Safety of classical swine fever virus vaccine strain LOM in pregnant sows and their offspring

Efficacy of an orally administered classical swine fever live marker vaccine (Flc-LOM-BErns strain) in pigs

In several countries, classical swine fever (CSF) has not been detected in domestic pigs, but has been detected in wild boars, making the disease difficult to control. To overcome this problem, we inoculated pigs with a CSF live marker vaccine (Flc-LOM-BErns strain), which has "distinguish infection from vaccinated animals (DIVA)" function, to determine whether it is suitable as an oral vaccine specifically for wild boars. Pigs inoculated intramuscularly or orally with the Flc-LOM-BErns vaccine were challenged 2 or 4 weeks later, respectively, with virulent CSFV. Pigs administered the oral Flc-LOM-BErns strain (105.0 and 6.0 TCID50/dose), and those vaccinated intramuscularly (103.0 TCID50/dose), had normal numbers of leukocytes and normal body temperature. Also, they generated protective neutralizing antibodies and anti-BVDV Erns antibodies. In addition, all pigs in these groups survived, with no CSFV RNA detected in feces, spleen, or other organs. Thus, the Flc-LOM-BErns vaccine shows excellent safety and efficacy, while having DIVA function and suitability for oral inoculation.

Safety and immunogenicity in piglets of two immunization schedules initiated at two or three weeks of age with PorvacÒ, a classical swine fever subunit marker vaccine

Classical swine fever is a highly contagious viral disease with a significant impact on food production worldwide. It currently represents one of the main limitations for the development of the pig industry in Cuba. PorvacÒ is a subunit marker vaccine that confers a very rapid onset of protection. Since there are different production systems in pig breeding, readjustments in the vaccination program are often required. This study compares the safety and efficacy in piglets of two vaccination schedules with PorvacÒ (0-2 weeks and 0-3 weeks), initiated at two or three weeks of age. Clinical monitoring was conducted, and a neutralization peroxidase-linked assay was used to measure the neutralization titers. All immunization regimens were safe and well-tolerated, without local or systemic adverse reactions in the vaccinated animals. Geometric mean neutralizing antibody titers higher than 1/1500 were detected in all groups during the six months of the trial. One month after the second immunization, piglets primed at two weeks of age, and boostered three weeks later, developed significantly higher neutralization titers (1/15644) compared to those vaccinated at a similar age but with a two-week interval between doses (1/5760). However, no significant differences in the titers were found three and six months after vaccination among the four regimens. In summary, all the variants studied are effective, but it is recommended to start vaccination at two weeks old, with the second dose at either two or three weeks later, depending on the production system and the purpose of the farm.

E2-CD154 vaccine candidate is safe and immunogenic in pregnant sows, and the maternal derived neutralizing antibodies protect piglets from classical swine fever virus challenge

E2-CD154 subunit vaccine candidate is safe and protects swine from Classical Swine Fever (CSF). However, its safety and immunogenicity in pregnant sows, and the capacity of maternal derived neutralizing antibodies (MDNA) to protect the offspring is yet to be demonstrated. The aim of this study was to evaluate the safety and immunogenicity of E2-CD154 in pregnant sows, and the capacity of MDNA to protect the offspring. Seventeen pregnant sows were vaccinated twice with E2-CD154 in either the first or the second third of pregnancy. Pregnancy and litter parameters were compared with a control group of non-vaccinated sows. Neutralizing antibodies (NAb) were monitored. The time course of MDNA was assessed in a group of six piglets born to an E2-CD154 immunized sow, and the animals were challenged with CSFV at day 63 after birth. No local or systemic adverse effects were found. Neither abortions, nor congenital malformations, nor stillbirths were observed. All sows develop high NAb titers after the first immunization. Piglets born to an E2-CD154 vaccinated sow still showed MDNA titers of 1:100 at day 63 after birth. Five animals were negative for virus isolation after challenge, and showed neither signs of CSF, nor macroscopic lesions in the organs. The other piglet was positive for CSFV isolation, and macroscopic lesions were observed in the spleen, although no clinical signs of CSF other than fever were detected. E2-CD154 vaccine candidate was safe and immunogenic in pregnant sows, and the passive immunity transmitted to the offspring was still protective by day 63 after birth.

Comparative Analysis of the Productivity and Immunogenicity of an Attenuated Classical Swine Fever Vaccine (LOM) and an Attenuated Live Marker Classical Swine Fever Vaccine (Flc-LOM-BErns) from Laboratory to Pig Farm

Herein, we compared the productivity of pigs inoculated with one of two classical swine fever (CSF) vaccines (low virulent of Miyagi (LOM) or Flc-LOM-BE^rns) plus the swine erysipelothrix rhusiopathiae (SE) vaccine. The feed intake and weight increase of the pigs inoculated with Flc-LOM-BE^rns + SE were normal. However, the feed intake of the pigs inoculated with LOM + SE dropped sharply from four days post-vaccination (dpv). In addition, the slaughter date was an average of eight days later than that of the pigs inoculated with Flc-LOM-BE^rns + SE. All pigs inoculated with the Flc-LOM-BE^rns + SE vaccine were completely differentiated at 14 days against CSF E^rns antibody and at approximately 45 days against the bovine viral diarrhea virus (BVDV) E^rns antibody; the titers were maintained until slaughter. Leucopenia occurred temporarily in the LOM + SE group, but not in the Flc-LOM-BE^rns + SE group. Expression of tumor necrosis factor (TNF)-α and IFN-γ was significantly (p < 0.05) higher in the LOM + SE group than in the mock (no vaccine) group. When conducting the same experiment on a breeding farm, the results were similar to those of the laboratory experiments. In conclusion, the biggest advantage of replacing the CSF LOM vaccine with the Flc-LOM-BE^rns vaccine is improved productivity.

Evaluation of classical swine fever E2 (CSF-E2) subunit vaccine efficacy in the prevention of virus transmission and impact of maternal derived antibody interference in field farm applications

Background

Classical swine fever (CSF) is one of the most devastating pig diseases that affect the swine industry worldwide. Besides stamping out policy for eradication, immunization with vaccines of live attenuated CSF or the CSF-E2 subunit is an efficacious measure of disease control. However, after decades of efforts, it is still hard to eliminate CSF from endemically affected regions and reemerging areas. Most of previous studies demonstrated the efficacy of different CSF vaccines in laboratories under high containment conditions, which may not represent the practical performance in field farms. The inadequate vaccine efficacy induced by unrestrained factors may lead to chronic or persistent CSF infection in animals that develop a major source for virus shedding among pig populations. In this study, a vaccination-challenge-cohabitation trial on specific-pathogen-free (SPF) pigs and long-term monitoring of conventional sows and their offspring were used to evaluate the efficacy and the impact of maternally derived antibody (MDA) interference on CSF vaccines in farm applications.

Results

The trials demonstrated higher neutralizing antibody (NA) titers with no clinical symptoms and significant pathological changes in the CSF-E2 subunit vaccine immunized group after CSFV challenge. Additionally, none of the sentinel pigs were infected during cohabitation indicating that the CSF-E2 subunit vaccine could provoke adequately acquired immunity to prevent horizontal transmission. In field farm applications, sows immunized with CSF-E2 subunit vaccine revealed an average of higher and consistent antibody level with significant reduction of CSF viral RNA detection via saliva monitoring in contrast to those of live attenuated CSF vaccine immunized sows possessing diverse antibody titer distributions and higher viral loads. Furthermore, early application of the CSF-E2 subunit vaccine in 3-week-old piglets illustrated no MDA interference on primary immunization and could elicit consistent and long-lasting adequate antibody response suggesting the flexibility of CSF-E2 subunit vaccine on vaccination program determination.

Conclusions

The CSF-E2 subunit vaccine demonstrated significant efficacy and no MDA interference for immunization in both pregnant sows and piglets. These advantages provide a novel approach to avoid possible virus shedding in sow population and MDA interference in piglets for control of CSF in field farm applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40813-020-00188-6.

A classical swine fever virus E2 fusion protein produced in plants elicits a neutralizing humoral immune response in mice and pigs

Classical swine fever (CSF) is one of the most important viral diseases of swine worldwide. Although live or attenuated virus vaccines have been used to control CSFV, it is difficult to distinguish vaccinated pigs from infected pigs; this leads to restrictions on import and export. Subunit vaccines based on the CSFV E2 glycoprotein have been developed using baculovirus or insect cell systems, but some weaknesses remain. Here, we describe production of an E2 recombinant protein using a Nicotiana benthamiana plant expression system. To do this, we took advantage of the ability of the swine Fc domain to increase solubility and stability of the fusion protein and to strengthen immune responses in target animals. N. benthamiana expressed high amounts of pFc2-fused E2 proteins, which were isolated and purified by affinity chromatography to yield a high pure recombinant protein in a cost-effective manner. Native-polyacrylamide gel electrophoresis and size exclusion chromatography confirmed that the pmE2:pFc2 fusion exists as a multimer rather than as a dimer. Injection of recombinant pmE2 protein into mice or piglets generated anti-pmE2 antibodies with efficient neutralizing activity against CSFV. These results suggest that a purified recombinant E2 protein produced in N. benthamiana generates high titers of neutralizing antibodies in vivo; as such, the protein could be developed as a subunit vaccine against CSFV.

Adverse Effects of Classical Swine Fever Virus LOM Vaccine and Jeju LOM Strains in Pregnant Sows and Specific Pathogen-Free Pigs

In Jeju island of South Korea, a classical swine fever (CSF) non-vaccinated region, many pig farmers insisted on abortion and stillbirth in pregnant sows and high mortality of suckling/weaning piglets by circulating CSF virus from 2014 to 2018. We investigated whether CSF viruses isolated from pigs in Jeju Island (Jeju LOM) have recovered their pathogenicity by conducting experiments using pregnant sows and specific pathogen-free (SPF) pigs. The CSF modified live LOM vaccine (MLV-LOM) and Jeju LOM strains induced abortion and stillbirth in pregnant sows. Viral antigens were detected in the organs of fetuses and stillborn piglets in the absence of specific pathological lesions associated with the virulent CSF virus in both groups (MLV-LOM and Jeju LOM strain). However, antigen was detected in one newborn piglet from a sow inoculated with a Jeju LOM strain, suggesting that it may cause persistent infections in pigs. SPF pigs inoculated with the MLV-LOM or Jeju LOM strains were asymptomatic, but virus antigen was detected in several organ and blood samples. Virus shedding in both groups of animals was not detected in the feces or saliva until 21 days post inoculation. The serum concentration of the three major cytokines, IFN-α, TNF-α, and IL-10, known to be related to lymphocytopenia, were similar in both groups when the MLV-LOM or Jeju LOM strains were inoculated into SPF pigs. In conclusion, Jeju LOM strains exhibited most of the characteristics of the MLV-LOM in pigs and resulted in the same adverse effects as the MLV-LOM strain.

Impact of a Live Attenuated Classical Swine Fever Virus Introduced to Jeju Island, a CSF-Free Area

Here, we examine the effects of LOM(Low virulence of Miyagi) strains isolated from pigs (Jeju LOM strains) of Jeju Island, where vaccination with a live attenuated classical swine fever (CSF) LOM vaccine strain was stopped. The circulation of the Jeju LOM strains was mainly caused by a commercial swine erysipelas (Erysipelothrix rhusiopathiae) vaccine mixed with a LOM vaccine strain, which was inoculated into pregnant sows of 20 pig farms in 2014. The Jeju LOM strain was transmitted to 91 pig farms from 2015 to 2018. A histopathogenic investigation was performed for 25 farms among 111 farms affected by the Jeju LOM strain and revealed pigs infected with the Jeju LOM strain in combination with other pathogens, which resulted in the abortion of fetuses and mortality in suckling piglets. Histopathologic examination and immunohistochemical staining identified CSF-like lesions. Our results also confirm that the main transmission factor for the Jeju LOM strain circulation is the vehicles entering/exiting farms and slaughterhouses. Probability estimates of transmission between cohabiting pigs and pigs harboring the Jeju LOM strain JJ16LOM-YJK08 revealed that immunocompromised pigs showed horizontal transmission (r = 1.22). In a full genome analysis, we did not find genetic mutation on the site that is known to relate to pathogenicity between Jeju LOM strains (2014–2018) and the commercial LOM vaccine strain. However, we were not able to determine whether the Jeju LOM strain (2014–2018) is genetically the same virus as those of the commercial LOM vaccine due to several genetic variations in structure and non-structure proteins. Therefore, further studies are needed to evaluate the pathogenicity of the Jeju LOM strain in pregnant sow and SPF pigs and to clarify the characteristics of Jeju LOM and commercial LOM vaccine strains.

A multiplex RT-PCR assay for rapid and simultaneous detection of four RNA viruses in swine

A multiplex reverse transcription polymerase chain rection (mRT-PCR) was developed for simultaneous detection of four RNA viruses in swine. The conserved target sequences directed to classical swine fever virus (CSFV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis coronavirus (TGEV) were selected based on alignments of genomic sequences and then specific primers were designed. The mRT-PCR assay was developed and evaluated for its specificity and sensitivity. The expected product from the single viral template was amplified by mRT-PCR and no spurious PCR amplification occurred from the genomic RNA or DNA of other pathogens. For single virus or different combinations of two viruses the detection limit of mRT-PCR was consistent with a single RT-PCR wtith 1 × 10³ copies. For different combinations of the three viruses or four viruses, sensitivity of PEDV detection partially decreased. All of positive clinical specimens by the mRT-PCR were identically confirmed using Taqman RT-qPCR. Therefore, the mRT-PCR is a useful tool for epidemiological studies and laboratory diagnosis of single virus and/or mixed infections in swine.

Detection of classical swine fever virus (CSFV) E2 and Erns antibody (IgG, IgA) in oral fluid specimens from inoculated (ALD strain) or vaccinated (LOM strain) pigs

The objective of this study was to describe oral fluid and serum antibody (IgG, IgA) responses against classical swine fever virus (CSFV) E2 and E^rns proteins in pigs (n = 60) inoculated with a moderately virulent field strain (ALD, n = 30) or a modified live virus vaccine strain (LOM, n = 30). Oral fluid (n = 1391) and serum (n = 591) samples were collected from individually-housed pigs between day post inoculation (DPI) -14 to 28. Testing revealed the synchronous appearance of E2- and E^rns-specific IgG and IgA antibodies in serum and oral fluids over time, with E2 and E^rns IgG ELISAs providing better diagnostic performance than the IgA ELISAs. Overall the data suggest the feasibility of large-scale, cost-effective screening of populations for CSFV using oral fluid samples. Given the historic issues of cross-reactivity among pestiviruses, future research should focus on the development of CSFV-specific testing platforms for the detection of E2 and/or E^rns IgG in oral fluid, ideally to be used in combination with DIVA vaccines.

Classical Swine Fever Outbreak after Modified Live LOM Strain Vaccination in Naive Pigs, South Korea

We report classical swine fever outbreaks occurring in naive pig herds on Jeju Island, South Korea, after the introduction of the LOM vaccine strain. Two isolates from sick pigs had >99% identity with the vaccine stain. LOM strain does not appear safe; its use in the vaccine should be reconsidered.

Corrigendum to "Efficacy of E2 glycoprotein fused to porcine CD154 as a novel chimeric subunit vaccine to prevent classical swine fever virus vertical transmission in pregnant sows"

Here we evaluated the effect of double vaccination with a novel subunit marker vaccine candidate based in the CSFV E2 glycoprotein fused to the porcine CD154 to prevent CSFV vertical transmission. A lentivirus-based gene delivery system was used to obtain a stable recombinant HEK 293 cell line for the expression of E2 fused to porcine CD154 molecule. Six pregnant sows were distributed in two groups and at 64days of gestation animals numbered 1-4 (group 1) were vaccinated via intramuscular inoculation with 50μg of E2-CD154 subunit vaccine. Animals from group 2 (numbered 5 and 6, control animals) were injected with PBS. Seventeen days later sows from group 1 were boosted with the same vaccine dose. Twenty-seven days after the first immunization, the sows were challenged with a virulent CSFV Margarita strain and clinical signs were registered. Samples were collected during the experiment and at necropsy to evaluate immune response and virological protection. Between 14 and 18days after challenge, the sows were euthanized, the foetuses were obtained and samples of sera and tissues were collected. E2-CD154 vaccinated animals remained clinically healthy until the end of the study; also, no adverse reaction was shown after vaccination. An effective boost effect in the neutralizing antibody response after the second immunization and viral challenge was observed and supports the virological protection detected in these animals after vaccination. Protection against CSFV vertical transmission was found in the 100% of serums samples from foetus of vaccinated sows. Only two out of 208 samples (0.96%) were positive with Ct value about 36 corresponding to one tonsil and one thymus, which may be non-infective viral particles. Besides, its DIVA potential and protection from vertical transmission, the novel CSFV E2 bound to CD154 subunit vaccine, is a promising alternative to the live-attenuated vaccine for developing countries.

Efficacy of E2 glycoprotein fused to porcine CD154 as a novel chimeric subunit vaccine to prevent classical swine fever virus vertical transmission in pregnant sows

Here we evaluated the effect of double vaccination with a novel subunit marker vaccine candidate based in the CSFV E2 glycoprotein fused to the porcine CD154 to prevent CSFV vertical transmission. A lentivirus-based gene delivery system was used to obtain a stable recombinant HEK 293 cell line for the expression of E2 fused to porcine CD154 molecule. Six pregnant sows were distributed in two groups and at 64days of gestation animals numbered 1-4 (group 1) were vaccinated via intramuscular inoculation with 50μg of E2-CD154 subunit vaccine. Animals from group 2 (numbered 5 and 6, control animals) were injected with PBS. Seventeen days later sows from group 1 were boosted with the same vaccine dose. Twenty-seven days after the first immunization, the sows were challenged with a virulent CSFV Margarita strain and clinical signs were registered. Samples were collected during the experiment and at necropsy to evaluate immune response and virological protection. Between 14 and 18days after challenge, the sows were euthanized, the foetuses were obtained and samples of sera and tissues were collected. E2-CD154 vaccinated animals remained clinically healthy until the end of the study; also, no adverse reaction was shown after vaccination. An effective boost effect in the neutralizing antibody response after the second immunization and viral challenge was observed and support the virological protection detected in these animals after vaccination. Protection against CSFV vertical transmission was found in the 100% of serums samples from foetus of vaccinated sows. Only two out of 208 samples (0.96%) were positive with Ct value about 36 corresponding to one tonsil and one thymus, which may be non-infective viral particles. Besides, its DIVA potential and protection from vertical transmission, the novel CSFV E2 bound to CD154 subunit vaccine, is a promising alternative to the live-attenuated vaccine for developing countries.

Classical swine fever vaccines-State-of-the-art

Due to its impact on animal health and pig industry, classical swine fever (CSF) is still one of the most important viral diseases of pigs. To control the disease, safe and highly efficacious live attenuated vaccines exist for decades. These vaccines have usually outstanding efficacy and safety but lack differentiability of infected from vaccinated animals (DIVA or marker strategy). In contrast, the first generation of E2 subunit marker vaccines shows constraints in efficacy, application, and production. To overcome these limitations, new generations of marker vaccines are developed. A wide range of approaches have been tried including recombinant vaccines, recombinant inactivated vaccines or subunit vaccines, vector vaccines, and DNA/RNA vaccines. During the last years, especially attenuated deletion vaccines or chimeric constructs have shown potential. At present, especially two new constructs have been intensively tested, the adenovirus-delivered, Semliki Forest virus replicon-vectored marker vaccine candidate "rAdV-SFV-E2" and the pestivirus chimera "CP7_E2alf". The later was recently licensed by the European Medicines Agency. Under field conditions, all marker vaccines have to be accompanied by a potent test system. Particularly this point shows still weaknesses and it is important to embed vaccination in a well-established vaccination strategy and a suitable diagnostic workflow. In summary, conventional vaccines are a standard in terms of efficacy. However, only vaccines with DIVA will allow improved eradication strategies e.g. also under emergency vaccination conditions in free regions. To answer this demand, new generations of marker vaccines have been developed and add now to the tool box of CSF control.