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