2 protein was stably expressed in the recombinant MDV-infected cells and self-assembled into IBDV subviral particles. Insertion of the VP2 gene did not affect the replication phenotype of MDV in cell cultures, nor did it increase the virulence of the MDV vaccine strain in chickens. After challenge with very virulent IBDV, r814US2VP2 conferred full protection, whereas r814UL41VP2 and r814US10VP2 provided partial or no protection. All the three recombinant vaccines provided full protection against very virulent MDV challenge in chickens. These results demonstrated that r814US2VP2 could be used as a promising bivalent vaccine against both Marek’s and infectious bursal diseases in chickens.

Evaluation of two strains of Marek's disease virus serotype 1 for the development of recombinant vaccines against very virulent infectious bursal disease virus

Attenuated strains of Marek's disease virus serotype 1 (MDV1), and the closely related herpesvirus of turkeys, are among the most potent vectors for development of recombinant vaccines for poultry. To investigate the effects of MDV1 strain characteristics on the protective efficacy of the recombinant vaccines, we developed two recombinant MDV1 vaccines for expressing the VP2 gene of infectious bursal disease virus (IBDV) based on two different MDV1 strains, the attenuated strain 814 and the Meq gene-deleted recombinant MDV1 strain rLMS△Meq, as the viral vectors. The r814-VP2 virus based on the 814 strain exhibited higher replication efficiency in cell culture while lower viral titers in chickens, compared to rLMS△Meq-VP2 derived from the rLMS△Meq strain. Further studies indicated that r814-VP2 produced higher levels of VP2 protein in cells and elicited stronger immune responses against IBDV in chickens than rLMS△Meq-VP2. After IBDV challenge, rLMS△Meq-VP2 provided 50% protection against mortality, and the birds that survived developed bursal atrophy and gross lesions. In contrast, r814-VP2 conferred complete protection not only against development of clinical signs and mortality, but also against the formation of bursal lesions. The results indicate that different MDV1 vector influences the protective efficacy of recombinant MDV1 vaccines. The r814-VP2 has the potential to serve as a bivalent vaccine against two important lethal pathogens of chickens.

Effects of different promoters on the protective efficacy of recombinant Marek's disease virus type 1 expressing the VP2 gene of infectious bursal disease virus

The vaccine efficacy of recombinant viruses can be influenced by many factors. Accordingly, the activity of promoters has been one of the major factors affecting the antigen expression and protection rate. In the present study, two recombinant Marek's disease virus type 1 (MDV1) vaccines containing the VP2 gene of infectious bursal disease virus (IBDV) under control of different promoters were generated from overlapping fosmid DNAs. The rMDV-Pec-VP2 virus containing the VP2 gene under control of the Pec promoter (CMV enhancer and chicken β-actin chimera promoter) demonstrated higher VP2 expression and stronger antibody response against IBDV in chickens than the rMDV-CMV-VP2 virus using the CMV promoter. After IBDV lethal challenge in specific-pathogen-free chickens, rMDV-Pec-VP2 provided complete protection against developing mortality, clinical signs, and the formation of bursal lesions, which was better than that provided by rMDV-CMV-VP2. Our findings indicate that the protective efficacy of the recombinant MDV1 vaccine against IBDV highly correlates with VP2 expression. This recombinant MDV1 vaccine expressing VP2 could have significant potential as a bivalent vaccine against both virulent IBDV and MDV infections in chickens.

Farming of Plant-Based Veterinary Vaccines and Their Applications for Disease Prevention in Animals

Plants have been studied for the production of pharmaceutical compounds for more than two decades now. Ever since the plant-made poultry vaccine against Newcastle disease virus made a breakthrough and went all the way to obtain regulatory approval, research to use plants for expression and delivery of vaccine proteins for animals was intensified. Indeed, in view of the high production costs of veterinary vaccines, plants represent attractive biofactories and offer many promising advantages in the production of recombinant vaccine proteins. Furthermore, the possibility of conducting immunogenicity and challenge studies in target animals has greatly exaggerated the progress. Although there are no edible plant-produced animal vaccines in the market, plant-based vaccine technology has great potentials. In this review, development, uses, and advantages of plant-based recombinant protein production in various expression platforms are discussed. In addition, examples of plant-based veterinary vaccines showing strong indication in terms of efficacy in animal disease prevention are also described.

Toll-like receptor-based adjuvants: enhancing the immune response to vaccines against infectious diseases of chicken

Huge productivity loss due to infectious diseases in chickens is a major problem and, hence, robust development of the poultry industry requires control of poultry health. Immunization using vaccines is routine practice; however, to combat infectious diseases, conventional vaccines as well as new-generation recombinant vaccines alone, due to relatively weak immunogenicity, may not be effective enough to provide optimum immunity. With this in mind, there is a need to incorporate better and more suitable adjuvants in the vaccines to elicit the elevated immune response in the host. Over last few decades, with the increase in the knowledge of innate immune functioning, efforts have been made to enhance vaccine potency using novel adjuvants like Toll-like receptor based adjuvant systems. In this review, we will discuss the potential use of toll-like receptor ligands as an adjuvant in vaccines against the infectious diseases of chickens.

Safety, immunogenicity and duration of immunity elicited by an inactivated bovine ephemeral fever vaccine

Bovine ephemeral fever (BEF) is an economically important viral vector-borne cattle disease. Several live-attenuated, inactivated and recombinant vaccines have been tested, demonstrating varying efficacy. However, to the best of our knowledge, duration of immunity conferred by an inactivated vaccine has never been reported. In the last decade, Israel has faced an increasing number of BEF outbreaks. The need for an effective vaccine compatible with strains circulating in the Middle East region led to the development of a MONTANIDE™ ISA 206 VG (water-in-oil-in-water), inactivated vaccine based on a local strain. We tested the safety, immunogenicity and duration of immunity conferred by this vaccine. The induced neutralizing antibody (NA) response was followed for 493 days in 40 cows vaccinated by different protocols. The vaccine did not cause adverse reactions or a decrease in milk production. All cows [except 2 (6.7%) which did not respond to vaccination] showed a significant rise in NA titer of up to 1:256 following the second, third or fourth booster vaccination. Neutralizing antibody levels declined gradually to 1:16 up to 120 days post vaccination. This decline continued in cows vaccinated only twice, whereas cows vaccinated 3 or 4 times showed stable titers of approximately 1:16 for up to 267 days post vaccination. At least three vaccinations with the inactivated BEF vaccine were needed to confer long-lasting immunity. These results may have significant implications for the choice of vaccination protocol with inactivated BEF vaccines. Complementary challenge data should however be added to the above results in order to determine what is the minimal NA response conferring protection from clinical disease.

Codon optimization and woodchuck hepatitis virus posttranscriptional regulatory element enhance the immune responses of DNA vaccines against infectious bursal disease virus in chickens

The present study was undertaken to evaluate the protective efficacy of DNA vaccines against infectious bursal disease virus (IBDV) in chickens and to determine whether codon optimization and the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) could improve the immunogenicity of the DNA vaccines. The VP2, VP243 and codon-optimized VP243 genes of IBDV were cloned into pCAGGS vector, and designated as pCAGVP2, pCAGVP243 and pCAGoptiVP243, respectively. Plasmids pCAGWVP243 and pCAGWoptiVP243 carrying the WPRE elements were also constructed as DNA vaccines. To evaluate vaccine efficacy, 2-week-old chickens were injected intramuscularly with the constructed plasmids twice at 2-week intervals and challenged with very virulent IBDV 2 weeks post-boost. Plasmid pCAGVP243 induced better immune responses than pCAGVP2. Chickens immunized with pCAGoptiVP243 and pCAGWVP243 had higher levels of antibody titers, lymphoproliferation responses and cytokine production compared with pCAGVP243. Furthermore, plasmid pCAGWoptiVP243 induced the highest levels of immune responses among the groups. After challenged, DNA vaccines pCAGVP2, pCAGVP243, pCAGoptiVP243, pCAGWVP243 and pCAGWoptiVP243 conferred protection for 33%, 60%, 80%, 87% and 100% of chickens, respectively, as evidenced by the absence of clinical signs, mortality, and bursal atrophy. These results indicate that codon optimization and WPRE could enhance the protective efficacy of DNA vaccines against IBDV and these two approaches could work together synergistically in a single DNA vaccine.

A review of vaccine development and research for industry animals in Korea

Vaccination has proven to be the most cost-effective strategy for controlling a wide variety of infectious diseases in humans and animals. For the last decade, veterinary vaccines have been substantially developed and demonstrated their effectiveness against many diseases. Nevertheless, new vaccines are greatly demanded to effectively control newly- and re-emerging pathogens in livestock. However, development of veterinary vaccines is a challenging task, in part, due to a variety of pathogens, hosts, and the uniqueness of host-susceptibility to each pathogen. Therefore, novel concepts of vaccines should be explored to overcome the limitation of conventional vaccines. There have been greatly advanced in the completion of genomic sequencing of pathogens, the application of comparative genomic and transcriptome analysis. This would facilitate to open opportunities up to investigate a new generation of vaccines; recombinant subunit vaccine, virus-like particle, DNA vaccine, and vector-vehicle vaccine. Currently, such types of vaccines are being actively explored against various livestock diseases, affording numerous advantages over conventional vaccines, including ease of production, immunogenicity, safety, and multivalency in a single shot. In this articles, the authors present the current status of the development of veterinary vaccines at large as well as research activities conducted in Korea.

Evolution of host resistance and trade-offs between virulence and transmission potential in an obligately killing parasite

Standard epidemiological theory predicts that parasites, which continuously release propagules during infection, face a trade-off between virulence and transmission. However, little is known how host resistance and parasite virulence change during coevolution with obligate killers. To address this question we have set up a coevolution experiment evolving Nosema whitei on eight distinct lines of Tribolium castaneum. After 11 generations we conducted a time-shift experiment infecting both the coevolved and the replicate control host lines with the original parasite source, and coevolved parasites from generation 8 and 11. We found higher survival in the coevolved host lines than in the matching control lines. In the parasite populations, virulence measured as host mortality decreased during coevolution, while sporeload stayed constant. Both patterns are compatible with adaptive evolution by selection for resistance in the host and by trade-offs between virulence and transmission potential in the parasite.

Attenuation of very virulent infectious bursal disease virus and comparison of full sequences of virulent and attenuated strains

A very virulent strain of infectious bursal disease virus (IBDVks) was isolated from the bursae of Fabricius of IBDV-affected broiler chickens. Following 43 serial passages in specific pathogen-free embryonated eggs, an attenuated strain was established (IBDVmb). Dosages of IBDVmb in the range 10(2) to 10(4) embryo infective dose of 50% were found to be safe and protective for commercial chicks. Chickens vaccinated with live vaccine containing IBDVmb responded with precipitating and type-specific neutralizing antibodies, and were immune to subsequent challenge with a very virulent IBDV. IBDVmb has been used as an attenuated vaccine throughout the world since 1993. A comparison of the full sequences of the virulent and attenuated strains (IBDVks and IBDVmb, respectively) revealed seven nucleotides that were different, four of them leading to changes in the amino-acid sequence. Comparison of the protein sequence of these strains and published sequences of very virulent and attenuated phenotypes lead us to suggest that the novel difference responsible for virulence of the Israeli strains are: residue 272 (VP2, very conserved site) and residue 527 (VP4), both in segment A, and in segment B (VP1) residues 96 and 161 (both conserved). Our study strengthens the possibility that more than one protein is involved in IBDV attenuation. In all reports, including ours, virulence was reduced without affecting antigenicity of the neutralizing epitopes in VP2. This could have practical implications for attenuated-vaccine development.

Efficacy of DNA vaccines against infectious bursal disease virus in chickens enhanced by coadministration with CpG oligodeoxynucleotide

The objective of the present study was to investigate the feasibility of a DNA vaccine and CpG oligodeoxynucleotide (ODN) to protect chickens against infectious bursal disease virus (IBDV) infection. Two plasmids DNA carrying VP2 genes of the very virulent (vv) strain of IBDV were constructed with reverse transcription-polymerase chain reaction and designated as pcDNA3.1-VP2 and pCI-VP2. The VP2 protein expressed in COS-7 cells transfected with the plasmid was confirmed by indirect immunofluorescence assay. Seven-day-old chickens were intramuscularly injected with the plasmids alone or plus commercial attenuated infectious bursal disease (IBD) vaccine or synthetic CpG ODN twice at weekly intervals. Chickens at 5 wk old were orally inoculated with vvIBDV strain 99J1 and observed for 7 days after challenge. Immunization with plasmid plus commercial attenuated IBD vaccine or CpG ODN conferred protection for 70%-80% of chickens, as evidenced by the absence of dinical signs, mortality, and atrophy in the cloacal bursa. About 25%-45% of chickens vaccinated with commercial attenuated IBD vaccine or pcDNA3.1-VP2 or pCI-VP2 plasmid alone had dinical signs and died after challenge. Furthermore, there were significantly different histopathologic lesion scores in the clocal bursae between the pcDNA3.1-VP2 or pCI-VP2 plus CpG or live vaccine and pcDNA3.1-VP2, pCI-VP2, or live vaccine vaccinated group. Enzyme-linked immunosorbent assay antibody titers in chickens vaccinated the constructs DNA plus live vaccine or CpG ODN were significantly higher than in those inoculated with the constructs or the live vaccine alone. These results suggest that coadministration of the constructed plasmid pcDNA3.1-VP2 or pCI-VP2 with CpG ODN or commercial attenuated IBD vaccine could protect chickens efficiently from direct vvIBDV challenge.

DNA-mediated vaccination against infectious bursal disease in chickens

The objective of the present study was to investigate the feasibility of a DNA vaccine to protect chickens against infectious bursal disease virus (IBDV) infection. A plasmid DNA carrying VP2, VP4, and VP3 genes of the standard challenge (STC) strain of IBDV was constructed and designated as pCR3.1-VP243-STC. One-day-old chickens were intramuscularly injected with the plasmid pCR3.1-VP243-STC once (group D1), twice (group D2), or three times (group D3) at weekly intervals. Chickens at 3 weeks old were orally inoculated with IBDV strain STC and observed for 10 days after challenge. Immunization twice (group D2) or three times (group D3) with the plasmid pCR3.1-VP243-STC conferred protection for 50-100 or 80-100% of chickens, respectively, as evidenced by the absence of clinical signs, mortality, and bursal atrophy. Although chickens vaccinated once (group D1) with the plasmid pCR3.1-VP243-STC did not have clinical signs, they exhibited varying degree of bursal atrophy after challenge. Enzyme-linked immunosorbent assay (ELISA) antibody titers in chickens protected by the plasmid pCR3.1-VP243-STC were significantly lower (P<0.05) than those not protected 10 days after challenge. IBDV antigen was not detected in the bursae of chickens that were protected by receiving the plasmid pCR3.1-VP243-STC twice or three times. The results indicate that the constructed plasmid pCR3.1-VP243-STC as a DNA vaccine provided efficacious protection for chickens against IBDV infection.

Dual-viral vector approach induced strong and long-lasting protective immunity against very virulent infectious bursal disease virus

To induce strong protective immunity against very virulent infectious bursal disease virus (vvIBDV) in chickens, two viral vector systems, Marek's disease and Fowlpox viruses expressing the vvIBDV host-protective antigen VP2 (rMDV, rFPV), were used. Most of chickens vaccinated with the rFPV or rMDV alone, or vaccinated simultaneously with both at their hatch (rMDV-rFPV(1d)), were protected against developing clinical signs and mortality; however, only zero to 14% of the chickens were protected against gross lesions. In contrast, gross lesions were protected in 67% of chickens vaccinated primarily with the rMDV followed by boosting with the rFPV 2 weeks later (rMDV-rFPV(14d)). Protection against the severe histopathological lesions of rFPV, rMDV, rMDV-rFPV(1d), and rMDV-rFPV(14d) vaccine groups were 33, 42, 53, and 73%, respectively. Geometric mean antibody titers to VP2 of chickens vaccinated with the rFPV, rMDV, rMDV-rFPV(1d), and rMDV-rFPV(14d) before the challenge were 110, 202, 254, and 611, respectively. Persistent infection of the rMDV in chickens after the booster vaccination with rFPV was suggested by detection of the rMDV genes from peripheral blood lymphocyte DNA at 28 weeks of age. These results indicate that the dual-viral vector approach is useful for quickly and safely inducing strong and long-lasting protective immunity against vvIBDV in chickens.

Evolution of virulence in a heterogeneous host population

There is a large body of theoretical studies that investigate factors that affect the evolution of virulence, that is parasite-induced host mortality. In these studies the host population is assumed to be genetically homogeneous. However, many parasites have a broad range of host types they infect, and trade-offs between the parasite virulence in different host types may exist. The aim of this paper is to study the effect of host heterogeneity on the evolution of parasite virulence. By analyzing a simple model that describes the replication of different parasite strains in a population of two different host types, we determine the optimal level of virulence in both host types and find the conditions under which strains that specialize in one host type dominate the parasite population. Furthermore, we show that intrahost evolution of the parasite during an infection may lead to stable polymorphisms and could introduce evolutionary branching in the parasite population.

Protection of chickens against very virulent infectious bursal disease virus (IBDV) and Marek's disease virus (MDV) with a recombinant MDV expressing IBDV VP2

To develop a herpes virus vaccine that can induce immunity for an extended period, a recombinant Marek's disease (MD) virus (MDV) CVI-988 strain expressing infectious bursal disease virus (IBDV) host-protective antigen VP2 at the US2 site (rMDV) was developed under the control of an SV40 early promoter. Chickens vaccinated with the rMDV showed no clinical signs and no mortality and 55% of the chickens were considered protected histopathologically after challenge with very virulent IBDV (vvIBDV), whereas all of the chickens vaccinated with the conventional IBDV vaccine showed no clinical signs and were protected. Chickens vaccinated with the CVI-988 or chickens in the challenge control showed severe clinical signs and high mortality (70-75%) and none of them were protected. Also, the rMDV conferred full protection to chickens against vvMDV just as the CVI-988 strain did, whereas 90% of the challenge control chickens died of MD. Antibody levels against IBDV and MDV following the vaccination increased continuously for at least 10 weeks. No histopathological lesions in the rMDV-vaccinated chickens and no contact transmission of the rMDV to their penmates were confirmed. These results demonstrate that an effective and safe recombinant herpesvirus-based IBD vaccine could be constructed by expressing the VP2 antigen at the US2 site of the CVI-988 vaccine strain.

Experimental evolution of parasites

Serial passage experiments are a form of experimental evolution that is frequently used in applied sciences; for example, in vaccine development. During these experiments, molecular and phenotypic evolution can be monitored in real time, providing insights into the causes and consequences of parasite evolution. Within-host competition generally drives an increase in a parasite's virulence in a new host, whereas the parasite becomes avirulent to its former host, indicating a trade-off between parasite fitnesses on different hosts. Understanding why parasite virulence seldom escalates similarly in natural populations could help us to manage virulence and deal with emerging diseases.

Pathogenicity and immunosuppressive properties of infectious bursal disease virus field isolates and commercial vaccines in India

The pathogenicity and immunosuppressive properties of two field isolates of infectious bursal disease. virus (IBDV) and five commercial IBDV live virus vaccines marketed in India were evaluated in this study. The pathogenicity of the wild type viruses and vaccines were based on mortality, the bursa:body weight ratio and microscopic lesions in the bursa in 3-week-old chicks that received these viruses. The immunosuppressive effects of these viruses were evaluated by measuring the antibody responses to sheep red blood cells, Brucella abortus plain antigen and Newcastle disease virus (NDV) vaccine in one-day-old chicks. One field isolate (N35/93) was found to be more pathogenic and immunosuppressive than the other (N45/92) while none of the commercial mild Lukert type vaccines were found to be pathogenic. One of the vaccine strains marked as Mild Lukert type was highly immunosuppressive; one was moderate and one could be classified as mild. Both the intermediate vaccines tested were highly immunosuppressive.