Elsevier

Virus Research

Volume 203, 4 May 2015, Pages 24-35
Virus Research

Pathogenicity of three type 2 porcine reproductive and respiratory syndrome virus strains in experimentally inoculated pregnant gilts

https://doi.org/10.1016/j.virusres.2015.03.005Get rights and content

Highlights

  • PRRSV strain KS06-483 appeared less virulent than NVSL 97-7895 and KS06-72109.

  • High and low virulent PRRSV strains were equally able to cross the placental barrier.

  • Important cytokines in reproductive PRRS were CCL2, IFNα and IFNγ.

  • Higher PRRS viral loads in fetuses increased odds of fetal death.

  • Higher frequency of lesions in fetal and maternal tissues in dead versus live infected fetuses.

Abstract

Mechanisms of reproductive failure resulting from infection with porcine reproductive and respiratory syndrome virus (PRRSV) are still poorly understood. Presented herein are the results of a side-by-side evaluation of the pathogenicity of three type 2 PRRSV strains in a reproductive model, from a pilot study used to develop experimental conditions and laboratory methods for a larger experiment. Pregnant gilts were experimentally infected with PRRSV at gestation day 85 or served as uninfected negative controls. After 21 days, all gilts and fetuses were necropsied. Clinical signs, litter outcome, viral load, cytokine levels, and pathology were compared from samples collected among pigs exposed to the three PRRSV strains. Based on differences in histologic lesions, and fetal weights, and numeric differences in gilt serum cytokine levels, litter outcome and virus replication in fetal tissues KS06-483 appeared less virulent than NVSL 97-7895 and KS06-72109 isolates. Levels of chemokine ligand 2 (CCL2), interferon alpha (IFNα), and interferon gamma (IFNγ) were increased in PPRRSV-infected compared to non-infected gilts (0.01 > P < 0.06). Inoculation with NVSL 97-7895 induced higher levels of all three cytokines. All three PRRSV isolates were able to induce high mean viral load in individual litters, which was closely related to the proportion of PRRSV positive fetuses in the litter. Viral load in fetal samples was also positively associated with viral load at the maternal–fetal interface. All but one dead fetus were positive for PRRSV RNA, and higher concentrations of PRRSV RNA in fetal thymus increased the odds of fetal death. Our results suggest that virus replication in fetal tissues and the maternal–fetal interface, but not in other gilt tissues, are important for the outcome of reproductive PRRS. Additionally, our data indicate that umbilical lesions decreased corresponding to the use of pentobarbital sedation prior to euthanasia of pregnant gilts by captive bolt.

Introduction

Porcine reproductive and respiratory syndrome (PRRS) is one of the most costly diseases affecting the global swine industry (Holtkamp et al., 2013, Nieuwenhuis et al., 2012). Although reproductive disease associated with PRRS virus (PRRSV) contributes to over $300 million in losses annually in the USA alone (Holtkamp et al., 2013), a proportionately small amount of research has focused on the reproductive form of the disease, and the underlying mechanisms of PRRSV-induced reproductive failure are still poorly understood. The outcome of infection in pregnant sows and gilts largely depends on the stage of gestation. In early gestation, PRRSV can cause embryonic death (Prieto et al., 1996, Prieto et al., 1997), while in mid-gestation the virus does not readily cross transplacentally and does not induce reproductive failure (Christianson et al., 1993, Kranker et al., 1998). PRRSV infection in late gestation consistently results in transplacental infection of fetuses and clinical manifestation characterized by abortions, early farrowings, fetal death, and the birth of weak, congenitally infected piglets resulting in elevated pre-weaning mortality (Cheon and Chae, 2001, Kranker et al., 1998, Mengeling et al., 1994, Terpstra et al., 1991). The mechanisms of transplacental infection and fetal death remain unclear. It has been proposed that PRRSV-induced fetal death is not a direct result of PRRSV replication in the fetus, but is rather related to virus replication in the maternal–fetal interface resulting in apoptosis and necrosis of infected and surrounding cells leading to detachment and degeneration of the fetal placenta (Karniychuk et al., 2011, Karniychuk et al., 2012).

PRRSV can be divided into two distinct genotypes: type 1 (European) and type 2 (North American). PRRSV strains show a high degree of genetic diversity, both between and within genotypes, leading to the sub-classification into at least 3 European subtypes (Stadejek et al., 2008) and multiple North American clades (Shi et al., 2010a, Shi et al., 2010b). Different strains also display important biological differences, such as the ability and/or efficiency of propagation in different cell types (Benfield et al., 1992, de Abin et al., 2009), pathogenicity (Halbur et al., 1995, Halbur et al., 1996, Han et al., 2013a, Martinez-Lobo et al., 2011, Morgan et al., 2013) and antigenic properties (Magar et al., 1997, Nelson et al., 1993, Wensvoort et al., 1992). Investigations of the pathogenicity of different PRRSV isolates were mainly performed in young pigs using respiratory models whereas relatively few studies have evaluated differences in pathogenicity of PRRSV isolates in a reproductive model (Cheon and Chae, 2004, Mengeling et al., 1996).

The present experiment was a pilot study to develop experimental conditions and laboratory methods for a larger experiment investigating phenotypic and genotypic predictors of PRRSV resistance in pregnant gilts (Ladinig et al., 2014b). Yet it is one of a select few experiments enabling side-by-side comparison of distinct strains in a reproductive model. Three type 2 PRRSV isolates were used to inoculate late-term pregnant gilts. The first and main objective of this study was to compare the pathogenicity of the three PRRSV isolates by investigating clinical signs, litter outcome, and levels of PRRSV RNA in fetal tissues. This information was used to select the virus isolate used in the main project. Given the differences we found amongst the three isolates in objective 1 and results of cytokine responses we subsequently obtained in our main project (Ladinig et al., 2014a), the second objective was to explore strain differences in histologic lesion severity in fetal tissues and the maternal–fetal interface, as well as cytokine profiles in gilt serum. This enabled us to gain insights into potential mechanisms of PRRSV reproductive pathogenicity.

Section snippets

Animals

Purebred Landrace gilts from a high-health nucleus herd (free of PRRSV, Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae based on absence of clinical signs and routine serologic monitoring) were selected at approximately 150 days of age in two experimental repetitions (rep one: n = 7; rep two: n = 8). Gilts were vaccinated against porcine parvovirus (PPV), erysipelas, Leptospira spp. (Farrowsure Gold B, Zoetis Animal Health, Canada, Kirkland, QC) twice prior to breeding, porcine

Clinical signs and litter outcomes

No gilt demonstrated respiratory signs including dyspnea or persistent paroxysmal coughing, or showed signs of lethargy or depression following infection with any of the three virus isolates. Reduced daily feed intake was observed in 4/4 gilts infected with NVSL 97-7895, and 2/4 gilts each with KS06-483 and KS06-72109. While 3/4 NVSL 97-7895 infected gilts and the two KS06-483 infected gilts had reduced feed intake for only one to three days, the two gilts infected with KS06-72109 were more

Discussion

Since the present pilot study was used to set up experimental conditions and laboratory methods for a larger experiment investigating phenotypic and genotypic predictors of PRRSV resistance in pregnant gilts (Ladinig et al., 2014b), the number of experimental animals per treatment group was small, but comparable with other reproductive experiments in pigs. Therefore, differences among treatment groups in some of the measured parameters, although often remarkable, were not statistically

Acknowledgements

The authors wish to acknowledge the numerous technicians and students from the Western College of Veterinary Medicine, Vaccine and Infectious Disease Organization, Prairie Diagnostic Services, Inc. and University of Alberta who assisted with this project. We offer special thanks to those taking leadership roles including Stewart Walker, Don Wilson, Tuanjie Chang, Linda Ye and Lois Ridgeway, to Ian Dohoo for his guidance with statistical analyses, and to Graham Plastow for the overall

References (52)

  • S.B. Morgan et al.

    Increased pathogenicity of European porcine reproductive and respiratory syndrome virus is associated with enhanced adaptive responses and viral clearance

    Vet. Microbiol.

    (2013)
  • F.A. Osorio et al.

    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)
  • C. Prieto et al.

    Transplacental infection following exposure of gilts to porcine reproductive and respiratory syndrome virus at the onset of gestation

    Vet. Microbiol.

    (1997)
  • R.R. Rowland et al.

    Lymphoid tissue tropism of porcine reproductive and respiratory syndrome virus replication during persistent infection of pigs originally exposed to virus in utero

    Vet. Microbiol.

    (2003)
  • M. Shi et al.

    Molecular epidemiology of PRRSV: a phylogenetic perspective

    Virus Res.

    (2010)
  • K. Van Reeth et al.

    Differential production of proinflammatory cytokines in the pig lung during different respiratory virus infections: correlations with pathogenicity

    Res. Vet. Sci.

    (1999)
  • E. Albina et al.

    Interferon-alpha response to swine arterivirus (PoAV), the porcine reproductive and respiratory syndrome virus

    J. Interferon Cytokine Res.

    (1998)
  • D.A. Benfield et al.

    Characterization of swine infertility and respiratory syndrome (SIRS) virus (isolate ATCC VR-2332)

    J. Vet. Diagn. Invest.

    (1992)
  • W. Buddaert et al.

    In vivo and in vitro interferon (IFN) studies with the porcine reproductive and respiratory syndrome virus (PRRSV)

    Adv. Exp. Med. Biol.

    (1998)
  • W.T. Christianson et al.

    Pathogenesis of porcine reproductive and respiratory syndrome virus infection in mid-gestation sows and fetuses

    Can. J. Vet. Res.

    (1993)
  • M.F. de Abin et al.

    Comparative infection efficiency of Porcine reproductive and respiratory syndrome virus field isolates on MA104 cells and porcine alveolar macrophages

    Can. J. Vet. Res.

    (2009)
  • P.L. Delputte et al.

    IFN-alpha treatment enhances porcine Arterivirus infection of monocytes via upregulation of the porcine Arterivirus receptor sialoadhesin

    J. Interferon Cytokine Res.

    (2007)
  • V. Dwivedi et al.

    Evaluation of immune responses to porcine reproductive and respiratory syndrome virus in pigs during early stage of infection under farm conditions

    Virol. J.

    (2012)
  • J. Felsenstein

    PHYLIP – phylogeny inference package (version 3.2)

    Cladistics

    (1989)
  • W. Feng et al.

    In utero infection by porcine reproductive and respiratory syndrome virus is sufficient to increase susceptibility of piglets to challenge by Streptococcus suis type II

    J. Virol.

    (2001)
  • J. Gomez-Laguna et al.

    Changes in lymphocyte subsets and cytokines during European porcine reproductive and respiratory syndrome: increased expression of IL-12 and IL-10 and proliferation of CD4(-)CD8(high)

    Viral Immunol.

    (2009)
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