Assessment of efficacy of a bivalent BTV-2 and BTV-4 inactivated vaccine by vaccination and challenge in cattle

Co-expression of VP2, NS1 and NS2-Nt proteins by an MVA viral vector induces complete protection against bluetongue virus

Introduction

Bluetongue (BT), caused by bluetongue virus (BTV), is an important arthropod-borne livestock disease listed by the World Organization for Animal Health. Live-attenuated and inactivated vaccines have permitted to control BT but they do not simultaneously protect against the myriad of BTV serotypes. Recently, we identified the highly conserved BTV nonstructural protein NS1 and the N-terminal region of NS2 as antigens capable of conferring multiserotype protection against BTV.

Methods

Here, we designed Modified Vaccinia Ankara (MVA) viral vectors that expressed BTV-4 proteins VP2 or VP7 along with NS1 and NS2-Nt as well as MVAs that expressed proteins VP2, VP7 or NS1 and NS2-Nt.

Results

Immunization of IFNAR(-/-) mice with two doses of MVA-NS1-2A-NS2-Nt protected mice from BTV-4M infection by the induction of an antigen-specific T cell immune response. Despite rMVA expressing VP7 alone were not protective in the IFNAR(-/-) mouse model, inclusion of VP7 in the vaccine formulation amplified the cell-mediated response induced by NS1 and NS2-Nt. Expression of VP2 elicited protective non-cross-reactive neutralizing antibodies (nAbs) in immunized animals and improved the protection observed in the MVA-NS1-2A-NS2-Nt immunized mice when these three BTV antigens were co-expressed. Moreover, vaccines candidates co-expressing VP2 or VP7 along with NS1 and NS2-Nt provided multiserotype protection. We assessed protective efficacy of both vaccine candidates in sheep against virulent challenge with BTV-4M.

Discussion

Immunization with MVA-VP7-NS1-2A-NS2-Nt partially dumped viral replication and clinical disease whereas administration of MVA-VP2-NS1-2A-NS2-Nt promoted a complete protection, preventing viraemia and the pathology produced by BTV infection.

Pentavalent Disabled Infectious Single Animal (DISA)/DIVA Vaccine Provides Protection in Sheep and Cattle against Different Serotypes of Bluetongue Virus

Bluetongue (BT) is a midge-borne OIE-notifiable disease of ruminants caused by the bluetongue virus (BTV). There are at least 29 BTV serotypes as determined by serum neutralization tests and genetic analyses of genome segment 2 encoding serotype immunodominant VP2 protein. Large parts of the world are endemic for multiple serotypes. The most effective control measure of BT is vaccination. Conventionally live-attenuated and inactivated BT vaccines are available but have their specific pros and cons and are not DIVA compatible. The prototype Disabled Infectious Single Animal (DISA)/DIVA vaccine based on knockout of NS3/NS3a protein of live-attenuated BTV, shortly named DISA8, fulfills all criteria for modern veterinary vaccines of sheep. Recently, DISA8 with an internal in-frame deletion of 72 amino acid codons in NS3/NS3a showed a similar ideal vaccine profile in cattle. Here, the DISA/DIVA vaccine platform was applied for other serotypes, and pentavalent DISA/DIVA vaccine for “European” serotypes 1, 2, 3, 4, 8 was studied in sheep and cattle. Protection was demonstrated for two serotypes, and neutralization Ab titers indicate protection against other included serotypes. The DISA/DIVA vaccine platform is flexible in use and generates monovalent and multivalent DISA vaccines to combat specific field situations with respect to Bluetongue.

Safety and efficacy of a Bluetongue inactivated vaccine (serotypes 1 and 4) in sheep

A new inactivated vaccine against Bluetongue virus (BTV) serotypes 1 and 4, was developed from field isolates. Safety and efficacy of the vaccine were evaluated in sheep by serological monitoring and virus nucleic acid detection after experimental infection of vaccinated animals. Seroconversion was observed in vaccinated animals at day 14 post vaccination (pv) with neutralizing antibody titer of 1.9 and 1.8 for serotypes 1 and 4, respectively. The titer increase significantly after the booster reaching 2.7 and persist one year >1.5 for both serotypes. After challenge with virulent isolates, vireamia was recorded in control animals, as evident by q-PCR with threshold cycles (Ct) ranging from 24 to 31 and peaked at day 10 post challenge, while no vireamia was detected in vaccinated animals. Vaccinated sheep were fully protected against the disease and infection.

Development of an inactivated combined vaccine for protection of cattle against lumpy skin disease and bluetongue viruses

Lumpy Skin Disease (LSD) and Bluetongue (BT) are the main ruminants viral vector-borne diseases. LSD is endemic in Africa and has recently emerged in Europe and central Asia as a major threat to cattle industry. BT caused great economic damage in Europe during the last decade with a continuous spread to other countries. To control these diseases, vaccination is the only economically viable tool. For LSD, only live-attenuated vaccines (LAVs) are commercially available, whilst for BT both LAVs and inactivated vaccines are available with a limited number of serotypes. In this study, we developed an inactivated, oil adjuvanted bivalent vaccine against both diseases based on LSDV Neethling strain and BTV4. The vaccine was tested for safety and immunogenicity on cattle during a one-year period. Post-vaccination monitoring was carried out by VNT and ELISA. The vaccine was completely safe and elicited high neutralizing antibodies starting from the first week following the second injection up to one year. Furthermore, a significant correlation (R = 0.9040) was observed when comparing VNT and competitive ELISA in BTV4 serological response. Following BTV4 challenge, none of vaccinated and unvaccinated cattle were registered clinical signs, however vaccinated cattle showed full protection from viraemia. In summary, this study highlights the effectiveness of this combined vaccine as a promising solution for both LSD and BT control. It also puts an emphasis on the need for the development of other multivalent inactivated vaccines, which could be greatly beneficial for improving vaccination coverage in endemic countries and prophylaxis of vector-borne diseases.

Alternative methods to reduce the animal use in quality controls of inactivated BTV8 Bluetongue vaccines

The acceptance of serology data instead of challenge for market release of new batches of commercial vaccine is under evaluation by regulatory agencies in order to reduce the use of animals and costs for manufacturers. In this study two vaccines for Bluetongue virus serotype 8 were submitted to quality controls required by the European Pharmacopoeia and tested on sheep in comparison with a commercial inactivated vaccine. Body temperature, antibody titres and viraemia of vaccinated and controls sheep were recorded. In addition IL4 and IFNγ in sera and supernatant derived from in vitro stimulation of blood cells were also quantified using two commercial ELISA kit. The outer-capsid protein VP2 contained in vaccine formulations was quantified using a home-made capture-ELISA. Results obtained indicates that in-lab evaluation of cell-mediated and humoral immune response are useful parameters to predict the efficacy of BTV inactivated vaccines avoiding the challenge phase required to release new batches of vaccines with proven clinical efficacy and safety. The correlation observed between serology data and VP2 protein concentration of final product could be useful in-process control to predict if a new vaccine batch of BTV must be discarded or released to the market.

Comparative Evaluation of T-Cell Immune Response to BTV Infection in Sheep Vaccinated with Pentavalent BTV Vaccine When Compared to Un-Vaccinated Animals

Recent invasion of multiple bluetongue virus serotypes (BTV) in different regions of the world necessitates urgent development of efficient vaccine that is directed against multiple BTV serotypes. In this experimental study, cell mediated immune response and protective efficacy of binary ethylenimine (BEI) inactivated Montanide^™ ISA 206 adjuvanted pentavalent (BTV-1, 2, 10, 16 and 23) vaccine was evaluated in sheep and direct challenge with homologous BTV serotypes in their respective group. Significant (P < 0.05) up-regulation of mRNA transcripts of IFN-α, IL-2, IL-6, IL-12, IFN-γ and TNF-α in PBMCs of vaccinated animals as compared to control (un-vaccinated) animals at certain time points was observed. On the other hand, there was a significant increase in mean ± SD percentage of CD8⁺ T cells after 7 days post challenge (DPC) but, the mean ± SD percentage of CD4⁺ T-cell population slightly declined at 7 DPC and enhanced after 14 DPC. Significant differences (P < 0.05) of CD8⁺ and CD4⁺T cells population was also observed between vaccinated and unvaccinated sheep. The vaccine also significantly (P < 0.05) reduced BTV RNA load in PBMCs of vaccinated animals than unvaccinated animals following challenge. There were no significant difference (P > 0.05) in cytokine induction, BTV RNA load and CD8⁺ and CD4⁺cell count among BTV-1, 2, 10, 16 and 23 serotype challenges except significant increase in mean ± SD percentage of CD8⁺ in BTV-2 group. These findings put forwarded that binary ethylenimine inactivated montanide adjuvanted pentavalent bluetongue vaccine has stimulated cell mediated immune response and most importantly reduced the severity of BTV-1, 2, 10, 16 and 23 infections following challenge in respective group.

Beta-propiolactone inactivated bivalent bluetongue virus vaccine containing Montanide ISA-71VG adjuvant induces long-term immune response in sheep against serotypes 4 and 16 even after 3 years of controlled vaccine storage

In this study, we developed and evaluated the beta-propiolactone inactivated bivalent bluetongue virus (BTV) serotypes 4 and 16 vaccine delivered with Montanide™ ISA-71VG adjuvant. The safety, stability and immunological profile of the fresh and after three years of long-term storage of the vaccine formulation was analyzed. We observed after long-term storage that the vaccine emulsion was stable as indicated by unchanged pH and viscosity. The stored vaccine formulation induced virus neutralizing antibodies (VNA) in sheep against both the bluetongue virus serotypes at 7-10 day post-vaccination (dpv). VNA titers reached the peak by 60 dpv and detectable during the entire study period. Antibodies against bluetongue virus structural protein VP7 were detected by ELISA in all BTV vaccinated experimental animal groups. Partial clinical protection was observed in vaccinates against challenge virulent BTV-4 and BTV-16 serotypes by 10 dpv, while complete protection was observed at 14 dpv. The levels of viremia was decreased in challenged sheep by 10 dpv while the viremia was undetectable by 14 dpv. In summary, our newly formulated bivalent BTV (BTV-4 and BTV-16) vaccine delivered with Montanide™ ISA-71VG adjuvant was found safe and stable for over three years and induced protective response in sheep.

Estimation of French cattle herd immunity against bluetongue serotype 8 at the time of its re-emergence in 2015

BACKGROUND

From 2006 to 2010, France experienced two bluetongue epidemics caused by serotype 1 (BTV-1) and 8 (BTV-8) which were controlled by mass vaccination campaigns. After five years without any detected cases, a sick ram was confirmed in August 2015 to be infected by a BTV-8 strain almost identical to that circulating during the previous outbreak. By then, part of the French cattle population was expected to be still protected, since bluetongue antibodies are known to last for many years after natural infection or vaccination. The objective of this study was to estimate the proportion of cattle in France still immune to BTV-8 at the time of its re-emergence in 2015.

RESULTS

We used BTV group-specific cELISA results from 8525 cattle born before the vaccination ban in 2013 and 15,799 cattle born after the ban. Samples were collected from January to April 2016 to estimate seroprevalence per birth cohort. The overall seroprevalence in cattle at national and local levels was extrapolated from seroprevalence results per birth cohort and their respective proportion at each level. To indirectly assess pre-immune status of birth cohorts, we computed prevalence per birth cohort on infected farms in autumn 2015 using 1377 RT-PCR results. These revealed limited BTV circulation in 2015. Seroprevalence per birth cohort was likely to be connected to past exposure to natural infection and/or vaccination with higher seroprevalence levels in older animals. A seroprevalence of 95% was observed for animals born before 2008, of which > 90% were exposed to two compulsory vaccination campaigns in 2008-2010. None of the animals born before 2008 were found to be infected, unlike 19% of the young cattle which had never been vaccinated. This suggests that most ELISA-positive animals were pre-immune to BTV-8. We estimated that 18% (from 12% to 32% per département) of the French cattle population was probably pre-immune in 2015.

CONCLUSIONS

These results strongly suggest a persistence of antibodies for at least 5-6 years after natural infection or vaccination. The herd immunity of the French cattle population probably limited BTV circulation up to 2015, by which time more than 80% of cattle were naive.

Bluetongue: control, surveillance and safe movement of animals

The performance of different bluetongue control measures related to both vaccination and protection from bluetongue virus (BTV) vectors was assessed. By means of a mathematical model, it was concluded that when vaccination is applied on 95% of animals even for 3 years, bluetongue cannot be eradicated and is able to re‐emerge. Only after 5 years of vaccination, the infection may be close to the eradication levels. In the absence of vaccination, the disease can persist for several years, reaching an endemic condition with low level of prevalence of infection. Among the mechanisms for bluetongue persistence, the persistence in the wildlife, the transplacental transmission in the host, the duration of viraemia and the possible vertical transmission in vectors were assessed. The criteria of the current surveillance scheme in place in the EU for demonstration of the virus absence need revision, because it was highlighted that under the current surveillance policy bluetongue circulation might occur undetected. For the safe movement of animals, newborn ruminants from vaccinated mothers with neutralising antibodies can be considered protected against infection, although a protective titre threshold cannot be identified. The presence of colostral antibodies interferes with the vaccine immunisation in the newborn for more than 3 months after birth, whereas the minimum time after vaccination of animal to be considered immune can be up to 48 days. The knowledge about vectors ecology, mechanisms of over‐wintering and criteria for the seasonally vector‐free period was updated. Some Culicoides species are active throughout the year and an absolute vector‐free period may not exist at least in some areas in Europe. To date, there is no evidence that the use of insecticides and repellents reduce the transmission of BTV in the field, although this may reduce host/vector contact. By only using pour‐on insecticides, protection of animals is lower than the one provided by vector‐proof establishments.

This publication is linked to the following EFSA Supporting Publications article: http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1182/full, http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1171/full

Current and next-generation bluetongue vaccines: Requirements, strategies, and prospects for different field situations

Bluetongue virus (BTV) causes the hemorrhagic disease bluetongue (BT) in ruminants. The best way to control outbreaks is vaccination. Currently, conventionally modified-live and inactivated vaccines are commercially available, which have been successfully used to control BT, but nonetheless have their specific shortcomings. Therefore, there is a need for improved BT vaccines. The ideal BT vaccine is efficacious, safe, affordable, protective against multiple serotypes and enables the differentiation of infected from vaccinated animals. Different field situations require specific vaccine profiles. Single serotype outbreaks in former BT-free areas need rapid onset of protection against viremia of the respective serotype. In contrary, endemic multiple serotype situations require long-lasting protection against all circulating serotypes. The ideal BT vaccine for all field situations does not exist and balancing between vaccine properties is needed. Many new vaccines candidates, ranging from non-replicating subunits to replicating next-generation reverse genetics based vaccines, have been developed. Some have been tested extensively in large numbers of ruminants, whereas others were developed recently and have only been tested in vitro and in mice models. Most vaccine candidates are promising, but have their specific shortcomings and advantages. In this review, current and next-generation BT vaccines are discussed in the light of prerequisites for different field situations.

Comparative analysis of cellular immune responses and cytokine levels in sheep experimentally infected with bluetongue virus serotype 1 and 8

Protective immunity in sheep with bluetongue virus (BTV) infection as well as the role of BTV-induced cytokines during immune response remains unclear. Understanding the basis immunological mechanisms in sheep experimentally infected with serotypes 1 and 8 (BTV-1 and -8) was the aim of this study. A time-course study was carried out in order to evaluate cell-mediated immune response and serum concentrations of cytokines (IL-1β, TNFα, IL-12, IFNγ, IL-4 and IL-10) with inflammatory and immunological functions. Depletion of T cell subsets (mainly CD4(+), γδ and CD25(+)) together with the absence of cytokines (IFNγ and IL-12) involved in the regulation of cell-mediated antiviral immunity at the first stage of the disease suggested that both BTV-1 and BTV-8 might impair host's capability against primary infections which would favor viral replication and spreading. However, cellular immune response and cytokines elicited an immune response in sheep that efficiently reduced viremia in the final stage of the experiment. Recovery of T cell subsets (CD4(+) and CD25(+)) together with a significant increase of CD8(+) T lymphocytes in both infected groups were observed in parallel with the decrease of viremia. Additionally, the recovery of CD4(+) T lymphocytes together with the significant increase of IL-4 serum levels at the final stage of the experiment might contribute to humoral immune response activation and neutralizing antibodies production against BTV previously described in the course of this experiment. These results suggested that both cellular and humoral immune response may contribute to protective immunity against BTV-1 and BTV-8 in sheep. The possible role played by IL-10 and CD25(+) cells in controlling inflammatory and immune response in the final stage of the experiment has also been suggested.

Evaluation of adaptive immune responses and heterologous protection induced by inactivated bluetongue virus vaccines

Eradication of bluetongue virus is possible, as has been shown in several European countries. New serotypes have emerged, however, for which there are no specific commercial vaccines. This study addressed whether heterologous vaccines would help protect against 2 serotypes. Thirty-seven sheep were randomly allocated to 7 groups of 5 or 6 animals. Four groups were vaccinated with commercial vaccines against BTV strains 2, 4, and 9. A fifth positive control group was given a vaccine against BTV-8. The other 2 groups were unvaccinated controls. Sheep were then challenged by subcutaneous injection of either BTV-16 (2 groups) or BTV-8 (5 groups). Taken together, 24/25 sheep from the 4 experimental groups developed detectable antibodies against the vaccinated viruses. Furthermore, sheep that received heterologous vaccines showed significantly reduced viraemia and clinical scores for BTV-16 when compared to unvaccinated controls. Reductions in clinical signs and viraemia among heterologously vaccinated sheep were not as common after challenge with BTV-8. This study shows that heterologous protection can occur, but that it is difficult to predict if partial or complete protection will be achieved following inactivated-BTV vaccination.

Factors Affecting Seroconversion Rates in Cattle Vaccinated with Two Commercial Inactivated BTV-8 Vaccines Under Field Conditions

The immunogenicity of two inactivated bluetongue virus serotype 8 (BTV-8) vaccines was evaluated in 880 cattle under field conditions. The effect of selected factors on vaccine performance was also analysed at the herd and animal levels (vaccine, herd size and production, age, sex, time interval between vaccination and blood sampling and veterinary training). The immunogenicity elicited by vaccination with the two vaccines was monitored with the aid of a competitive enzyme-linked immunosorbent assay (c-ELISA) and serum neutralization test (SNT). To investigate whether the selected factors influenced seroconversion at the herd and animal levels, a multilevel logistic regression model developed in a mixed model was applied. Of the 880 cattle vaccinated, 76.0% yielded BTV c-ELISA antibodies, whereas only 25.0% seroconverted based on SNT. Type of vaccine (odds ratio [OR] 4.5; 95% confidence interval [CI], 2.2-9.0 for SNT and OR 3.5; 95% CI, 2.1-5.9 for c-ELISA), veterinary training in vaccine administration (OR 8.1; 95% CI, 4.7-14.1 for SNT and OR 2.4; 95% CI, 1.3-4.2 for c-ELISA), animal age (OR 1.4; 95% CI, 1.1-1.8 for SNT and OR 1.7; 95% CI, 1.4-2.1 for c-ELISA) and days between first vaccine administration and blood collection (OR 1.9; 95% CI, 1.1-3.1 for SNT and OR 2.6; 95% CI, 1.7-3.8 for c-ELISA) were the major factors affecting vaccine performance under field conditions. This is the first study to use multilevel logistic regression in the evaluation of selected risk factors affecting BTV-8 vaccine performance in cattle.

Vector independent transmission of the vector-borne bluetongue virus

Bluetongue is an economically important disease of ruminants. The causative agent, Bluetongue virus (BTV), is mainly transmitted by insect vectors. This review focuses on vector-free BTV transmission, and its epizootic and economic consequences. Vector-free transmission can either be vertical, from dam to fetus, or horizontal via direct contract. For several BTV-serotypes, vertical (transplacental) transmission has been described, resulting in severe congenital malformations. Transplacental transmission had been mainly associated with live vaccine strains. Yet, the European BTV-8 strain demonstrated a high incidence of transplacental transmission in natural circumstances. The relevance of transplacental transmission for the epizootiology is considered limited, especially in enzootic areas. However, transplacental transmission can have a substantial economic impact due to the loss of progeny. Inactivated vaccines have demonstrated to prevent transplacental transmission. Vector-free horizontal transmission has also been demonstrated. Since direct horizontal transmission requires close contact of animals, it is considered only relevant for within-farm spreading of BTV. The genetic determinants which enable vector-free transmission are present in virus strains circulating in the field. More research into the genetic changes which enable vector-free transmission is essential to better evaluate the risks associated with outbreaks of new BTV serotypes and to design more appropriate control measures.

Pulmonary artery haemorrhage in newborn calves following bluetongue virus serotype 8 experimental infections of pregnant heifers

The emergence of bluetongue disease (BT) among livestock in Europe in 2006 raised many questions including the occurrence and epidemiological significance of foetal infections in cattle. To clarify these aspects, vaccinated and unvaccinated pregnant heifers were sequentially infected twice in an isolation facility (biosafety level 3) with a northern European outbreak strain of Bluetongue virus serotype 8 (BTV-8). The study was terminated 2 months after calving with necropsy of the dams and their offspring. The cattle were monitored throughout the study by clinical scoring and for the presence of circulating neutralising antibodies, and after calving for the presence of infectious virus and viral RNA in blood and milk. Four calves, one born from a vaccinated dam and three from non-vaccinated ones, that were infected at 120 days of gestation had obvious haemorrhage of the pulmonary artery at necropsy. Although haemorrhage of the pulmonary artery is highly characteristic of BT, viral RNA was not detected in any of these calves. Furthermore, although none of the calves born from heifers infected prior to mid-gestation had teratogenic BTV typical brain lesions, some had lesions at birth suggestive of in utero BTV infection. Despite the lack of viral RNA detection, the presence of haemorrhage of the pulmonary artery deserves to be reported as a new observation in the context of the multiple investigations having as main subject the BTV placental crossing in cattle.

Rapid generation of replication-deficient monovalent and multivalent vaccines for bluetongue virus: protection against virulent virus challenge in cattle and sheep

Since 1998, 9 of the 26 serotypes of bluetongue virus (BTV) have spread throughout Europe, and serotype 8 has suddenly emerged in northern Europe, causing considerable economic losses, direct (mortality and morbidity) but also indirect, due to restriction in animal movements. Therefore, many new types of vaccines, particularly subunit vaccines, with improved safety and efficacy for a broad range of BTV serotypes are currently being developed by different laboratories. Here we exploited a reverse genetics-based replication-deficient BTV serotype 1 (BTV-1) (disabled infectious single cycle [DISC]) strain to generate a series of DISC vaccine strains. Cattle and sheep were vaccinated with these viruses either singly or in cocktail form as a multivalent vaccine candidate. All vaccinated animals were seroconverted and developed neutralizing antibody responses to their respective serotypes. After challenge with the virulent strains at 21 days postvaccination, vaccinated animals showed neither any clinical reaction nor viremia. Further, there was no interference with protection with a multivalent preparation of six distinct DISC viruses. These data indicate that a very-rapid-response vaccine could be developed based on which serotypes are circulating in the population at the time of an outbreak.

A one-year follow-up of antibody response in cattle and sheep after vaccination with serotype 8- and serotype 1-inactivated BT vaccines

Sixteen sheep and 18 cattle were followed up during 1 year to estimate the duration of immunity induced by inactivated bluetongue virus serotype 8 (BTV-8) vaccines (sheep and cattle) and a bluetongue virus serotype 1 (BTV-1) vaccine (cattle) under field conditions using cELISA and seroneutralization test (SNT). Four sheep never seroconverted. Those that seroconverted were all seronegative by BTV-8 SNT at the date of last sampling [378 days post-vaccination (dpv)]. Eight sheep were still positive by competitive ELISA (cELISA) 378 dpv. All the cattle seroconverted. At the end of the study, eight and 11 cattle were still positive by BTV-8 SNT and cELISA, respectively (335 dpv); and nine were still positive by BTV-1 SNT (301 dpv).

Detection of neutralizing antibodies against Bluetongue virus serotype 8 by an optimized plasma neutralization test

The neutralization test is used commonly for quantifying neutralizing antibodies and for distinguishing among different virus serotypes (serotyping). Due to the co-circulation of multiple serotypes of Bluetongue virus (BTV), the neutralization test has become an important surveillance method in Europe. However, the existence of different protocols makes test standardization and interpretation of results difficult. The current paper describes the development of a neutralization test using plasma and addresses the factors critical for detection of neutralizing antibodies against BTV serotype 8 (BTV-8), such as virus propagation, stability of virus infectivity and origin of the BTV-8 strain. The results indicated that animals exposed to the Northern European BTV-8 strain developed low neutralizing antibody titers, particularly after vaccination and experimental infection. Although clearly ELISA-positive, these samples often yielded false negative results when tested by the neutralization test using the OIE recommended virus concentration of 100 TCID₅₀/50 μl. The sensitivity of the neutralization test could be improved significantly with retained specificity by using a reduced TCID₅₀ and the homologous European BTV-8 strain instead of the South African reference strain.

Characterization of the immune response induced by a commercially available inactivated bluetongue virus serotype 1 vaccine in sheep

The protective immune response generated by a commercial monovalent inactivated vaccine against bluetongue virus serotype 1 (BTV1) was studied. Five sheep were vaccinated, boost-vaccinated, and then challenged against BTV1 ALG/2006. RT-PCR did not detect viremia at any time during the experiment. Except a temperature increase observed after the initial and boost vaccinations, no clinical signs or lesions were observed. A specific and protective antibody response checked by ELISA was induced after vaccination and boost vaccination. This specific antibody response was associated with a significant increase in B lymphocytes confirmed by flow cytometry, while significant increases were not observed in T lymphocyte subpopulations (CD4⁺, CD8⁺, and WC1⁺), CD25⁺ regulatory cells, or CD14⁺ monocytes. After challenge with BTV1, the antibody response was much higher than during the boost vaccination period, and it was associated with a significant increase in B lymphocytes, CD14⁺ monocytes, CD25⁺ regulatory cells, and CD8⁺ cytotoxic T lymphocytes.

Characterization of protection afforded by a bivalent virus-like particle vaccine against bluetongue virus serotypes 1 and 4 in sheep

Background

Bluetongue virus (BTV) is an economically important, arthropod borne, emerging pathogen in Europe, causing disease mainly in sheep and cattle. Routine vaccination for bluetongue would require the ability to distinguish between vaccinated and infected individuals (DIVA). Current vaccines are effective but are not DIVA. Virus-like particles (VLPs) are highly immunogenic structural mimics of virus particles, that only contain a subset of the proteins present in a natural infection. VLPs therefore offer the potential for the development of DIVA compatible bluetongue vaccines.

Methodology/Principal Findings

Merino sheep were vaccinated with either monovalent BTV-1 VLPs or a bivalent mixture of BTV-1 VLPs and BTV-4 VLPs, and challenged with virulent BTV-1 or BTV-4. Animals were monitored for clinical signs, antibody responses, and viral RNA. 19/20 animals vaccinated with BTV-1 VLPs either alone or in combination with BTV-4 VLPs developed neutralizing antibodies to BTV-1, and group specific antibodies to BTV VP7. The one animal that showed no detectable neutralizing antibodies, or group specific antibodies, had detectable viral RNA following challenge but did not display any clinical signs on challenge with virulent BTV-1. In contrast, all control animals' demonstrated classical clinical signs for bluetongue on challenge with the same virus. Six animals were vaccinated with bivalent vaccine and challenged with virulent BTV-4, two of these animals had detectable viral levels of viral RNA, and one of these showed clinical signs consistent with BTV infection and died.

Conclusions

There is good evidence that BTV-1 VLPs delivered as monovalent or bivalent immunogen protect from bluetongue disease on challenge with virulent BTV-1. However, it is possible that there is some interference in protective response for BTV-4 in the bivalent BTV-1 and BTV-4 VLP vaccine. This raises the question of whether all combinations of bivalent BTV vaccines are possible, or if immunodominance of particular serotypes could interfere with vaccine efficacy.

A side effect of decreased fertility associated with vaccination against bluetongue virus serotype 8 in Holstein dairy cows

Inactivated virus vaccines have been widely used to control bluetongue after introduction of serotype 8 of the bluetongue virus (BTV) in northern Europe in 2006. To evaluate vaccination, quantitative knowledge of its possible side effects is needed. One current adverse reaction with inactivated vaccines is a rise in body temperature, which could reduce cow reproductive performance. The objective of this study was to quantify a possible side effect of vaccination on fertility before the implantation of the embryo of dairy cows under field conditions. The study was performed on herds that were not exposed to BTV. Fertility was assessed by return-to-service following artificial insemination (AI). Biological assumptions for a possible side effect of vaccination were conception failure and embryonic death. Associations between return-to-service rates and vaccine injections were assessed using mixed-logistic regression models and survival analysis. Two models were considered: a 3-week-return-to-service model comparing cows vaccinated between 3 days before and 16 days after AI and unvaccinated cows (assuming an effect on conception failure or early embryonic death), and a 90-day-return-to-service model comparing cows vaccinated between 3 days before and 42 days after AI and unvaccinated cows (assuming an effect on conception failure, early or late embryonic death). Only cows receiving a second vaccine injection between 2 and 7 days after AI had a significantly higher risk of 3-week-return-to-service (RR=1.19 [1.07-1.33]). This corresponds to an increase of return-to-service by 4 percentage points. A side effect of vaccination could be due to early embryonic death. The slight side effect on fertility associated with vaccination was low compared to effects of BTV-8 exposure on fertility.

A comparison of intradermal and intravenous inoculation of bluetongue virus serotype 23 in sheep for clinico-pathology, and viral and immune responses

The pathogenesis of bluetongue (BT) could vary with route of inoculation. Using laboratory-passaged moderately virulent bluetongue virus serotype 23 (BTV-23), one of the most prevalent Indian serotype, we investigated the pathogenesis of BT in intradermally (ID) and intravenously (IV) inoculated native sheep. The ID inoculation resulted in relatively increased clinical signs and lesions in many organs as compared to IV inoculation. BTV-23 detection by real-time RT-PCR and isolation studies revealed that ID inoculation can be more efficient than IV ones in disseminating and spreading virus to systemic organs, including pre-scapular draining lymph node, spleen, lungs and pulmonary artery. Furthermore, the ID inoculation resulted in early onset and increased humoral response with significant increase (P<0.01) in antibody titre at various intervals. Taken together, these data suggest that ID inoculation can be more potent in reproducing many aspects of natural infection, including clinical disease, viral and immune responses, and may be useful route in setting up experimental infections for challenge or pathogenesis studies using laboratory passaged BTVs.

Evaluation of humoral response and protective efficacy of three inactivated vaccines against bluetongue virus serotype 8 one year after vaccination of sheep and cattle

The long-term efficacy of three commercially available inactivated vaccines against bluetongue virus serotype 8 (BTV-8) (BLUEVAC) 8, Zulvac 8, and BTVPUR AlSap 8) was evaluated in a seroprevalence study and challenge experiments. Seroprevalences 1 year after vaccination ranged from 75% to 100%. In two infection experiments, groups of vaccinated sheep and cattle selected either randomly or for low antibody levels were challenged with a European BTV-8 strain 12 months after vaccination. With two exceptions, all animals, including those with low antibody levels prior to challenge, were protected from viral replication and clinical disease even at low initial antibody levels. Vaccination of susceptible ruminants in yearly intervals is thus considered an adequate scheme for BTV-8 control in Europe.

Use of inactivated bluetongue virus serotype 8 vaccine against virulent challenge in sheep and cattle

The immunisation properties of an inactivated bluetongue virus serotype 8 (BTV-8) vaccine were evaluated in sheep and cattle. Five sheep were vaccinated with one dose of vaccine and five cattle were vaccinated with two doses 28 days apart. Six sheep and five cattle served as unvaccinated controls. All animals were subjected to a virulent BTV-8 challenge, and safety and antibody responses were monitored. All control animals developed disease and viraemia, while vaccinated animals were clinically protected and viraemia was completely prevented.

Emergence of bluetongue serotypes in Europe, part 1: description and validation of four real-time RT-PCR assays for the serotyping of bluetongue viruses BTV-1, BTV-6, BTV-8 and BTV-11

The control of bluetongue virus (BTV) in Central-Western Europe is greatly complicated by the coexistence of several BTV serotypes. Rapid, sensitive and specific assays are therefore needed to correctly identify the currently circulating BTV serotypes in field samples. In the present study, four serotype-specific real-time RT-PCR assays (RT-qPCR) are described for the detection of the BTV-1, BTV-6, BTV-8 and BTV-11 serotypes. The analytical sensitivity of the BTV-1/S2, BTV-6/S2, BTV-8/S2 and BTV-11/S2 serotype-specific RT-qPCR assays is comparable to the earlier described serogroup-specific pan-BTV/S5 RT-qPCR assay. In silico and in vitro analyses indicated that none of the assays cross-react with viruses which are symptomatically or genetically related to BTV and only detect the intended BTV serotypes. All assays exhibited a linear range of at least 0.05-3.80 log(10) TCID(50) ml(-1) and a PCR-efficiency approaching the ideal amplification factor of two per PCR cycle. Both intra- and inter-run variations were found to be low with a total coefficient of variation of 1-2% for clear positive samples and <10% for very weak positive samples. Finally, the performance of the described assays was compared with commercially available kits for the detection of BTV-1, BTV-6 and BTV-8. Three in-house assays gave exactly the same diagnostic result (positive/negative) as the commercial assays and can thus be used interchangeably. Together with the earlier described serogroup-specific pan-BTV/S5, the serotype-specific RT-qPCR assays form a flexible and properly validated set of tools to detect and differentiate the BTV serotypes currently circulating in Central-Western Europe.

Efficacy of three inactivated vaccines against bluetongue virus serotype 8 in sheep

Bluetongue has become a major animal health issue in the European Union. The member states and Switzerland have agreed on a vaccination strategy. Three different inactivated monovalent vaccines against bluetongue virus serotype 8 were selected for the compulsory vaccination program carried out in Germany in 2008. The efficacy of these vaccines was evaluated in a pilot study in sheep immunised under field conditions by clinical, virological and serological examination before and after experimental challenge infection with a BTV-8 field isolate. Antibody levels prior to challenge infection differed between the vaccinated groups, but all seroconverted animals were fully protected against clinical disease and virus replication. Only one vaccinated animal was very weakly positive in the real-time RT-PCR at day 10 after challenge infection, and one seronegative sheep in one of the vaccine groups was not protected.

Bluetongue vaccines: the past, present and future

Bluetongue (BT) is a noncontagious and arboviral disease of both domestic and wild ruminants. The disease is enzootic in areas where reservoirs (cattle and wild ruminants) and vectors exist for the BT virus (BTV). A total of 24 BTV serotypes have been recognized worldwide. The major control measures include restriction of animal movement, vector control applying insecticides, slaughter of infected animals and vaccination. Prophylactic immunization of sheep against BT is the most practical and effective control measure to combat BT infection. At present, attenuated vaccines are used in the Republic of South Africa, the USA and other countries. However, EU countries were using attenuated vaccines, only recently shifting to inactivated vaccines owing to their safety and efficacy. In India, inactivated vaccines are in experimental stages and are expected to be on the market shortly. Inactivated vaccines generate serotype-specific long-lasting protective immunity after two injections, and may help in controlling epidemics. Differentiating infected from vaccinated animals (DIVA) is theoretically possible with inactivated vaccines but has not yet been developed, whereas the attenuated live vaccines are not candidates for DIVA. Attenuated live vaccines are efficacious but safety issues are of great concern. New-generation vaccines (subunit, virus-like particles, core-like particles and vectored) can be employed for DIVA. Recombinant vaccines, which generate cross-protection against multiple BTV serotypes, have great potential in BT vaccine regimens. Furthermore, new-generation vaccines are safe and efficacious experimentally, but large-scale field trials are warranted. Alternative areas, such as antivirals, siRNA, interferon and nanotechnology, may be of future use in the control of BT. We give an overview of BT vaccines, starting from conventional to recent developments, and their feasibility in controlling BT infection.

Prospects for improved bluetongue vaccines

Bluetongue has been recognized as a viral disease of livestock for more than 100 years. Repeated incursions of Bluetongue into Europe since 1998 have been particularly devastating for highly sensitive European fine-wool sheep breeds, and have resulted in a resurgence of interest in vaccine manufacture. Fortunately, the virus and its serology are well understood and vaccination prevents the disease. However, current vaccines are not without their problems, and many new approaches are being tested to improve the safety and breadth of protection afforded. This review describes the leading technologies for improved bluetongue vaccines and looks ahead to how advances in other viral vaccines might be applied to this disease.