Abstract
The objective of this study was to evaluate the efficacy of a modified-live virus (MLV) porcine reproductive and respiratory syndrome virus (PRRSV) vaccine against a heterologous PRRSV-2 challenge in late-term pregnancy gilts under experimental conditions. Eighteen gilts were randomly assigned to vaccinated-challenged, unvaccinated-challenged, and unvaccinated-unchallenged groups (n = 6 gilts per group). Pregnant gilts in the vaccinated-challenged and unvaccinated-unchallenged groups were able to carry their pregnancies to full term and farrowed after 114 to 115 days of gestation. In contrast, pregnant gilts in the unvaccinated-challenged group did not reach full term and farrowed early, after 104 to 108 days of gestation. Pregnant gilts vaccinated with the PRRSV-2 MLV vaccine exhibited a reduction in PRRSV-2 viremia. At the time of challenge with PRRSV-2, vaccinated gilts had relatively low levels of neutralizing antibody titers (≤ 1:16 titer), whereas the number of interferon-γ-secreting cells (IFN-γ-SC) was consistently at protective levels (IFN-γ-SC, ≥ 150 per million). Induction of cell-mediated immunity, as measured by PRRSV-2-specific IFN-γ-SC, correlated with a reduction in PRRSV-2 viremia. Duration of immunity was a minimum of 19 wk. Taken together, the results presented here suggest that vaccination of gilts with a PRRSV-2 MLV vaccine can protect against a heterologous PRRSV-2 challenge and improve the reproductive performance of late-term pregnancy gilts.
Résumé
L’objectif de la présente étude était d’évaluer dans des conditions expérimentales l’efficacité d’un vaccin à virus vivant modifié (MLV) du virus du syndrome reproducteur et respiratoire porcin (PRRSV) contre une infection défi avec un PRRSV-2 hétérologue chez des cochettes en fin de gestation. Dix-huit cochettes furent assignées de manière aléatoire à un des groupes suivants: vaccinées-infectées, non-vaccinées-infectées et non-vaccinées-non-infectées (n = 6 cochettes par groupe). Les cochettes gestantes dans les groupes vaccinées-infectées et non-vaccinées-non-infectées furent en mesure de mener leur gestation à terme et ont mis-bas après 114 à 115 jours de gestation. À l’opposé, les cochettes gestantes du groupe (témoin) non-vaccinées-infectées ne se sont pas rendues à terme et ont mis-bas plus tôt, après 104 à 108 jours de gestation. Les cochettes gestantes vaccinées avec le vaccin PRRSV-2 MLV ont montré une réduction de la virémie à PRRSV-2. Au moment de l’infection-défi avec le PRRSV-2, les cochettes vaccinées avaient des titres relativement bas d’anticorps neutralisants (titre ≤ 1:16), alors que le nombre de cellules secrétant de l’interféron-γ (IFN-γ-SC) était constamment à des niveaux de protection (IFN-γ-SC, ≥ 150 par million). L’induction de l’immunité à médiation cellulaire, telle que mesurée par l’IFN-γ-SC spécifique à PRRSV-2, corrélait avec une réduction de la virémie à PRRSV-2. La durée de l’immunité était d’un minimum de 19 sem. Pris dans son ensemble, les résultats présentés ici suggèrent que la vaccination des cochettes avec un vaccin PRRSV-2 MLV peut protéger contre une infection-défi avec un PRRSV-2 hétérologue et améliorer les performances de reproduction des cochettes en fin de gestation.
(Traduit par Docteur Serge Messier)
Introduction
The first case of porcine reproductive and respiratory syndrome (PRRS) in Korea was described in 1992 (1). Since then, the PRRS virus (PRRSV) has become endemic and PRRS has become one of the most economically devastating diseases to the Korean and worldwide swine industry. Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive, single-stranded ribonucleic acid (RNA) virus that belongs to the genus Porartevirus, in the Arteriviridae family and the order Nidovirales (2). Two distinct species have been extensively described: PRRSV-1 (formerly genotype 1 of European origin) and PRRSV-2 (formerly genotype 2 of North American origin) (3,4).
Infection with PRRSV causes reproductive disorders in breeding females and respiratory symptoms in weaners and young fattening pigs. Reproductive disorders are mainly characterized by late abortions, premature farrowing, litters with mummified, stillborn, or weak-born piglets, and high pre-weaning mortality rates. Infection of pregnant sows can have different outcomes depending on their time of gestation. While PRRSV has been known to cause embryonic death during early gestation (5,6), the virus does not readily cross the placenta during mid-gestation, which limits the impact on reproductive failure (7,8). During late gestation, however, PRRSV has the ability to cross the placenta, causing abortions, early farrowings, fetal death, and the birth of weak, congenitally infected piglets, all of which contribute to an increase in pre-weaning mortality (8–11).
Currently, PRRSV vaccines are the most common tool used to control respiratory diseases in growing pigs and reproductive disorders in sows. However, vaccine regimens vary depending on the country. In Korea, swine producers vaccinate sows in order to protect against reproductive disorders. In North America, PRRSV vaccines are mainly used to control respiratory disease by vaccinating growing pigs (personal communication, Dr. Aaron J. Lower, Carthage Veterinary Service, Carthage, Illinois, USA). The Korean Animal Health Products Association has estimated that approximately 75% of Korean sows have been administered a PRRSV modified-live virus (MLV). In Korea, vaccination is currently the most important and cost-effective strategy for controlling PRRSV infection and reproductive failure, particularly in gilts and sows.
A PRRSV-2 MLV vaccine was licensed in 2015 in the U.S. and in 2017 in Korea for protection against reproductive failure for gilts and sows. A previous study under field conditions showed that this MLV vaccine improved the reproductive performance of sows on farms that were endemically infected with PRRSV-2 (12). When a farm is endemically infected with PRRSV-2, it is important to note that the immunological and virological responses to a vaccine cannot be accurately evaluated as vaccinated sows are continuously exposed to circulating field viruses. Studies with a controlled challenge could better assess the efficacy of the vaccine, but there have been no such studies to date.
Assessing the efficacy of a PRRSV-2 MLV vaccine against PRRSV-2 is clinically relevant because PRRSV-2 is still the predominant species circulating on Korean farms and PRRSV-2 MLV vaccines are the most commonly used by Korean pig farmers. The objective of this study was to evaluate a PRRSV-2 MLV vaccine against heterologous PRRSV-2 challenge in pregnant gilts, in terms of reproductive failure.
Materials and methods
PRRSV isolates
Porcine reproductive and respiratory syndrome virus-2 (PRRSV-2) (SNUVR090851, lineage 1, GenBank no. JN315685) was used as inocula (13). A sequence analysis of ORF5 determined an 87.2% sequence identity between the challenge stain and P129 vaccine virus (PRRSV-2, lineage 8, GenBank no. AF494042).
Experimental design
The gilts used in this study were purchased from a PRRSV-free commercial farm that had not vaccinated against PRRSV. Enzyme-linked immunosorbent assay (ELISA) and real-time polymerase chain reaction (RT-PCR) were used to confirm that all subjects were negative for PRRSV (14). Of the 24 healthy gilts purchased, only 18 were pregnant and were used in this study. The 18 pregnant gilts were randomly assigned to 3 groups (n = 6 gilts per group) according to treatment using the random number generation function (Excel; Microsoft Corporation, Redmond, Washington, USA) (Figure 1). Each group was housed in a different room until the day of challenge in order to avoid transmission of shed vaccine virus to unvaccinated individuals.
Figure 1.
Experimental design. Gilts were administered a porcine reproductive and respiratory syndrome virus (PRRSV)-2 modified-live virus (MLV) vaccine against a heterologous PRRSV-2 challenge in late-term pregnancy under experimental conditions on certain days as shown. Gilts in the Vac2/ Ch2 group were injected intramuscularly with 2.0 mL of PRRS MLV vaccine at −135 d post-challenge (dpc, 42 d before breeding). Pregnant gilts in Vac2/Ch2 and UnVac/Ch2 groups were inoculated intranasally with PRRSV-2 at 0 dpc (93 d of gestation). Blood samples (
) were collected from all pregnant gilts by jugular venipuncture at −135 (42 d before breeding) −93 (0 d of gestation), −65 (28 d of gestation), 0 (93 d of gestation, 3 wk before farrowing), 7 (100 d of gestation), and 21 (114 d of gestation) dpc.
At −135 d post-challenge (dpc) (42 d before breeding), gilts in the vaccinated/challenged (Vac2/Ch2) group were injected intramuscularly with a 2.0-mL dose of PRRS MLV vaccine (Fostera PRRS; Zoetis, Parsippany, New Jersey, USA, Lot No. 169588, Serial No. 163540/159469, Expiration date 28 Nov 17). Gilts in the unvaccinated/challenged (UnVac/Ch2) and the unvaccinated/ unchallenged (UnVac/UnCh) groups were injected intramuscularly with 2.0 mL of phosphate buffered saline (PBS, 0.01M, pH 7.4) as control (Figure 1).
At 0 dpc (93 d of gestation), pregnant gilts in the Vac2/Ch2 and UnVac/Ch2 groups were inoculated intranasally with 6 mL of tissue culture supernatant containing 104 50% tissue culture infectious dose (TCID50)/mL of PRRSV-2 (SNUVR090851, 2nd passage in alveolar macrophages). Pregnant gilts in the UnVac/UnCh group were similarly inoculated with 6 mL of uninfected cell culture supernatant as control (Figure 1).
After challenge, the gilts in the Vac2/Ch2 and UnVac/Ch2 groups were randomly assigned to 3 out of 5 total rooms, with at least 1 gilt from each group in each room. Pigs in the UnVac/UnCh group were randomly placed in 6 pens in the 2 remaining rooms. Each room had 4 pens and each gilt was individually housed in a pen. The experimental unit for the treatment was the individual animal.
Blood samples were collected from all pregnant gilts by jugular venipuncture at −135 (42 d before breeding), −93 (0 d of gestation), −65 (28 d of gestation), 0 (93 d of gestation, 3 wk before farrowing), 7 (100 d of gestation), and 21 (114 d of gestation) dpc (Figure 1). Stillborn piglets were collected from each group and lung, lymph node, heart, tonsil, and thymus tissues were harvested from each stillborn individual and tested for the presence of PRRSV. The tissues from each stillborn piglet were harvested under clean conditions and stored and tested separately. All methods and protocols used in this study were approved by the Seoul National University Institutional Animal Care and Use Committee (Protocol number SNU-180621-13).
Reproductive performance
After the virus challenge, all pregnant gilts were monitored daily for rectal temperature by the same personnel. Farrowing data were also collected, including litter size [total number of piglets born, live birth, stillborn, mummified, and lightweight < 1 kg body weight (BW) per litter] at birth, and on the day of weaning at 21 d old. A period of less than 111 d is considered premature farrowing.
Quantification of PRRSV RNA in blood
Serum samples were collected from pregnant gilts to test for the presence of PRRSV RNA. Ribonucleic acid (RNA) was extracted from the samples and RT-PCR was used to quantify genomic complementary DNA (cDNA) copy numbers for PRRSV-1 or PRRSV-2, as previously described (14,15). The primers for the challenge virus were designed based on the highly conserved ORF7 region. For PRRSV-2 challenge strain, the forward and reverse primers were 5′-TGGCCAGTCAGTCAATCAAC-3′ and 5′-AATCGATTGCAAGCAGAGGGAA-3′, respectively (14,15). For the vaccine virus, the forward and reverse primers were 5′-CTTGACACAGTTGGTCTGGTTACT-3′ and 5′-GTTCTTCGCAAGCCTAATAACG-3′, respectively, based on ORF5 (16).
Serology
The serum samples were tested for total PRRSV-specific antibodies using a commercially available PRRSV enzyme-linked immunosorbent assay (ELISA) (HerdCheck PRRS X3 Ab test; IDEXX Laboratories, Westbrook, Maine, USA). Serum virus neutralization tests were also carried out with challenge PRRSV-2 strain, as previously described (17). Serum samples were considered to be positive for neutralizing antibody (NA) if the titer was greater than 2.0 (log2) (17).
Enzyme-linked immunospot (ELISPOT) assay
Porcine reproductive and respiratory syndrome virus (PRRSV)-specific interferon-γ-secreting cells (IFN-γ-SCs) were quantified in peripheral blood mononuclear cells (PBMCs), as previously described (18–20). In a typical PRRSV-specific reaction, the background caused by the non-PRRSV stimulated wells (negative control) did not exceed 5 spots per cell and the obtained values were subtracted from the respective counts obtained from the stimulated wells. The IFN-γ positive spots on the membranes were imaged, analyzed, and counted using an automated ELISPOT Reader (AID ELISPOT Reader; AID GmbH, Strassberg, Germany). ELISPOT assay was done in duplicate.
In-situ hybridization
In-situ hybridization (ISH) was conducted to detect the presence of PRRSV-1 and PRRSV-2 nucleic acid in fetal tissue, as previously described (13,21). The number of PRRSV-positive cells per tissue area unit (0.25 mm2) was counted using NIH Image J 1.45s program (13,21).
Statistical analysis
Prior to statistical analysis, reverse transcriptase polymerase chain reaction (RT-PCR) and neutralizing antibody data were logtransformed to reduce variance and positive skewness (base 10 and 2, respectively). One-way analysis of variance (ANOVA) was used to examine whether there were statistically significant differences in PRRSV RNA in sera, reproductive performance parameters, PRRSV RNA, serology data, ELISPOT, and ISH analysis among the 3 groups, for each time point. When a test result from 1-way ANOVA showed a statistical significance, a post-hoc test was conducted for a pairwise comparison with Tukey’s adjustment. For neutralizing antibody measurements, a Kruskal-Wallis test was employed, since normality assumption was not met for neutralizing antibody data (Shapiro-Wilk test was applied to test normality). When the result from the Kruskal-Wallis test showed statistical significance, a Mann-Whitney test with Tukey’s adjustment was carried out to determine the significance of group differences at each time point. A P-value < 0.05 was considered significant.
Results
Reproductive performance
The reproductive performance of the vaccinated gilts was assessed based on the length of the gestation period and the number of piglets born alive, stillborn, mummified, and lightweight (< 1 kg body weight). All the pregnant gilts from the vaccinated/challenged (Vac2/Ch2) and the unvaccinated/unchallenged control (UnVac/ UnCh) groups carried their pregnancies to full term and farrowed at 114 to 115 d of gestation. Pregnant gilts from the unvaccinated/ challenged (UnVac/Ch2) group farrowed prematurely, with a gestation period of 104 to 108 d. The gestation period of pregnant gilts from the Vac2/Ch2 and UnVac/UnCh groups was significantly longer (P < 0.05) than the UnVac/Ch2 group. In addition, gilts from the Vac2/Ch2 and UnVac/UnCh groups had a significantly higher (P < 0.05) number of live-born and weaned piglets and a significantly lower (P < 0.05) number of stillborn and mummified fetuses than the UnVac/Ch2 group (Table I). No adverse systemic or local reactions relative to vaccination were observed throughout the entire pregnancy period, which confirmed the safety of the vaccine.
Table I.
Reproductive parameters (mean ± standard deviation) of gilts among 3 groups
| Vac2/Ch2 | UnVac/Ch2 | UnVac/UnCh | |
|---|---|---|---|
| Vaccination | Yes | None | None |
| Challenge | PRRSV-2 | PRRSV-2 | None |
| Gilts | |||
| Gestation length | 114.33 ± 0.52a | 105.67 ± 1.37b | 114.17 ± 0.41a |
| Premature farrowing | 0/6 | 6/6 | 0/6 |
| Piglets/litter | |||
| Total born | 11.67 ± 1.63 | 11.83 ± 1.72 | 11.33 ± 1.21 |
| Live-born | 10.33 ± 1.75a | 2.33 ± 0.82b | 10.67 ± 0.82a |
| Stillborn | 1.33 ± 0.52b | 8.83 ± 0.98a | 0.50 ± 0.55b |
| Mummified | 0 ± 0b | 0.67 ± 0.52a | 0.17 ± 0.41a,b |
| Lightweight (< 1 kg) | 0.50 ± 0.55 | 0 ± 0 | 0.33 ± 0.52 |
| Splay-legs | 0.33 ± 0.52 | 0 ± 0 | 0.17 ± 0.41 |
| Weaned | 9.17 ± 1.17a | 2.17 ± 0.4b | 10.00 ± 0.89a |
Significant (P < 0.05) difference among 3 groups.
Quantification of PRRSV RNA in sera
To evaluate the reduction of viremia, serum samples were collected from pregnant gilts and the number of PRRSV genomic copies was quantified by RT-PCR. All pregnant gilts were negative at the time of vaccination (−135 dpc, 6 wk before breeding) and challenge (0 dpc, 93 d of gestation), confirming that they were PRRSV-free. At 7 and 21 dpc, pregnant gilts from the Vac2/Ch2 group had a significantly (P < 0.05) reduced number of genomic copies of PRRSV-2 RNA in their sera compared to the UnVac/Ch2 group. As expected, all pregnant gilts from the Vac2/Ch2 and UnVac/Ch2 groups were negative for PRRSV-1 and no PRRSV-1 or PRRSV-2 genomes were detected in the sera of pregnant gilts from the UnVac/ UnCh group throughout the study (Figure 2).
Figure 2.
Mean values of the genomic copy number of PRRSV-2 RNA in serum from Vac2/Ch2 (
), UnVac/Ch2 (
), and UnVac/UnCh (
) groups. Variation is expressed as the standard deviation. One-way analysis of variance (ANOVA) was used to examine whether there were statistically significant differences in the genomic copy number of PRRSV-2 RNA in serum. When a test result from 1-way ANOVA showed a statistical significance, a post-hoc test was conducted for a pairwise comparison with Tukey’s adjustment to determine the significance of group differences at each time point. Different superscripts (a, b, and c) indicate significant (P < 0.05) difference among 3 groups.
Immune responses
Porcine reproductive and respiratory syndrome virus-2 (PRRSV-2)-specific antibody titers were quantified using an ELISA kit. At the time of vaccination (−135 dpc, 42 d before breeding), pregnant gilts in all 3 groups were seronegative, which again confirmed that they were PRRSV-free. At −93, −65, 0, 7, and 21 dpc, pregnant gilts from the Vac2/Ch2 group had elicited a significantly higher (P < 0.05) PRRSV-specific antibody titer than the UnVac/Ch2 group. As expected, gilts in the UnVac/UnCh group were negative for PRRSV throughout the study (Figure 3A).
Figure 3.
Humoral immune responses. A — Mean values of the ELISA S/P ratio in serum from Vac2/Ch2 (
), UnVac/Ch2 (
), and UnVac/UnCh (
) groups. One-way analysis of variance (ANOVA) was used to examine whether there were statistically significant differences in the ELISA S/P ratio in serum. When a test result from 1-way ANOVA showed a statistical significance, a post-hoc test was conducted for a pairwise comparison with Tukey’s adjustment to determine the significance of group differences at each time point. B — Neutralizing antibody (NA) titers against PRRSV-2 in serum from Vac2/Ch2 (
), UnVac/Ch2 (
), and UnVac/UnCh (
) groups. A Kruskal-Wallis test was used to examine whether there were statistically significant differences in NA titers against PRRSV-2 in serum. When the result from the Kruskal-Wallis test showed statistical significance, a Mann-Whitney test with Tukey’s adjustment was done to determine the significance of group differences at each time point. Variation is expressed as the standard deviation. Different superscripts (a, b, and c) indicate significant (P < 0.05) difference among 3 groups.
Neutralizing antibody (NA) titers against PRRSV-2 were only detected in pregnant gilts from the Vac2/Ch2 group before challenge. In addition, at −93, −65, 0, 7, and 21 dpc, pregnant gilts from the Vac2/Ch2 group had significantly higher (P < 0.05) NA titers against PRRSV-2 than the UnVac/Ch2 group. All pregnant gilts from the UnVac/UnCh group were negative for PRRSV-2 NAs throughout the study (Figure 3B).
Spot size distribution showed that the majority of spots were the same size and that overall spot size spread was in the same range for Vac2/Ch2 and UnVac/Ch2 groups (Figure 4A). No spot was observed in the UnVac/UnCh group (Figure 4A). Cell-mediated immune responses were evaluated using ELISPOT to quantify the number of PRRSV-2-specific IFN-γ-SCs. Only pregnant gilts from the Vac2/Ch2 group had detectable PRRSV-2-specific IFN-γ-SCs before challenge. At −93, −65, 0, 7, and 21 dpc, the number of PRRSV-2-specific IFN-γ-SCs was significantly higher (P < 0.05) in pregnant gilts from the Vac2/Ch2 group than those in the UnVac/Ch2 group (Figure 4B). No PRRSV-2-specific IFN-γ-SCs were detected in pregnant gilts from the UnVac/UnCh group throughout the study.
Figure 4.
Cell-mediated immune responses. A — Results of ELISPOT assay with peripheral blood mononuclear cells (PBMCs) from Vac2/Ch2, UnVac/Ch2, and UnVac/UnCh groups at 21 d post-challenge. The number of PRRSV-specific spots corresponds to the number of interferon-γ-secreting cells (IFN-γ-SCs) in 1 × 106 PBMCs. B — Frequency of PRRSV-2-specific IFN-γ-SCs/106 in PBMCs. One-way analysis of variance (ANOVA) was used to examine whether there were statistically significant differences in the frequency of PRRSV-2-specific IFN-γ-SCs/106 in PBMCs. When a test result from 1-way ANOVA showed a statistical significance, a post-hoc test was conducted for a pairwise comparison with Tukey’s adjustment to determine the significance of group differences at each time point. Variation is expressed as the standard deviation. Different superscripts (a, b, and c) indicate significant (P < 0.05) difference among 3 groups.
Pathology
In-situ hybridization (ISH) was used to detect the presence of PRRSV nucleic acid in fetal tissue, including thymus, lung, tonsil, lymph node, and heart (Figure 5A). Litters from the vaccinated challenged (Vac2/Ch2) group had significantly lower (P < 0.05) scores from all harvested tissues than the unvaccinated challenged (UnVac/ Ch2) group (Figure 5B). As expected, no PRRSV-1 positive cells were detected in any of the fetal tissues from pregnant gilts in the Vac2/ Ch2 and UnVac/Ch2 groups and no PRRSV-1 or PRRSV-2 positive cells were detected in any of the fetal tissues from pregnant gilts in the UnVac/UnCh group throughout the study.
Figure 5.
In-situ hybridization (ISH). A — PRRSV-2 nucleic acids were detected in fetal tissue of the thymus, lung, tonsil, lymph node, and heart from the Vac2/Ch2 and UnVac/Ch2 groups by ISH. Bar = 11 μm. B — Means scores for ISH with fetal lung, lymph node, thymus, tonsil, and heart from Vac2/Ch2 (
), UnVac/Ch2 (
), and UnVac/UnCh (
) groups. One-way analysis of variance (ANOVA) was used to examine whether there were statistically significant differences in the PRRSV-positive cells. When a test result from 1-way ANOVA showed a statistical significance, a post-hoc test was conducted for a pairwise comparison with Tukey’s adjustment to determine the significance of group differences. Different superscripts (a, b, and c) indicate significant (P < 0.05) difference among 3 groups.
Discussion
The results of this study show that a PRRSV-2 MLV vaccine is efficacious against heterologous PRRSV-2 challenge in late-term pregnancy gilts. Our findings are consistent with the results reported from a previous field trial study (12), in which vaccination with the same PRRSV-2 MLV vaccine resulted in an improved reproductive performance at farms that were endemic with PRRSV-2 infection. Reproductive performance is severely affected by PRRSV-2 infection, which causes enormous losses to the swine industry. The improvement in reproductive performance in gilts and sows is one of the most important criteria for evaluating the efficacy of a PRRSV MLV vaccine (12). In our study, vaccination resulted in a higher number of live-born and weaned piglets and a lower number of stillborn or mummified piglets. This suggests that the vaccine was able to protect against reproductive failure caused by PRRSV-2 infection.
Efficacious vaccines are also able to prevent or at least reduce viremia following challenge. While the PRRSV-2 MLV vaccine used in this study was not able to fully prevent viremia, it was able to significantly reduce it. A previous study had reported that the same vaccine was able to prevent viremia against homologous challenge (22). The mechanism by which the PRRSV MLV vaccine protects against reproductive failure in pregnant gilts is not well understood. Previous studies have shown that, although the degree of maternal viremia does not directly cause abortions (23), it plays an important role in PRRSV crossing the placenta and replicating in the endometrium (24). Placental infection with PRRSV most likely induces histopathological lesions in the maternal-fetal interface (25), which cause placental degradation and deterioration of placental function (26). This enables the spread of PRRSV to the fetal tissue where it can also replicate, resulting in fetal death (23).
To understand how the vaccine was able to reduce viremia, we looked at the immune response as a result of vaccination. At the time of PRRSV-2 challenge, very few vaccinated gilts had protective levels of neutralizing antibody titer (≥ 1:16 titer), which was previously shown to prevent abortion in pregnant sows under experimental conditions (27). In contrast, vaccinated gilts exhibited high frequencies of IFN-γ-SC (≥ 150 per million), which is typically associated with effective prevention of abortion in pregnant sows under field conditions (19). In addition, the reduction of PRRSV-2 viremia coincided with the increase of IFN-γ-SC levels in vaccinated gilts following PRRSV-2 challenge, suggesting it is a result of T-cell response. These results are in agreement with several previous studies, where IFN-γ T-cell response results in protective immunity (18,28–30).
The duration of immunity (DOI) is also a very important parameter when selecting a PRRSV MLV vaccine as it can play a critical role in production management if the vaccine can protect gilts and sows from pregnancy to farrowing. The PRRSV MLV vaccine used in this study is effective in improving reproductive performance from 6 wk before breeding all the way to farrowing. This is consistent with the 19 wk of DOI claimed by the vaccine manufacturer (https://www.zoetis.com).
To the authors’ knowledge, this is the first experimental study to assess the efficacy of a PRRSV-2 MLV vaccine against a heterologous PRRSV-2 challenge in pregnant gilts. Even though the PRRSV-2 MLV vaccine provided heterologous protection, it is important to note that it was tested against only 1 PRRSV-2 strain and, therefore, does not guarantee protection against other PRRSV-2 strains. More studies are necessary to evaluate its efficacy against a broad range of PRRSV field viruses. Reproductive failures caused by PRRSV result in devastating losses to swine farmers (31). Because PRRSV-2 is still the predominant virus circulating on Korean farms, an efficacious vaccine against PRRSV-2 can help swine practitioners and producers to control PRRSV infection in gilts and sows.
In conclusion, the PRRSV-2 MLV vaccine was able to induce protective immunity, which resulted in reduced PRRSV-2 viremia. The duration of immunity of the PRRSV-2 MLV vaccine used in the study is 19 wk, as claimed by the manufacturer. The improved reproductive performance as a result of vaccination can bring tremendous economic benefits to swine producers.
Acknowledgments
The authors’ research was supported by contract research funds (Grant no. 550-20160024) from the Research Institute for Veterinary Science (RIVS) of the College of Veterinary Medicine, Seoul National University and by the BK 21 Plus Program (Grant no. 5260-20150100) for Creative Veterinary Science Research.
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