- Open Access
Vaccination of piglets at 2 and 3 weeks of age with Ingelvac PRRSFLEX® EU provides protection against heterologous field challenge in the face of homologous maternally derived antibodies
© The Author(s). 2016
- Received: 12 December 2015
- Accepted: 14 July 2016
- Published: 14 August 2016
Due to difficulties in eradicating porcine reproductive and respiratory syndrome (PRRS) linked to biosecurity challenges, transmission of the virus and the lack of efficient DIVA vaccines, successful control of PRRS requires a combination of strict management measures and vaccination of both sows and piglets. The present study aimed to assess the efficacy of a recently developed MLV vaccine (Ingelvac PRRSFLEX® EU) in piglets at 2 and 3-weeks of age in the presence of homologous maternally derived antibodies as the dams were vaccinated with the same vaccine strain (ReproCyc® PRRS EU).
The study was carried out on a Hungarian farrow to finish farm naturally infected with PRRSv. The study was designed as a blind, placebo controlled side by side trial. ORF5 sequence similarity of the vaccine strain and the resident field strain was 87.8 %. PRRS specific real-time quantitative PCR was performed from serum samples to measure both the viral load and the frequency of virus positive animals.
At the time of the natural infection observed in the control group at 10–12 weeks of age, the number of viraemic animals did not increase significantly in the vaccinated group. To understand the infection dynamics, positive PCR samples with low Ct values were sequenced (ORF5) and the data analysis indicated the circulation of wild type virus in both groups, however wild type virus was only found in non-vaccinated animals.
Our data indicate that piglets vaccinated at as early as 2 weeks of age with Ingelvac PRRSFLEX® EU were protected both in terms of proportion of viraemic animals and viraemia levels. It has to be highlighted that these results were achieved in piglets with high levels of homologous maternally derived antibodies (MDA) at the time of vaccination.
- Modified live virus
- Maternally derived antibodies
Porcine reproductive and respiratory syndrome (PRRS) is one of the most widespread, and economically devastating disease in swine industry. It is characterized by reproductive losses in breeding herds, increased mortality in newborn pigs and respiratory disorders in growing pigs [1, 2].
The disease emerged almost at the same time in Europe  and North America , and since then, it has rapidly spread throughout the world, and become endemic in almost every major swine producing country.
PRRS virus (PRRSV) is a member of the Arteriviridae family within the order of Nidovirales . The relatively small, enveloped virus has a positive-sense single-stranded RNA genome of approximately 15.1 kb in length and encodes 10 ORFs [6–8]. In the last years new ORFs (TF) and −1/−2 programmed ribosomal frameshift signals were discovered in ORF1a, expressing two novel proteins, nsp2TF and nsp2N [9, 10]. Moreover alternative reading frames were identified on the structural protein coding regions: ORF2, ORF5 and most recently on ORF7 coding for GP2a, GP5a and ORF7ap, respectively [11, 12].
Soon after the first isolation, marked genetic differences were identified between these strains and they were classified in two distinct genotypes (Type I, formerly EU, and Type II, formerly NA) [5, 13]. Recent phylogenetic studies performed on Lithuanian, Belarussian and Russian Type I strains revealed unexpectedly high degree of variability within this genotype and led to the definition of four subtypes [14, 15].
The current strategies used to control, or eliminate PRRS require strict management implications including the application of strict biosecurity measures, whole herd depopulation and repopulation, test and removal of seropositive animals, closure of the breeding herd, and vaccination .
The marked genetic differences observed among various PRRSV isolates can have a negative effect on the efficacy of modified live vaccines (MLV) , however the degree of genetic similarity between the resident strain and the vaccine does not predict the degree of protection conferred after vaccination .
Previous studies using Type I MLV and a natural exposure of growing pigs to a field strain of the same genotype reported a reduction of clinical signs, and improved activation of cell mediated immunity in vaccinated animals, but the vaccination did not reduce the incidence of viraemic animals and the levels of viraemia. The duration of viraemia however was shorter in vaccinated piglets . In an other study, performed on PRRSV exposed pregnant sows using commercially available attenuated and farm-specific inactivated vaccine the authors found significantly lower number of viraemic piglets born to sows of vaccinated groups compared to mock-vaccinated ones. Their results indicated that both vaccines could be useful tools in the control of PRRS in the breeding herd .
Therapeutic use of a Type II MLV vaccine as an intervention in an acute outbreak was reported to reduce the duration of viral shedding. Also reduced respiratory disease and improved production parameters were recorded when the challenged-vaccinated animals were re-infected with a highly virulent challenge strain . In a recent study the therapeutic, post-infection use of the same Type II MLV was reported to reduce significantly the viral shedding as measured by oral fluid analysis and cumulative PRRSV presence in the air .
The objective of the present work was to determine and compare the efficacy of a recently developed MLV vaccine (Ingelvac PRRSFLEX® EU) to a mock (PBS) vaccinated cohort in 2 and 3 weeks old piglets born to sows mass-vaccinated twice with a homologous MLV ReproCyc PRRS® EU and assess the possible interference with homologous maternally derived antibodies with vaccine efficacy.
This study was conducted in on a commercial, farrow to finish, closed system farm in Hungary. Monitoring over several years showed an ongoing PRRS wild type strain circulation on the farm, confirmed by an actual screening shortly before study initiation. The pre-screening of the herd was performed as a cross sectional ELISA seroprofiling and PCR on serum samples obtained from 80 animals (10 samples of pigs at the age of 2, 4, 6, 8, 10, 12, 14, 16 weeks). The results revealed an ongoing field virus circulation starting in 6-weeks-old animals. Sequences obtained from the study site over time are included in Fig. 4. In total 475 piglets at 2 weeks of age and 551 piglets at 3 weeks of age were included in the study. The batches of piglets were divided into a vaccinated group (Ingelvac PRRSFLEX® EU) and a non-vaccinated control group (246/229 and 351/200 vaccinated/non-vaccinated animals in the 2two- and 3-weeks of age group, respectively). Piglets were vaccinated under the sow and then at 4 weeks of age transferred to one barn that was surrounded by fattening units and farrowing barns. Groups were held in separate rooms and not commingled until the age of 12-weeks of life. The study was blinded for treatment and randomized by farrowing units to prevent cross contamination of non-vaccinated piglets. The piglets originated from sows and gilts that were previously vaccinated with ReproCyc® PRRS EU (Boehringer Ingelheim Vetmedica GmbH, Germany).
The Ingelvac PRRSFLEX® EU vaccine strain (PRRS 94881, full genome Gen Bank accession number KT988004) is attenuated from a field virus first isolated in 2002 from a farm in Germany with clinical symptoms of PRRS. The parental strain belongs to the European Type I, subtype 1 lineage of PRRS viruses. The vaccine strain shared 87.8 % nucleotide identity in the ORF5 gene with the circulating resident wild type PRRSV strain.
Piglets were vaccinated with one dose of Ingelvac PRRSFLEX® EU vaccine with a minimum immunizing dose as indicated on the vaccine label instructions at 2-weeks of age or 3-weeks of age. Control animals were administered one dose of vaccine solvent (PBS) without antigen content. No other vaccinations or treatments were administered to the animals on at least 3 days before and after the PRRS vaccine treatment.
Sample analysis and assessment of viremia and serology
In each group, 20 % of animals were designated at random as sample animals for blood collection. Blood samples were collected pre-vaccination and then weekly in weeks four to ten after vaccination for the 2 weeks of age groups and weekly in weeks three to nine after vaccination in the 3 weeks of age groups. After drawing, blood samples were allowed to clot at room temperature, were centrifuged and serum harvested. Serum samples were held at −80 °C and for serology and qPCR testing, respectively.
For ELISA the IDDEX PRRS X3 test was used following the manufacturer’s instructions (HerdChek* Porcine Reproductive and Respiratory Syndrome Antibody Test Kit X3 – IDEXX Laboratories Inc., Westbrook, ME, USA). Results were reported as negative (ELISA sample to positive [S/P] ratio of < 0.4) or positive (ELISA S/P ratio of ≥ 0.4).
PRRS serum qPCR
For detection of PRRS virus RNA a validated TaqMan probe based quantitative reverse transcription real time PCR targeting the viral ORF7 was used (bioScreen EVDMC GmbH, Hannover, Germany). Results were reported as negative (n.d.), positive (not quantifiable, <3.0 log10 genome equivalents (GE)/ml) and quantifiable log10 GE/mL. A qPCR result of n.d. (not detected) was assigned a value of 0 log10 GE/mL and a positive qPCR result was assigned a log10 value of 3.0 GE/mL for statistical purposes.
Serum samples tested positive by qPCR and that exceeded a virus concentration of 3.0 GE/ml were selected for ORF5 sequence analysis. Sequencing of ORF5 was done by amplifying the respective region of the PRRSV genome by PCR directly from the samples, followed by Sanger-sequencing of the purified PCR product according to Balka et al. . The sequencing data obtained was aligned with ORF5 sequences of PRRSV reference strains (i.e. wild type strain circulation on the farm, vaccine strains of commercially available vaccines used on the farm) and subsequent calculation of sequence similarities between the sequences. Sequence analysis was done with CLC Main Workbench v4.1.1.
Phylogenetic analyses were performed using the CLUSTAL X 1.81 software employing IUB DNA weight matrix with 0.5 transition ratio. Bootstrap resampling was carried out on 100 replicate data sets. Phylogenetic trees were plotted with the TREEVIEW (Win32 version 106 1.6.6.) software.
To prove efficacy of a vaccine protection has to be proven by a challenge with virulent wild type PRRS virus. A virulent PRRS virus strain was circulating on the farm before initiation of the study as proven by virus isolation and subsequent sequencing over the past years. Previous cross sectional screening results performed less than 4 months before the start of the study showed an active PRRS circulation in pigs as early as 6 weeks of age, with a peak at 10 weeks of age (data not shown). ORF5 sequence similarity of the field strain and the vaccine strain was 87.8 %. Field challenge in the study animals was also controlled by sequencing of PRRS virus positive blood samples. First signs of a field challenge occurred at 9 weeks of age in non-vaccinated animals. Since the study investigation ended at 12-weeks of age for all animals the peak of field infection might have not been reached at that time.
Frequency of viremic animals after vaccination and at field challenge
Detection of serum viral copies after vaccination and at field challenge
Low levels of PRRS viraemia was detected by qPCR in the vaccinated groups shortly after vaccination, while the control groups remained PRRS virus negative in this period (Fig. 3). The maximum mean titer, including all investigated animals within a group, was measured 5 weeks after vaccination at 0.278 log10 GE/mL. All but one positive tested animals at all time points before field challenge showed only non-quantifiable qPCR results. Field challenge occurred at 9 weeks post vaccination as indicated by the appearance of the first PRRSV positive animals in the control group. During the field challenge the mean titer of the vaccinated group peaked at 0.472 log10 GE/mL in the 2-weeks of age group and 0.46 log10 GE/mL in the 3-weeks of age group, while the non-vaccinated animals reached 2.23 log10 GE/mL and 1.48 log10 GE/mL in the 2-weeks and 3-weeks of age groups, respectively. The difference at the end of the study was highly significant for the 2-weeks of age group (p < 0.0001) and 3-weeks of age group (p = 0.0006).
Sequence analysis of ORF5 gene
Group assortment of sequenced isolates
2-weeks of age
3-weeks of age
10 weeks of life
11 weeks of life
12 weeks of life
The present study aimed to assess the field efficacy of a recently developed MLV PRRS vaccine (Ingelvac PRRSFLEX® EU) in piglets at 2 and 3 weeks of age in the presence of homologous maternally derived antibodies. At the time of the field challenge infection, observed in the control group (10–12 weeks of age, WOA), the number of viraemic animals did not increase in the vaccinated group. It has to be highlighted that results were achieved in piglets with high levels of homologous maternally derived antibodies at the time of vaccination since the dams were vaccinated with the same vaccine strain (ReproCyc® PRRS EU).
The protective effect of the MLV against natural challenge is either related to the successful induction of neutralizing antibodies (NAs), and/or boost of the cellular immunity. Previous studies have shown that passive transfer of NAs can protect pregnant sows from viraemia, transplacental shedding and reproductive failure, whereas titres higher that 1:32 can confer sterilizing immunity in growing pigs [24, 25]. The protection however will be less effective in case of heterologous challenge even in the same genotype . The genetic differences however (usually expressed as ORF5 similarities) between the immunizing and the challenging strains cannot predict the degree of the protective immunity conferred . Moreover marked differences have been observed among PRRSV isolates both in terms of their susceptibility to neutralization by NAs induced by other strains and also in their ability to induce NAs that are neutralizing a broad spectrum of isolates .
Other authors found that vaccine efficacy in their heterologous challenge model was attributed to the ability of the MLV to boost the cellular immunity by the induction of IFN-γ secreting cells that inversely correlated with the production of IL-10 by PBMCs .
Our results indicate proper efficacy of the vaccine strain in terms of proportion of viraemic animals as well as levels of viraemia, however further studies are needed to assess the neutralization ability and spectrum of the NAs induced by Ingelvac PRRSFLEX® EU, and it’s ability to induce specific cellular immunity.
Another critical part of the results was the presence of relative high amounts of homologous maternal antibodies at the time of vaccination. Evidence can be found in the literature proving the negative effect of the MDAs on the antibody responses after vaccination (reviewed in [28, 29]). For example, piglets vaccinated in the presence of MDAs had lower levels of antibody response in case of Aujeszky disease , swine influenza virus , and Mycoplasma hyopneumoniae . On the other hand, the development of the specific cell mediated immunity seems to be less affected or even enhanced by the MDAs. In a recent study piglets vaccinated at 7 days of age in the presence of MDAs against Mycoplasma hyopneumoniae showed an earlier cell mediated immunity even though they failed to show vaccine-induced antibody response , however results obtained in the case of other pathogens can not be directly adapted to PRRSV infection.
Titre dependent interaction between MDAs and vaccination has been observed by van Woensel et al. , who observed high proportion of vaccine takes either at high and low antibody titres, but not at medium titres. The authors explained this observation by the effect of opsonization as an alternative mechanism of PRRSV to enter in the macrophages at high antibody titres. However this Fc receptor based entry route for PRRSV has been questioned lately . However it has to be highlighted, that S/P ratios as measured by ELISA test in our study do not necessarily correspond to antibody titre values and even less to neutralizing antibody levels.
The partial interference of the MDAs with the vaccination cannot be excluded in our case as MDA positive and negative groups were not compared in this study. Certainly, the vaccinated group developed immunity allowing to keep the proportion of the viraemic piglets under 10 % despite the 30–45 % found in the control group.
In our experiment we found that correct timing of the vaccination is more dependent on the presumed natural challenge, than the amount of MDA in the piglets. The vaccine was able to overcome the suppressing effects of the homologous MDA and the time frame between the vaccination and the challenge – observed at 8–10 weeks of age in the control group – was sufficient to develop effective immunity and preventing infection. According to our results, cross sectional seroprofiling and virus detection in the affected age groups is recommended in order to identify the time of the infection and ensure ideal timing for vaccination.
Comparative phylogenetic analyses were conducted after sequencing the ORF5 region of 42 positive samples with sufficient amounts of viral copies (39 from non-vaccinated and five from vaccinated group). 40/42 strains were identical or very closely related (>98 % similarity) to the resident wild type strain. The presence of the wild type virus in the vaccinated group (4 cases) confirmed the infection pressure by the resident virus on the farm. Control and vaccinated groups were located in different rooms in the same barn to prevent the possible transmission of the MLV strain, but also the rooms were randomly distributed in the same building to provide the same environment in terms of virological pressure. One control animal carried a derivative strain of a commercial vaccine strain with ORF5 98.4 % sequence identity, and from one vaccinated animal the Ingelvac PRRSFLEX® EU vaccine strain was isolated.
Summarizing the data we can conclude that piglets vaccinated at 2 and 3 weeks of age with Ingelvac PRRSFLEX® EU were protected against natural infection both in terms of viraemia levels and proportion of viraemic animals.
The authors thank Dr. M. Vanselow for statistical services, and Borbala Vegh for technical laboratory support.
Availability of data and materials
ORF5 sequences published in this study are deposited in the GenBank under the Accession Numbers: KU168911-KU168955
GenBank Accession Number of the full length genome sequence of the PRRSFLEX® EU MLV strain is: KT988004.
GyB was the investigator of the trial, and wrote the manuscript, GyP conducted the whole trial, CK, KD designed and monitored the study, analyzed the results and reviewed the manuscript. All authors read and approved the final manuscript.
Boehringer Ingelheim Vetmedica GmbH (Ingelheim, Germany) funded the trial through the Szent-Istvan University, Faculty of Veterinary Science, Department of Pathology and Forensic Veterinary Medicine. All trial materials were supplied by the funding body and blind serological tests were performed in the laboratories of bioScreen GmbH (Münster, Germany). Two of the authors (CK, KD) are employees of the funding body.
Ethics approval and consent to participate
The study protocol, including ethical and welfare aspects was approved and authorized by the Hungarian National Food Chain Safety Office Directorate of Veterinary Medical Products (no. VU-05623-AM01) according to the Decree No. 128/2009.(X.6.) of the Ministry of Agriculture and Rural Development.
According to the contracts Boehringer Ingelheim Vetmedica has the exclusive and unlimited right to publish data and results obtained within the framework of the study or to release such information to third parties for publication.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
- Lunney JK, Benfield DA, Rowland RR. Porcine reproductive and respiratory syndrome virus: an update on an emerging and re-emerging viral disease of swine. Virus Res. 2010;154:1–6.View ArticlePubMedGoogle Scholar
- Holtkamp DJ, Kliebenstein JB, Neumann EJ, et al. Assessment of the economic impact of porcine reproductive and respiratory syndrome virus on United States pork producers. J Swine Health Prod. 2013;21:72–84.Google Scholar
- Wensvoort G, Terpstra C, Pol JM, ter Laak EA, Bloemraad M, de Kluyver EP, et al. Mystery swine disease in The Netherlands: the isolation of Lelystad virus. Vet Quart. 1991;13:121–8.View ArticleGoogle Scholar
- Keffaber KK. Reproductive failure of unknown etiology. Am Association Swine Pract Newsl. 1989;1:1–9.Google Scholar
- Faaberg KS, Balasuriya UB, Brinton MA, Gorbalenya AE, Leung FC-C, et al. Arteriviridae. In: Virus taxonomy, 9th report of the International Committee on taxonomy of viruses. London: Elsevier/Academic Press; 2012.Google Scholar
- Snijder EJ, Meulenberg JJ. The molecular biology of arteriviruses. J Gen Virol. 1998;79:961–9.View ArticlePubMedGoogle Scholar
- Johnson CR, Griggs TF, Gnanandarajah J, Murtaugh MP. Novel structural protein in porcine reproductive and respiratory syndrome virus encoded by an alternative ORF5 present in all arteriviruses. J Gen Virol. 2011;92:1107–16.View ArticlePubMedPubMed CentralGoogle Scholar
- Firth AE, Zevenhoven-Dobbe JC, Wills NM, Go YY, Balasuriya UB, Atkins JF, et al. Discovery of a small arterivirus gene that overlaps the GP5 coding sequence and is important for virus production. J Gen Virol. 2011;92:1097–106.View ArticlePubMedPubMed CentralGoogle Scholar
- Fang Y, Treffers EE, Li Y, Tas A, Sun Z, van der Meer Y, de Ru AH, van Veelen PA, Atkins JF, Snijder EJ, Firth AE. Efficient −2 frameshifting by mammalian ribosomes to synthesize an additional arterivirus protein. Proc Natl Acad Sci U S A. 2012;109:E2920–8.View ArticlePubMedPubMed CentralGoogle Scholar
- Li Y, Treffers EE, Napthine S, Tas A, Zhu L, Sun Z, Bell S, Mark BL, van Veelen PA, van Hemert MJ, Firth AE, Brierley I, Snijder EJ, Fang Y. Transactivation of programmed ribosomal frameshifting by a viral protein. Proc Natl Acad Sci U S A. 2014;111:E2172–81.View ArticlePubMedPubMed CentralGoogle Scholar
- Snijder EJ, Kikkert M, Fang Y. Arterivirus molecular biology and pathogenesis. J Gen Virol. 2013;94:2141–63.View ArticlePubMedGoogle Scholar
- Olasz F, Denes B, Balint A, Magyar T, Belak S, Zadori Z. Immunological and biochemical characterization of 7ap, a short protein translated from an alternative frame of ORF7 of PRRSV. Acta Vet Hung. 2016;64:273–87.View ArticlePubMedGoogle Scholar
- Allende R, Lewis TL, Lu Z, Rock DL, Kutish GF, Ali A, et al. North American and European porcine reproductive and respiratory syndrome viruses differ in non-structural protein coding regions. J Gen Virol. 1999;80:307–15.View ArticlePubMedGoogle Scholar
- Stadejek T, Oleksiewicz MB, Scherbakov AV, Timina AM, Krabbe JS, Chabros K, Potapchuk D. Definition of subtypes in the European genotype of porcine reproductive and respiratory syndrome virus: nucleocapsid characteristics and geographical distribution in Europe. Arch Virol. 2008;153:1479–88.View ArticlePubMedGoogle Scholar
- Stadejek T, Stankevicius A, Murtaugh MP, Oleksiewicz MB. Molecular evolutionof PRRSV in Europe: current state of play. Vet Microbiol. 2013;165:21–8.View ArticlePubMedGoogle Scholar
- Corzo CA, Mondaca E, Wayne S, Torremorell M, Dee S, Davies P, Morrison RB. Control and elimination of porcine reproductive and respiratory syndrome virus. Virus Res. 2010;154:185–92.View ArticlePubMedGoogle Scholar
- Labarque G, Reeth KV, Nauwynck H, Drexler C, Van Gucht S, Pensaert M. Impact of genetic diversity of European-type porcine reproductive and respiratory syndrome virus strains on vaccine efficacy. Vaccine. 2004;22:4183–90.View ArticlePubMedGoogle Scholar
- Prieto C, Alvarez E, Martínez-Lobo FJ, Simarro I, Castro JM. Similarity of European porcine reproductive and respiratory syndrome virus strains to vaccine strain is not necessarily predictive of the degree of protective immunity conferred. Vet J. 2008;175:356–63.View ArticlePubMedGoogle Scholar
- Martelli P, Gozio S, Ferrari L, Rosina S, De Angelis E, Quintavalla C, Bottarelli E, Borghetti P. Efficacy of a modified live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine in pigs naturally exposed to a heterologous European (Italian cluster) field strain: Clinical protection and cell-mediated immunity. Vaccine. 2009;27:3788–99.View ArticlePubMedGoogle Scholar
- Geldhof MF, Van Breedam W, De Jong E, Lopez Rodriguez A, Karniychuk UU, Vanhee M, Van Doorsselaere J, Maes D, Nauwynck HJ. Antibody response and maternalimmunity upon boosting PRRSV-immune sows with experimental farm-specific andcommercial PRRSV vaccines. Vet Microbiol. 2013;167:260–71.View ArticlePubMedGoogle Scholar
- Cano JP, Dee SA, Murtaugh MP, Pijoan C. Impact of a modified-live porcine reproductive and respiratory syndrome virus vaccine intervention on a population of pigs infected with a heterologous isolate. Vaccine. 2007;25:4382–91.View ArticlePubMedGoogle Scholar
- Linhares DC, Cano JP, Wetzell T, Nerem J, Torremorell M, Dee SA. Effect of modified-live porcine reproductive and respiratory syndrome virus (PRRSv) vaccine on the shedding of wild-type virus from an infected population of growing pigs. Vaccine. 2012;30:407–13.View ArticlePubMedGoogle Scholar
- Balka G, Hornyák A, Bálint A, Kiss I, Kecskeméti S, Bakonyi T, Rusvai M. Genetic diversity of porcine reproductive and respiratory syndrome virus strains circulating in Hungarian swine herds. Vet Microbiol. 2008;127:128–35. 5.View ArticlePubMedGoogle Scholar
- Osorio FA, Galeota JA, Nelson E, Brodersen B, Doster A, Wills R, Zuckermann F, Laegreid WW. Passive transfer of virus-specific antibodies confers protection against reproductive failure induced by a virulent strain of porcine reproductive and respiratory syndrome virus and establishes sterilizing immunity. Virology. 2002;302:9–20.View ArticlePubMedGoogle Scholar
- Lopez OJ, Oliveira MF, Garcia EA, Kwon BJ, Doster A, Osorio FA. Protection against porcine reproductive and respiratory syndrome virus (PRRSV) infection through passive transfer of PRRSV-neutralizing antibodies is dose dependent. Clin Vaccine Immunol. 2007;14:269–75.View ArticlePubMedPubMed CentralGoogle Scholar
- Martínez-Lobo FJ, Díez-Fuertes F, Simarro I, Castro JM, Prieto C. Porcine reproductive and respiratory syndrome virus isolates differ in their susceptibility to neutralization. Vaccine. 2011;29:6928–40.PubMedGoogle Scholar
- Díaz I, Darwich L, Pappaterra G, Pujols J, Mateu E. Different European-type vaccines against porcine reproductive and respiratory syndrome virus have different immunological properties and confer different protection to pigs. Virology. 2006;351:249–59.View ArticlePubMedGoogle Scholar
- Hodgins DC, Shewen PE. Vaccination of neonates: problem and issues. Vaccine. 2012;30:1541–59.View ArticlePubMedGoogle Scholar
- Niewiesk S. Maternal antibodies: clinical significance, mechanism of interference with immune responses, and possible vaccination strategies. Front Immunol. 2014;5:446. doi:10.3389/fimmu.2014.00446.View ArticlePubMedPubMed CentralGoogle Scholar
- Wittmann G, Ohlinger V. Aujeszky’s disease vaccination and infection of pigs with maternal immunity: effects on cell- and antibody-mediated immunity. Arch Virol. 1987;92:87–101.View ArticlePubMedGoogle Scholar
- Kitikoon P, Nilubol D, Erickson BJ, Janke BH, Hoover TC, Sornsen SA, Thacker EL. The immune response and maternal antibody interference to a heterologous H1N1 swine influenza virus infection following vaccination. Vet Immunol Immunopathol. 2006;112:117–28.View ArticlePubMedGoogle Scholar
- Hodgins DCS, Shewen PE, Dewey CE. Influence of age and maternal antibodies on antibody responses of neonatal piglets vaccinated against Mycoplasma hyopneumoniae. J Swine Health Prod. 2004;12:10–6.Google Scholar
- Bandrick M, Theis K, Molitor TW. Maternal immunity enhances Mycoplasma hyopneumoniae vaccination induced cell-mediated immune responses in piglets. BMC Vet Res. 2014;10:124.View ArticlePubMedPubMed CentralGoogle Scholar
- van Woensel PAM, Liefkens K, Demaret S. Vaccination of MDA positive pigs with a live modified PRRSV vaccine. Vet Res. 2000;31:96.View ArticleGoogle Scholar
- Murtaugh MP, Genzow M. Immunological solutions for treatment and prevention of porcine reproductive and respiratory syndrome (PRRS). Vaccine. 2011;29:8192–204.View ArticlePubMedGoogle Scholar