An octavalent vaccine provides pregnant gilts protection against a highly virulent porcine parvovirus strain

Isolation and characterization of porcine parvovirus in Vietnam

Background and Aim

No study has successfully isolated parvovirus in Vietnam. This study aimed to isolate and characterize parvovirus strains indigenous in Vietnam for vaccine development against porcine parvovirus (PPV).

Materials and Methods

We collected serum and stillbirth samples from six provinces in Vietnam, and PPV-positive samples were identified using a polymerase chain reaction. Parvovirus isolation was attempted using the PK-15 cells maintained in a minimum essential medium supplemented with 5% fetal bovine serum and 1% antibiotics (Penicillin-streptomycin). The cells were incubated at 37°C with 5% CO2. Virulence experiments were conducted on white primiparous sows to evaluate the virulence of the PPV strain through hemagglutination inhibition (HI) titers and fetus lesions.

Results

We analyzed 360 serum and 32 stillbirth (liver and lungs) samples, revealing that 32/392 (8.2% ) of them were PPV-positive, all belonging to PPV1. Thirty-two PPV-positive samples were successfully isolated, with 100% identity as VP2 sequences. The phylogenetic tree revealed a close relationship with the Kresse strain (isolated from Canada in 1996) and the PPV1-0225-L-SD strain (isolated from China in 2022). Two PPV isolates (VC5 from Dongnai and TX7 from Thanhhoa) that exhibited high 50% tissue culture infectious dose titers were selected for the virulence experiment. On day 21, after injection, the HI antibody titers ranged from 10log2 to 12log2. On day 90, 71%-80% of fetuses were mummified.

Conclusion

This study showed that the PPV infection rate in Vietnam was 8.2%. Thirty-two isolates belonged to PPV1. Two PPV strains, VC5 and TX7, were determined to be highly virulent by the results of HI titers after injection into gilts. VC5 and TX7 were determined to be good candidates for further research on PPV vaccines.

Duration of maternally derived antibodies of porcine parvovirus in growing pigs and presence of antibodies in gilts and sows vaccinated with three different parvovirus vaccines

While gilts and sows are regularly vaccinated against the porcine parvovirus (PPV), little is known on the presence of antibodies in vaccinated sows nor the decline of maternally derived antibodies (MDA) in their offspring. On twelve farms serum samples were taken from 180 gilts and sows vaccinated at least twice with one of three different commercial PPV vaccines. On nine farms, additional 270 serum samples were collected from growing pigs of three different age categories. All 450 samples were examined for PPV antibodies (Abs) by ELISA and haemagglutination inhibition (HI) assay. In total, 65% of all gilts vaccinated twice with either vaccine 1 or vaccine 3 were seronegative by HI assay. In each farm, there were at least three animals with high Ab titres (≥ 1:1280) indicating the presence of PPV in all twelve study farms. However, PPV DNA could not be detected in collected faecal samples. While low to moderately high Ab titres (1:10-1:640) were measured in 98% of twelve-weeks-old pigs, ELISA was only positive in 30% of the same pigs. Though, the statement on the duration of MDA may depend on the applied test, we could confirm an exponential decay of MDA. In addition, we could demonstrate that applied serological tools are insufficient for the confirmation of successful vaccination.

A Subunit Vaccine Based on the VP2 Protein of Porcine Parvovirus 1 Induces a Strong Protective Effect in Pregnant Gilts

Porcine parvovirus 1 (PPV1) is one of the most prevalent pathogens that can cause reproductive disorder in sows. The VP2 protein of PPV1 is the most important immunogenic protein that induces neutralizing antibodies and protective immunity. Thus, VP2 is considered an ideal target antigen for the development of a genetically engineered PPV1 vaccine. In this study, the baculovirus transfer vector carrying the HR5-P10-VP2 expression cassette was successfully constructed with the aim of increasing the expression levels of the VP2 protein. The VP2 protein was confirmed using SDS‒PAGE and Western blot analyses. Electronic microscope analysis showed that the recombinant VP2 proteins were capable of self-assembling into VLPs with a diameter of approximately 25 nm. The immunogenicity of the VP2 subunit vaccine was evaluated in pigs. The results showed that VP2 protein emulsified with ISA 201VG adjuvant induced higher levels of HI antibodies and neutralizing antibodies than VP2 protein emulsified with IMS 1313VG adjuvant. Furthermore, the gilts immunized with the ISA 201VG 20 μg subunit vaccine acquired complete protection against PPV1 HN2019 infection. In contrast, the commercial inactivated vaccine provided incomplete protection in gilts. Therefore, the VP2 subunit vaccine is a promising genetically engineered vaccine for the prevention and control of PPV1.

Allele-Specific Dual PCRs to Identify Members of the 27a Cluster of PPV

Porcine Parvovirus (PPV) is one of the most important infectious agents causing severe reproductive failure in pigs. In the last two decades a particular, a novel genotype emerged in Europe and PPV-27a was named as the prototype of this genetic cluster. It was suggested that members of the PPV-27a cluster may adversely influence effective vaccination against PPV. For a reliable updated 27a definition, we aligned 93 databank-deposited partial or full nucleotide and protein sequences of the VP2 of different PPV isolates. We confirmed that the 27a cluster could indeed be distinguished from other members of the species, however, some divergences were identified compared to earlier defined genetic markers. Based on genetic differences, we developed a dual allele-specific polymerase chain reaction for the easy and quick discrimination of members of the 27a cluster from other PPV strains. The detection limit of dual PCR was found <1.66 × 104 copies/reaction. To sensitize and make it more user friendly, the method was further developed for qPCR application with fluorescent probes. Regarding the detection limit of the two PCRs (<1.66 × 104 copies/reaction of the dual PCR versus <2.40 × 102 copy/reaction of the dual qPCR), approximately two log improvement was achieved in the sensitivity of the method.

Molecular Epidemiology of Porcine Parvovirus Type 1 (PPV1) and the Reactivity of Vaccine-Induced Antisera Against Historical and Current PPV1 Strains

Porcine Parvovirus Type 1 (PPV1) contributes to important losses in the swine industry worldwide. During a PPV1 infection, embryos and fetuses are targeted, resulting in stillbirth, mummification, embryonic death, and infertility (SMEDI syndrome). Even though vaccination is common in gilts and sows, strains mainly belonging to the 27a-like group have been spreading in Europe since early 2000s, resulting in SMEDI problems and requiring in-depth studies into the molecular epidemiology and vaccination efficacy of commercial vaccines. Here, we show that PPV1 has evolved since 1855 [1737, 1933] at a rate of 4.71 × 10⁻⁵ nucleotide substitutions per site per year. Extensive sequencing allowed evaluating and reassessing the current PPV1 VP1-based classifications, providing evidence for the existence of four relevant phylogenetic groups. While most European strains belong to the PPV1a (G1) or PPV1b (G2 or 27a-like) group, most Asian and American G2 strains and some European strains were divided into virulent PPV1c (e.g. NADL-8) and attenuated PPV1d (e.g. NADL-2) groups. The increase in the swine population, vaccination degree, and health management (vaccination and biosafety) influenced the spread of PPV1. The reactivity of anti-PPV1 antibodies from sows vaccinated with Porcilis^© Parvo, Eryseng^© Parvo, or ReproCyc^© ParvoFLEX against different PPV1 field strains was the highest upon vaccination with ReproCyc^© ParvoFLEX, followed by Eryseng^© Parvo, and Porcilis^© Parvo. Our findings contribute to the evaluation of the immunogenicity of existing vaccines and support the development of new vaccine candidates. Finally, the potential roles of cluster-specific hallmark amino acids in elevated pathogenicity and viral entry are discussed.

Effects of three commercial vaccines against porcine parvovirus 1 in pregnant gilts

Porcine parvovirosis is a common and important cause of reproductive failure in naïve dams. Even though vaccination is generally effective at preventing disease occurrence, the homology between the vaccine and challenge strains has been recently suggested to play a role in protection. Therefore, the purpose of this study was to evaluate and compare the efficacy of three currently available commercial vaccines against porcine parvovirus genotype 1 (PPV1) in an experimental model using pregnant gilts. Seventy-seven PPV1-negative gilts were included in the trial and randomly allocated to four groups. In group 1, gilts received two doses, three weeks apart, of a PPV1 subunit vaccine (ReproCyc® ParvoFLEX). Following the same scheme, gilts from group 2 received two doses of a PPV1 bivalent vaccine (ERYSENG® PARVO). In group 3, gilts received two doses, four weeks apart, of a PPV1 octavalent vaccine (Porcilis® Ery + Parvo + Lepto). Lastly, gilts from group 4 were left untreated and were used as challenge controls. All gilts were artificially inseminated three weeks after completion of vaccination. Pregnant animals were subsequently challenged around 40 days of gestation with a heterologous PPV1 strain. Foetuses were harvested at around day 90 of gestation and evaluated for their macroscopic appearance (i.e., normal, mummified, or autolytic). Along the study, safety parameters after vaccination, antibody responses against PPV1 and viremia in gilts were also measured. All the foetuses in the challenge control group were mummified, which validated the challenge model, whereas the three evaluated vaccines protected the progeny against PPV1 by preventing the appearance of clinical manifestations associated to parvovirosis. Remarkably, the PPV1 subunit vaccine induced an earlier seroconversion of gilts and was the only vaccine that could prevent viremia after challenge. This vaccine also achieved the largest average litter size accompanied with a high average proportion of clinically healthy foetuses.

Duration of immunity against heterologous porcine parvovirus 1 challenge in gilts immunized with a novel subunit vaccine based on the viral protein 2

BACKGROUND

Porcine parvovirus 1 (PPV1) is widespread in commercial pig farms worldwide and has a significant impact to the swine industry. Long-lasting immunity achieved by means of vaccination is the main tool to prevent PPV1 infection and its associated clinical signs. Here we evaluated the duration of immunity (DOI) conferred by a novel subunit vaccine based on the viral protein (VP) 2 of PPV1, named ReproCyc® ParvoFLEX. The DOI was assessed at 6 months post-vaccination following the standard vaccination scheme (phase I) or after re-vaccination (phase II) with a single injection administered 24 weeks after the basic vaccination scheme. A total of 46, five to six-month-old gilts, free of PPV1 and porcine reproductive and respiratory syndrome virus (PRRSV), were randomly assigned to 6 groups (three in each phase): the negative control groups were inoculated with sodium chloride (NaCl), the vaccinated groups were immunized with the PPV1 subunit vaccine and the strict controls were neither treated nor challenged. Subsequently, the negative control and vaccinated groups from each phase were challenged with a heterologous PPV1 strain. Infection of fetuses was the primary outcome parameter for efficacy, though other supportive parameters were PPV1 viremia and serological status of the gilts and the condition of their fetuses (i.e. normal, autolytic, or mummified).

RESULTS

All gilts vaccinated against PPV1 tested seropositive at challenge and viremia after challenge was detectable only in the non-vaccinated animals. In this regard, fetuses positive to PPV1 by PCR were only found in litters from non-vaccinated sows.

CONCLUSIONS

These results point out that the immunity developed by the PPV1 subunit vaccine is effective in terms of preventing viremia, transplacental infection of fetuses and fetal death caused by PPV1 infection. ReproCyc® ParvoFLEX was demonstrated to protect fetuses against heterologous PPV1 challenge with a DOI of 6 months after vaccination.