The PRRS virus belongs to the genus Arterivirus, the only genus in the family Arteriviridae, placed with the families Mesoniviridae, Roniviridae and Coronaviridae in the order Nidovirales.
EAV: Equine arteritis virus
WPDV: Wobbly possum disease virus
SHFV: Simian haemorrhagic fever virus
LDV: Lactate dehydrogenase-elevating virus (mice)
PRRSV: Porcine reproductive and respiratory syndrome virus
All species in the genus Arterivirus have a nucleotide similarity of around 55-65%. Although arteriviruses share an evolutionary relationship with other members of the order, several of their characteristics are unique: genomic organisation; high genetic variability; morphology of the virion; limited cellular specificity (all replicate in alveolar macrophages, apart from WPDV, for which the cell type is yet unknown); and capacity to cause persistent or, in some cases, asymptomatic infections.
PRRS virus contains a non-segmented, single-stranded, positive-sense RNA genome. The RNA genome is composed of ten open reading frames (ORFs); its size is approximately 15 kilobases.
Open reading frames (ORF) 1a and 1b encode non-structural proteins (Nsp’s). These proteins are expressed only during the replication and they are not part of the virion.
The eight viral structural proteins are encoded by ORFs 2-7. They include major and minor structural proteins:
Minor structural proteins: three N-glycosylated: GP2a, GP3 and GP4, and one non-glycosylated: E or 2b. They are essential only for viral infectivity.
Major structural proteins: major envelope glycoprotein GP5, membrane non-glycosylated protein M, and nucleocapsid protein N. They are essential for particle formation and viral infectivity. Also, a novel structural protein called ORF5a has been recently described; it may be essential for cell tropism and viability of the virion.
The virion RNA serves as both genome and viral RNA messenger; therefore, it can directly encode the proteins.
The PRRS virion is spherical, about 45-80 nm in diameter. Recently, cryo-electron tomographic studies have suggested that the viral nucleocapsid has an asymmetric shape, rather than the isometric core previously assumed. The nucleocapsid, which is formed by the N protein and the RNA genome, is surrounded by the lipid-containing envelope.
As an RNA virus, it is considered that PRRS virus is a fairly labile virus in the environment. It is important to note that temperature and humidity greatly affect its viability. This data should be taken account in the control of the disease and when samples are stored or sent to the lab.
PRRS virus is easily inactivated by phenol, formaldehyde and most common detergents and disinfectants. Also, it is very sensitive to pH values lower than 6 or higher than 7.5. Therefore,routine clean-up and disinfection procedures (and drying) can inactivate it.
Since the beginning, two PRRS virus genotypes were established: European or Type 1 (prototype Lelystad virus) and North-American or Type 2 (prototype VR-2332). Interestingly, similarity between both prototype nucleotide sequences is as low as 55%. More importantly, intratype pairwise nucleotide sequence variation exceeds 20% (up to 30% in type 1 and around 21% in type 2).
One hallmark of PRRS virus is its extremely high genetic diversity.
A significant quantity of subtypes or clades has been identified in both genotypes. Within type 1, there are at least 4 different subtypes; subtype I is predominant in Western Europe, whereas subtypes II-IV are isolated only in countries eastwards of Poland. It seems that genetic variability within subtypes II-IV is higher than within subtype I (18.2% vs. 11.9% using ORF5, respectively). Regarding type 2, although subtypes have not been clearly defined, several lineages exist in two well-defined clades; seven lineages made up by North-American isolates and two lineages exclusively consisting of isolates from South-East Asia.
It has been described that the allocation of genetic diversity is not constant along the PRRS virus genome. Thus, it seems that diversity occurs more frequently in ORF1a, ORF3 and ORF4.
It is important to note that phylogenetic analyses to determine subtypes and lineages have generally been based on ORF5, which represents a low percentage of the total viral genome. We cannot rule out that this classification may change in the future using the whole genome.
Random mutations. As an RNA virus, PRRS virus has an RNA polymerase that does not have the ability to correct the inherent common errors that occur during the transcription of RNA. Since these errors appear every 100-1,000 nucleotides, each new virus may be different from the previous one. The PRRS virus mutation rate is assumed to be the highest so far for a virus, being up to 40 times higher than the mutation rates of well-known viruses, such as avian influenza virus or human immunodeficiency virus. As a consequence of these mutations, a number of new PRRS viral variants with nucleotide sequences different to the parents' one are produced; phenomenon known as quasi species.
Recombination. Mutation phenomena on their own cannot explain the presence of deletions and insertions in numerous locations all along the PRRS virus genome. When the full-length genome sequences have been examined, recombination has been detected across the genome. Recombination among PRRS virus strains has frequently been demonstrated in both genotypes, indicating that this phenomenon is also very important in PRRS virus genetic diversity.
Immune selection. During the infection, predominant variants would be those variants that possess changes in their genome which represent an advantage compared to the parental strain, such as ability to escape from the immune response. Therefore, as PRRS virus variants are constantly created, producing new forms of antigens, host immune responses could cause selection pressure.
ELISA. Antibodies against both types 1 and 2 can be recognised in the same ELISA but serological differentiation is also possible using ELISA made for this purpose. Nevertheless, some authors have already reported that using most common commercial ELISAs, it is possible to observe individual false negatives when antibodies against some strains are analysed, especially those belonging to subtypes II-IV.
PCR. There are few conserved regions in the PRRS virus genome. Obviously, this can complicate the PCR design. Thus, small changes in the genome region used as a target for the PCR may mean that a variable percentage of strains will not be detected.
New strains. In the last ten years, highly pathogenic strains, among them HP-PRRSV in Asia, have emerged. The ever-expanding diversity of the virus makes it likely that new strains will emerge, it being impossible to predict whether new strains will be more pathogenic or not and their impact on the host’s immunity.
Immune responses. During an infection, quasispecies phenomenon might contribute to the creation of immune escape mutants. We assume that PRRS virus genetic diversity is one of the major causes for the partial or complete lack of protection against re-infections. Also, it is one of the reasons behind vaccines not affording universal protection. Nevertheless, specific regions of the genome involved in the protective immunity still remain unknown, so the cross-protection between two strains cannot be predicted by comparing their genomic sequences.
Since the disease emerged, PRRS virus has disseminated worldwide, gained genetic diversity and, in particular cases, increased its virulence. The basis for the virulence, however, remains unknown, despite being investigated with a variety of different approaches.
Generally speaking, it seems that type 1 strains induce less severe respiratory disease than strains belonging to type 2. Also, the most pathogenic outbreaks reported until now have always been related to type 2 isolates. However, it is important to emphasize that there is a severe lack of data on the pathogenesis of Eastern European isolates (subtypes II, III and IV). Therefore, we cannot completely rule out the possibility that highly pathogenic strains could exist within type 1. Independent of subtypes and genotypes, PRRS virus isolates vary markedly in virulence. Thus, at a regional level, it is possible to detect more and less virulent strains.
Atypical pathogenic PRRS virus strains have emerged around the world. The risk of a new highly-pathogenic PRRS virus emerging and/or its worldwide spreading is a real threat.
Some examples of virulent strains are described below:
Subtype III (Type 1) in Europe:
Karniychuk and collaborators (2010) investigated the pathogenesis and antigenic characteristics of type 1 Lena strain, which was isolated from a Belarusian farm with reproductive and respiratory clinical signs. The main conclusion of the study was that Lena strain is a highly pathogenic PRRS virus strain belonging to subtype III (type 1). Compared to a conventional subtype I strain from Belgium, Lena infection caused obvious clinical signs. In Lena-infected pigs, high fever, anorexia and depression were the most evident signs. Eventually, four pigs out of ten died after the Lena inoculation. It seems that bacterial complications were enhanced in Lena-infected pigs.
Also, significant virological differences were observed between the subtype I strain from Belgium and Lena. Viral titres in sera and tissues from PRRSV (Lena)-infected pigs were statistically higher. The higher replication observed for the Lena strain could be due to its capacity to replicate in a widerange of macrophage subpopulations.
Type 2 in North-America:
Numerous severe PRRS outbreaks have been reported from North-American farms, mainly in the U.S.A. In the last twenty years, three outbreaks have been particularly significant:
Isolates 142 (1996). Atypical PRRS clinical outbreaks known as Swine abortion and mortality syndrome. It was mainly characterised by mid- or late-term abortions of 10-50% of the sows in a period of a few weeks. Sows and gilts had fever and were anorexic. Mortality was from 5% to 10% in the breeding herds. An increase in preweaning mortality and a decrease in nursery pig performance were also observed. The wild-type isolates involved were known as 142, because of their RFLP patterns.
Isolates 184 (2001 and 2006). RFLP 1-8-4 or Strain MN184. It caused high morbidity (50%) and mortality (20%) rates. ORF5 nucleotide sequence analysis and comparison with other type 2 PRRS virus strains demonstrated that MN184 were significantly different from previous strains. Three quite variable regions were identified, corresponding to nsp1β, nsp2 and ORF5. Nsp2 shared only 66-70% amino acid similarity to other North-American PRRS virus nsp2 proteins.
Type 2 in Asia:
Highly-pathogenic PRRS virus (HP-PRRSV): a special case. In June 2006, a very severe disease called Porcine high fever syndrome emerged in Jiangxi province (People’s Republic of China) and rapidly spread throughout the country. By the end of that year, the disease had already been detected in 16 provinces affecting over 2 million pigs with about 400,000 fatal cases. Subsequent analysis demonstrated that the disease was caused by an atypical highly virulent strain of PRRS virus. Since then, highly-pathogenic PRRS virus (HP-PRRSV) can be isolated around the East, South and North of Asia, including China, Vietnam, Bhutan, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Russia, Singapore, etc. Nowadays, it has become endemic in some of these countries and continues to have a devastating economic impact in all of them. According to ORF5 phylogenetic analysis, it seems that HP-PRRSV might have evolved from strains previously detected in China.
Although the basis for HP-PRRSV virulence remains unknown, some authors claimed that Nsp2, Nsp9 and/or Nsp10 may be involved in the fatal virulence of HP-PRRSV. Other studies have concluded that aberrant immune responses triggered by HP-PRRSV infection could be closely related to acute lung injury.
Allende R, Laegreid WW, Kutish GF, Galeota JA, Wills RW, Osorio FA. Porcine reproductive and respiratory syndrome virus: description of persistence in individual pigs upon experimental infection. J Virol. 2000, 74:10834-7.
Allende R, Kutish GF, Laegreid W, Lu Z, Lewis TL, Rock DL, Friesen J, Galeota JA, Doster AR, Osorio FA. Mutations in the genome of porcine reproductive and respiratory syndrome virus responsible for the attenuation phenotype. Arch Virol. 2000, 145:1149-61.
Chang CC, Yoon KJ, Zimmerman JJ, Harmon KM, Dixon PM, Dvorak CM, Murtaugh MP. Evolution of porcine reproductive and respiratory syndrome virus during sequential passages in pigs. J Virol. 2002, 76:4750-63.
Darwich L, Gimeno M, Sibila M, Diaz I, de la Torre E, Dotti S, Kuzemtseva L, Martin M, Pujols J, Mateu E. Genetic and immunobiological diversities of porcine reproductive and respiratory syndrome genotype I strains. Vet Microbiol. 2011, 150:49-62.
Dea S, Gagnon CA, Mardassi H, Pirzadeh B, Rogan D. Current knowledge on the structural proteins of porcine reproductive and respiratory syndrome (PRRS) virus: comparison of the North American and European isolates. Arch Virol. 2000, 145:659-88.
Díaz I, Gimeno M, Darwich L, Navarro N, Kuzemtseva L, López S, Galindo I, Segalés J, Martín M, Pujols J, Mateu E. Characterization of homologous and heterologous adaptive immune responses in porcine reproductive and respiratory syndrome virus infection. Vet Res. 2012, 19:43:30.
Dokland T. The structural biology of PRRSV. Virus Res. 2010, 154:86-97.
Domingo E, Holland JJ. RNA virus mutations and fitness for survival. Annu Rev Microbiol. 1997, 51:151-78
Dunowska M1, Biggs PJ, Zheng T, Perrott MR. Identification of a novel nidovirus associated with a neurological disease of the Australian brushtail possum (Trichosurus vulpecula). Vet Microbiol. 2012, 156:418-24.
Gimeno M, Darwich L, Diaz I, de la Torre E, Pujols J, Martín M, Inumaru S, Cano E, Domingo M, Montoya M, Mateu E. Cytokine profiles and phenotype regulation of antigen presenting cells by genotype-I porcine reproductive and respiratory syndrome virus isolates. Vet Res. 2011, 18:42:9.
Goldberg TL1, Lowe JF, Milburn SM, Firkins LD. Quasispecies variation of porcine reproductive and respiratory syndrome virus during natural infection. Virology. 2003, 317:197-207.
Halbur P, Bush E. Update on abortion storms and sow mortality. Swine Health Prod. 1997, 5:73.
Han J, Wang Y, Faaberg KS. Complete genome analysis of RFLP 184 isolates of porcine reproductive and respiratory syndrome virus. Virus Res. 2006, 122:175-82.
ICTV. Virus taxonomy: classification and nomenclature of viruses: Ninth Report of the International Committee on Taxonomy of Viruses. Ed: King AMQ, Adams MJ, Carstens EB, Lefkowitz EJ. Elsevier Academic Press. 2012.
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.
Le Gall A, Albina E, Magar R, Gauthier JP. Antigenic variability of porcine reproductive and respiratory syndrome (PRRS) virus isolates. Influence of virus passage in pig. Vet Res. 1997, 28:247-57.
Kapur V, Elam MR, Pawlovich TM, Murtaugh MP. Genetic variation in porcine reproductive and respiratory syndrome virus isolates in the midwestern United States. J Gen Virol. 1996, 77:1271-6.
Lee C, Yoo D. The small envelope protein of porcine reproductive and respiratory syndrome virus possesses ion channel protein-like properties. Virology. 2006, 355:30-43.
Martín-Valls GE, Kvisgaard LK, Tello M, Darwich L, Cortey M, Burgara-Estrella AJ, Hernández J, Larsen LE, Mateu E. Analysis of ORF5 and full-length genome sequences of porcine reproductive and respiratory syndrome virus isolates of genotypes 1 and 2 retrieved worldwide provides evidence that recombination is a common phenomenon and may produce mosaic isolates. J Virol. 2014, 88:3170-81.
Mengeling WL, Lager KM, Vorwald AC. Clinical consequences of exposing pregnant gilts to strains of porcine reproductive and respiratory syndrome (PRRS) virus isolated from field cases of “atypical” PRRS. Am J Vet Res. 1998, 59:1540–4.
Meulenberg JJ, Petersen-den Besten A, De Kluyver EP, Moormann RJ, Schaaper WM, Wensvoort G. Characterization of proteins encoded by ORFs 2 to 7 of Lelystad virus. Virology. 1995, 206:155-63.
Meulenberg JJ, Petersen den Besten A, de Kluyver E, van Nieuwstadt A, Wensvoort G, Moormann RJ. Molecular characterization of Lelystad virus. Vet Microbiol. 1997, 55:197-202.
Morgan SB, Frossard JP, Pallares FJ, Gough J, Stadejek T, Graham SP, Steinbach F, Drew TW, Salguero FJ. Pathology and virus distribution in the lung and lymphoid tissues of pigs experimentally inoculated with three distinct type 1 prrs virus isolates of varying pathogenicity. Transbound Emerg Dis. 2014. doi: 10.1111/tbed.12272.
Murtaugh MP, Elam MR, Kakach LT. Comparison of the structural protein coding sequences of the VR-2332 and Lelystad virus strains of the PRRS virus. Arch Virol. 1995, 140:1451-60.
Murtaugh MP, Yuan S, Faaberg KS. Appearance of novel PRRSV isolates by recombination in the natural environment. Adv Exp Med Biol. 2001, 494:31-6.
Murtaugh MP, Stadejek T, Abrahante JE, Lam TT, Leung FC. The ever-expanding diversity of porcine reproductive and respiratory syndrome virus. Virus Res. 2010, 154:18-30.
Music N, Gagnon CA. The role of porcine reproductive and respiratory syndrome (PRRS) virus structural and non-structural proteins in virus pathogenesis. Anim Health Res Rev. 2010, 11:135-63.
Oleksiewicz MB, Stadejek T, Maćkiewicz Z, Porowski M, Pejsak Z. Discriminating between serological responses to European-genotype live vaccine and European-genotype field strains of porcine reproductive and respiratory syndrome virus (PRRSV) by peptide ELISA. J Virol Methods. 2005, 129:134-44.
Rowland RR, Steffen M, Ackerman T, Benfield DA. The evolution of porcine reproductive and respiratory syndrome virus: quasispecies and emergence of a virus subpopulation during infection of pigs with VR-2332. Virology. 1999, 259:262-6.
Shi M, Lam TT, Hon CC, Hui RK, Faaberg KS, Wennblom T, Murtaugh MP, Stadejek T, Leung FC. Molecular epidemiology of PRRSV: a phylogenetic perspective. Virus Res. 2010, 154:7-17.
Snijder EJ, Meulenberg JJ. The molecular biology of arteriviruses. J Gen Virol. 1998, 79:961-79. Snijder EJ, Dobbe JC, Spaan WJ. Heterodimerization of the two major proteins is essential for arterivirus infectivity. J Virol. 2003, 77:97-104. Snijder EJ, Kikkert M, Fang Y. Arterivirus molecular biology and pathogenesis. J Gen Virol. 2013, 94:2141-63.
Spilman MS, Welbon C, Nelson E, Dokland T. Cryo-electron tomography of porcine reproductive and respiratory syndrome virus: organization of the nucleocapsid. J Gen Virol. 2009, 90:527-35.
Stadejek T, Oleksiewicz MB, Potapchuk D, Podgórska K. Porcine reproductive and respiratory syndrome virus strains of exceptional diversity in Eastern Europe support the definition of new genetic subtypes. J Gen Virol. 2006, 87:1835-41.
Stadejek T, Stankevicius A, Murtaugh MP, Oleksiewicz MB. Molecular evolution of PRRSV in Europe: current state of play. Vet Microbiol. 2013, 165:21-8.
Sun L, Li Y, Liu R, Wang X, Gao F, Lin T, Huang T, Yao H, Tong G, Fan H, Wei Z, Yuan S. Porcine reproductive and respiratory syndrome virus ORF5a protein is essential for virus viability. Virus Res. 2013, 171:178-85.
Tian D, Wei Z, Zevenhoven-Dobbe JC, Liu R, Tong G, Snijder EJ, Yuan S. Arterivirus minor envelope proteins are a major determinant of viral tropism in cell culture. J Virol. 2012, 86:3701-12.
Truong HM, Lu Z, Kutish GF, Galeota J, Osorio FA, Pattnaik AK. A highly pathogenic porcine reproductive and respiratory syndrome virus generated from an infectious cDNA clone retains the in vivo virulence and transmissibility properties of the parental virus. Virology. 2004, 325:308–19.
Van Vugt JJ, Storgaard T, Oleksiewicz MB, Bøtner A. High frequency RNA recombination in porcine reproductive and respiratory syndrome virus occurs preferentially between parental sequences with high similarity. J Gen Virol. 2001, 82:2615-20.
Weiland E, Wieczorek-Krohmer M, Kohl D, Conzelmann KK, Weiland F. Monoclonal antibodies to the GP5 of porcine reproductive and respiratory syndrome virus are more effective in virus neutralization than monoclonal antibodies to the GP4. Vet Microbiol. 1999, 66:171-86.
Wissink EH, Kroese MV, van Wijk HA, Rijsewijk FA, Meulenberg JJ, Rottier PJ. Envelope protein requirements for the assembly of infectious virions of porcine reproductive and respiratory syndrome virus. J Virol. 2005, 79:12495-506.
Zimmerman JJ, Benfield DA, Dee SA, Murtaugh MP, Stadejek T, Stevenson GW, Torremorell M. Porcine reproductive and respiratory syndrome virus (porcine arterivirus). In: 10th ed. Diseases of swine, Ed. Wiley-Blackwell. 2012, 31:463-86.