Transboundary and Emerging Diseases
REVIEW ARTICLE
Challenges and Opportunities for the Control and Elimination of Porcine Reproductive and Respiratory Syndrome Virus R. R. R. Rowland1 and R. B. Morrison2 1 2
Department of Diagnostic Medicine and Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS, USA Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN, USA
Keywords: porcine reproductive and respiratory syndrome virus; PPRS; regional control; elimination Correspondence: R. R. R. Rowland. Department of Diagnostic Medicine and Pathobiology, College of Veterinary Medicine, 1800 Denison Ave, Kansas State University, Manhattan, KS 66506, USA. Tel.: +785 532 4631; Fax: +785 532 4481; E-mail: [email protected]
Summary The control and elimination of porcine reproductive and respiratory syndrome virus (PRRSV) represent two of the most challenging tasks facing the pig industry worldwide. Several factors related to the biology of the virus make disease detection and elimination difficult. Efforts are further hampered by the lack of vaccines that can protect naı¨ve herds from infection. With this in mind, elimination efforts are being initiated which incorporate existing tools and knowledge. A new approach extends herd control strategies to the level of a region. One example of success in PRRSV regional elimination is the Stevens County project in Minnesota.
Received for publication July 28, 2011 doi:10.1111/j.1865-1682.2011.01306.x
Biology epidemiology and ecology Porcine reproductive and respiratory syndrome (PRRS), initially described in the late 1980s as ‘Mystery Swine Disease’, is associated with reproductive failure in sows, respiratory distress in nursing pigs, and poor growth performance during finishing. Porcine reproductive and respiratory syndrome has become endemic worldwide, with only the continents of Australia and Antarctica free from the virus. The causative agent of PRRS, the PRRS virus (PRRSV), was first isolated and identified by investigators in the Netherlands in 1991 (Wensvoort et al., 1991) and shortly thereafter in the United States (Benfield et al., 1992). North American and European viruses share only about 67% identity at the nucleotide level; therefore, European isolates are designated as Type 1 genotype viruses and North American isolates as Type 2. Type 1 viruses of European origin were first identified in US herds in 1999. The presence of two distinct genotypes
with diverse antigenic properties further complicates efforts to control PRRS (Fang et al., 2007). The properties of the virus that present the greatest challenges to PRRS control and elimination include (i) a complex virion surface topology and composition, (ii) the ability of the virus to remain persistent in a population, (iii) avoidance and subversion of innate and host immune responses, and (iv) the capacity to generate a large degree of genetic diversity in both structural and non-structural proteins (Lunney et al., 2010). Porcine reproductive and respiratory syndrome virus is an enveloped, positive polarity, non-segmented, single-stranded RNA virus belonging to the family Arteriviridae, within the order Nidovirales. The 15–15.5 kb PRRSV genome contains at least 11 ORFs and two untranslated regions flanking the 5¢ and 3¢ ends of the genome (Wootton et al., 2000; Wu et al., 2005; Johnson et al., 2011). The principal non-structural proteins, encoded by ORF1a and ORF1ab, have protease and replicase functions. The 3¢
ª 2012 Blackwell Verlag GmbH • Transboundary and Emerging Diseases. 59 (Suppl. 1) (2012) 55–59
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R. R. R. Rowland and R. B. Morrison
end of the genome codes for at least eight structural proteins. The virion structural protein composition and organization is reviewed in Dokland (2010). The major structural proteins, GP5, matrix (M) and nucleocapsid (N), are derived from ORFs 5, 6 and 7, respectively. GP2, GP3 and GP4 are minor structural proteins critical for virus replication, that represent important targets for neutralization by antibody. The most recently identified proteins are 2b and 5a, which are translated from ORF2 and ORF5, respectively (Wu et al., 2005; Johnson et al., 2011). Resistance to antibody is conferred by the presence of sugar residues on surface proteins, such as GP2, GP3, GP4 and GP5, combined with peptide sequence hypervariability (Fang et al., 2007; Shi et al., 2010; Vu H. et al., 2011). Viral proteins, such as nsp1, participate in the blocking of early innate responses, such as the induction of type 1 interferon (Yoo et al., 2010). At the level of a population, PRRSV is efficiently transmitted both horizontally and vertically. After the initial acute phase of infection, pigs may become subclinical carriers for several months, thus further perpetuating the virus (Rowland et al., 2003). The continued maintenance of the virus as a subclinical infection circulating within a production system is periodically interrupted by outbreaks of acute disease accompanied by high morbidity and mortality. Outbreaks are often linked to the introduction of a new virus combined with a lack of sufficient protection by existing herd immunity. Another possibility is antibody-dependent enhancement of infection, first described in 1996 (Tirado and Yoon, 2003). The most prevalent forms of viral entry into a production system are through the introduction of infected pigs, PRRSV-contaminated semen, and contaminated equipment, such as transportation vehicles. The risk of transmission by aerosols, also known as area spread, remains poorly understood; however, a recent report indicates that certain virus isolates can travel up to 9.8 km under the right experimental and environmental conditions (Otake et al., 2010). Modified live virus (MLV) vaccines became available in the United States in 1994. In general, MLV vaccines have been shown to reduce clinical signs and can aid in recovery following outbreaks of acute PRRS. However, several deficiencies have been noted, including incomplete protection, inability to distinguish infected from vaccinated pigs, and the potential for reversion to virulence (Botner et al., 1997; Mengeling et al., 1999; Darwich et al., 2010). An alternative to vaccination is the intentional infection of naive animals with wild-type live PRRSV either through contact with infected animals or exposure to infectious material. Acclimation with a live virus is an attempt to induce herd immunity against farm-specific strains. However, infection with a virulent virus presents several potential unintended consequences, such as the 56
continuous spread of the virus within a production system and the risk of introducing other pathogens. Environmental instability: a PRRSV Achilles’ Heel A complex biology, immunology and ecology combined with the absence of a vaccine sufficient to meet the needs of virus elimination make control and elimination of PRRSV seemingly impossible tasks. However, there are properties of the virus that can be exploited. For example, PRRSV is relatively unstable under normal environmental conditions and is especially sensitive to UV radiation, changes in pH, and increased temperature (Cutler et al., 2011). Jacobs et al. (2010) calculated T1/2 values of 1.6, 27.4, 84.8, and 155.5 h for temperatures of 30, 20, 10, and 4C, respectively. The virus is completely inactivated after a short incubation at temperatures >56C (Bloemraad et al., 1994). The application of common virucides, disinfectants, or steam is sufficient to completely inactivate PRRSV on surfaces. Furthermore, the virus does not survive in properly cured or cooked pork products. Even though PRRSV has been reported to travel for up to 9.8 km under experimental conditions, aerial transmission of virus over long distances appears to be a low risk and dependent on a set of ‘ideal’ environmental conditions (Otake et al., 2010). And finally, even though PRRSV is considered to be a persistent virus, the level of infection in individual animals eventually decays over time to the point of extinction (Rowland et al., 2003). Control of PRRSV at the herd level Over the years, several approaches have been employed for the control and elimination of PRRSV in single herds (see Corzo et al., 2010 for review). Highly effective approaches include depopulation-repopulation and all-in-all-out methods. Both depend on the placement of PRRSV-free pigs into a facility that is ‘free’ of virus. The application of time and common disinfection methods are sufficient to remove PRRSV from buildings contaminated with virus. Herd closure and rollover is the most common method for eliminating virus from sow farms. The approach is based on the gradual extinction of PRRSV in herds that are closed for approximately 200 days. Sows that remain seropositive are removed and replaced with negative pigs. The most recent tool for pig-dense regions is the use of whole barn filtration to block aerosol entry (Dee et al., 2010). A regional approach to PRRSV control and elimination Using the methods described above, it is relatively easy to eliminate PRRSV from a single herd; however, an
ª 2012 Blackwell Verlag GmbH • Transboundary and Emerging Diseases. 59 (Suppl. 1) (2012) 55–59
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outbreak with a new virus is all but inevitable. A new approach for the stable control and elimination of PRRSV is the implementation of herd control methods that simultaneously target multiple farms within a relatively large region. The rationale for a regional approach is to lessen the risk of re-introduction of virus into any single farm. Current regional elimination projects in the United States are supported by private companies and the USDA-funded PRRS Coordinated Agricultural Project (PRRS CAP). The steps for regional elimination are summarized below. Detailed descriptions of useful tools and biosecurity protocols can be downloaded at the PRRS CAP website (http://www.prrs.org). 1 Define the boundaries of a ‘region’ suitable for conducting PRRSV elimination and determine the level of producer participation. A region for conducting PRRSV elimination can be defined by a set of boundaries consisting of natural and/or man-made barriers, such as lakes, cities, mountains, or areas where a cluster of farms is spatially separated from other pig production sites. The ultimate boundary lines of a regional project are often defined by the group of farms that have a high interest in conducting a regional elimination project. Unlike traditional disease eradication programmes, which involve some level of government enforcement, regional elimination efforts are strictly voluntary. At any time during an elimination project, participants may withdraw. Therefore, ongoing communication and producer engagement are important elements for success. The sociology and psychology of producer participation is an area in need of further study. 2 Record premises characteristics and herd density. Premises can consist of sow farms, boar studs, nurseries, grow-finish farms, etc. Location, population size of each site, and the overall farm density within a region are recorded. Porcine reproductive and respiratory syndrome virus elimination is less problematic in regions dominated by fairly homogenous farm types at relatively low densities as compared to densely populated regions with mixed farm types. 3 Determine PRRSV status at each site. Porcine reproductive and respiratory syndrome virus RT-PCR and serology, common diagnostic tests, are used as to determine the infection status of individual herds. The amount and frequency of testing are determined based on the farm type and level of confidence needed to obtain an accurate result. Holtkamp et al. (2011) describe herd status designations, ranging from PRRSV Positive Unstable (Category 1) to PRRSV Negative (Category 4). This common set of terminologies is useful for communicating information within a region and for developing standardized herd status reporting methods.
PRRS Control and Elimination
4
5
6
7
Assess overall herd biosecurity and risk for introduction of PRRSV. The web-based tool, Production Animal Disease Risk Assessment Program (PADRAP), is useful for assessing overall PRRS biosecurity at the herd level and can be used for estimating the success of a PRRSV elimination program (http://www.padrap.org). When applied at several time points, PADRAP can be used to measure changes in biosecurity over time. Map pig movements between farms within the region and from sources entering from outside the region. As discussed above, a major biosecurity risk for the entry of PRRSV is through the introduction of PRRSV-infected pigs and equipment, such as contaminated trucks. Therefore, a good prospect for PRRSV elimination is a situation where the principal sources of pigs and pig transport are confined to sites within the region (intraregional movement). Pigs that enter from sources outside of the region and possess an unknown PRRSV status represent one the largest threats to success. Implement herd control strategies and report progress. Starting with the herd elimination methods described above (Corzo et al., 2010), a combination of herd control strategies can be initiated that best fit the type and density of pig farms within the region. Regular status reports are important for updating participating producers on progress towards elimination. Open lines of communication, obtainable goals, and clear criteria related to progress are critical to keeping producers engaged in the process. Reported data include the number of pigs and the PRRSV status (Category 1–4) for each herd, as well as a general description of progress, including the identification of obstacles. Publicized progress provides an incentive for PRRSVpositive farms to make progress towards a negative status. Surveillance. Once Category 4 (PRRSV Negative) status is achieved, continued monitoring is important to ensure that farms remain PRRSV negative. The most common method is to monitor for the presence of PRRSV by standard diagnostic serology. The frequency of sampling is variable, but should be conducted at least twice a year. Even though syndromic monitoring is not always an accurate indicator of PRRS, herds are monitored for the appearance of PRRS-associated clinical signs.
Current progress and future technologies At this time, the PRRS CAP supports seven regional elimination projects in the United States, which enroll approximately 2.5 million pigs. Progress and updates for each project can be found at the PRRS CAP website (http://www.prrs.org). The Stevens County project, in
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Minnesota, is the longest running regional elimination project (Corzo et al., 2010). The county is 1490 km sq and contains 87 pig sites (164 000 pigs), including sow farms, boar studs, nurseries and grow-finish sites. Only four farms declined to participate in the project. As a region, Stevens County is relatively isolated from other pig-associated sites. At the beginning of the project in 2004, 29 sites were PRRSV positive, 19 sites PRRSV negative, and the remaining sites were of unknown status. As of 2010, all sites were negative for PRRSV, with only sporadic outbreaks in sow farms. The source of virus in all cases was linked to the import of PRRSV-positive pigs from outside the region. Recently, the project was expanded to include all of Minnesota north of Highway 212. At this time, approximately one million pigs are enrolled in the project. New technologies and methodologies are being employed to improve the effectiveness of PRRSV elimination and lessen the costs. One example is oral fluid testing, which can be used as a substitute for serum in the detection of PRRSV infection (Kittawornrat et al., 2010). Oral fluid contains about 1% serum. The oral fluid samples are collected by allowing pigs in a single pen to chew on a rope. Oral fluid is extracted by squeezing the contents of the rope into a collection container. With only a few modifications, the oral fluid sample is treated and processed in the same manner as a routine diagnostic serum sample, including the incorporation into standard molecular and serologic diagnostic tests. Advantages of oral fluid collection include the ease of collection, decreased stress on pigs, and the ability to easily survey an entire population. Another advancement in support of regional elimination is in the area of risk-based testing and surveillance. Current sampling methods include the application of the same sampling protocol for every herd in a regional project. In a risk-based approach, the historical biosecurity status of a farm and surrounding farms, combined with other information, can be used to create a herd-specific sampling regimen that can maintain accurate surveillance while minimizing sample collection and associated costs. The application of genomic and genetic approaches to identify genes associated with PRRS resistance, susceptibility, or tolerance has far-reaching implications in the control and elimination of PRRSV. One goal of a genetic approach is to perform marker-assisted selection to develop pig breeds with improved PRRS resistance, and to avoid the unintended selection of genes linked to disease susceptibility. The approach is to conduct deep phenotyping on large numbers of pigs following experimental infection with a single PRRSV isolate (Lunney et al., 2011). The experimental model incorporates the infection of nursery pigs. Serum, total RNA, and weights are collected over a 58
42-day period. Progress to date includes the experimental infection of 1600 pigs from five commercial genetic sources. Genome-wide association studies are underway to identify genes and markers linked to two phenotypic traits: growth performance and PRRS virus load. A successful regional elimination project can be defined on three levels. The first is the installation of a process that fosters communication, education and improved biosecurity awareness among producers who seek a common goal. The second level is the development and implementation of new technologies that lessen costs and improve efficiency. And finally, the demonstration that PRRSV has been eliminated, a process that can be expected to require a much longer-term commitment. Acknowledgement This work is supported by USDA NIFA Award 200855620-19132. Conflict of interest statement In conducting this work, the authors have no conflict of interest, including no financial interests. References Benfield, D., E. Nelson, J. Collins, L. Harris, S. Goyal, D. Robison, W. Christianson, R. Morrison, D. Gorcyca, and D. Chladek, 1992: Characterization of swine infertility and respiratory syndrome (SIRS) virus (isolate ATCC VR-2332). J. Vet. Diagn. Invest. 4, 127–133. Bloemraad, M., E. de Kluijver, A. Petersen, G. Burkhardt, and G. Wensvoort, 1994: Porcine reproductive and respiratory syndrome: temperature and pH stability of Lelystad virus and its survival in tissue specimens from viraemic pigs. Vet. Microbiol. 42, 361–371. Botner, A., B. Strandbygaard, K. Sorensen, P. Have, K. Madsen, E. Madsen, and S. Alexandersen, 1997: Appearance of acute PRRS-like symptoms in sow herds after vaccination with a modified live PRRS vaccine. Vet. Rec. 141, 497–499. Corzo, C., E. Mondaca, S. Wayne, M. Torremorell, S. Dee, P. Davies, and R. Morrison, 2010: Control and elimination of porcine reproductive and respiratory syndrome virus. Virus Res. 154, 185–192. Cutler, T., C. Wang, Q. Qin, F. Zhou, K. Warren, K. Yoon, S. Hoff, J. Ridpath, and J. Zimmerman, 2011: Kinetics of UV(254) inactivation of selected viral pathogens in a static system. J. Appl. Microbiol. 111, 389–395. Darwich, L., I. Dı´az, and E. Mateu, 2010: Certainties, doubts and hypotheses in porcine reproductive and respiratory syndrome virus immunobiology. Virus Res. 154, 123–132. Dee, S., S. Otake, and J. Deen, 2010: Use of a production region model to assess the efficacy of various air filtration
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systems for preventing airborne transmission of porcine reproductive and respiratory syndrome virus and Mycoplasma hyopneumoniae: results from a 2-year study. Virus Res. 154, 177–184. Dokland, T., 2010: The structural biology of PRRSV. Virus Res. 154, 86–97. Fang, Y., P. Schneider, W. Zhang, K. Faaberg, E. Nelson, and R. Rowland, 2007: Diversity and evolution of a newly emerged North American Type 1 porcine arterivirus: analysis of isolates collected between 1999 and 2004. Arch. Virol. 152, 1009–1017. Holtkamp, D., D. Polson, M. Torremorell, R. Morrison, D. Classen, L. Becton, S. Henry, M. Rodibaugh, R. Rowland, H. Snelson, B. Straw, P. Yeske, and J. Zimmerman, 2011: Terminology for classifying swine herds by porcine reproductive and respiratory syndrome virus status. J. Swine Health Prod. 19, 44–56. Johnson, C., T. Griggs, J. Gnanandarajah, and M. Murtaugh, 2011: Novel structural protein in porcine reproductive and respiratory syndrome virus encoded by an alternative ORF5 present in all arteriviruses. J. Gen. Virol. 92, 1107–1116. Kittawornrat, A., J. Prickett, W. Chittick, C. Wang, M. Engle, J. Johnson, D. Patnayak, T. Schwartz, D. Whitney, C. Olson, K. Schwartz, and J. Zimmerman, 2010: Porcine reproductive and respiratory syndrome virus (PRRSV) in serum and oral fluid samples from individual boars: will oral fluid replace serum for PRRSV surveillance? Virus Res. 154, 170–176. Lunney, J., D. Benfield, and R. Rowland, 2010: Porcine reproductive and respiratory syndrome virus: an update on an emerging and re-emerging viral disease of swine. Virus Res. 154, 1–6. Lunney, J., J. Steibel, J. Reecy, E. Fritz, M. Rothschild, M. Kerrigan, B. Trible, and R. Rowland, 2011: Probing genetic control of swine responses to PRRSV infection: current progress of the PRRS host genetics consortium. BMC Proc. 5, (Suppl. 4) S30. Mengeling, W., K. Lager, R. Wesley, D. Clouser, A. Vorwald, and M. Roof, 1999: Diagnostic Implications of concurrent infections with attenuated and virulent strains of porcine reproductive and respiratory syndrome virus. Am. J. Vet. Res. 60, 119. Otake, S., S. Dee, C. Corzo, S. Oliveira, and J. Deen, 2010: Long-distance airborne transport of infectious PRRSV and
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Mycoplasma hyopneumoniae from a swine population infected with multiple viral variants. Vet. Microbiol. 145, 198–208. Rowland, R., S. Lawson, K. Rossow, and D. Benfield:, 2003: Lymphotropism of porcine reproductive and respiratory syndrome virus replication during persistent infection of pigs originally exposed to virus in utero. Vet. Microbiol. 96, 219–235. Shi, M., T. Lam, C. Hon, R. Hui, K. Faaberg, T. Wennblom, M. Murtaugh, T. Stadejek, and F. Leung, 2010: Molecular epidemiology of PRRSV: a phylogenetic perspective. Virus Res. 154, 7–17. Tirado, S., and K. Yoon, 2003: Antibody dependent enhancement of virus infection and disease. Viral Immunol. 16, 69– 86. Vu, H., B. Kwon, K. Yoon, W. Laegreid, A. Pattnaik, and F. Osorio, 2011: Immune evasion of porcine reproductive and respiratory syndrome virus through glycan shielding involves both glycoprotein 5 as well as glycoprotein 3. J. Virol. 85, 5555–5564. Wensvoort, G., C. Terpstra, J. Pol, E. ter Laak, M. Bloemraad, E. de Kluyver, C. Kragten, L. van Buiten, A. den Besten, F. Wagenaar, J. Broekhuijsen, P. Moonen, T. Zetstra, E. de Boer, H. Tibben, M. de Jong, P. van’t Veld, G. Groenland, J. Gennep, M. Voets, J. Verheijden, and J. Braamskamp, 1991: Mystery swine disease in The Netherlands: the isolation of Lelystad virus. Vet. Q. 13, 121–130. Wootton, S., D. Yoo, and D. Rogan, 2000: Full-length sequence of a Canadian porcine reproductive and respiratory syndrome virus (PRRSV) isolate. Arch. Virol. 145, 2297–2323. Wu, W., Y. Fang, R. Rowland, S. Lawson, J. Christopher-Hennings, K. Yoon, and E. Nelson, 2005: The 2b protein as a minor structural component of PRRSV. Virus Res. 114, 177– 181. Yoo, D., C. Song, Y. Sun, Y. Du, O. Kim, and H. Liu, 2010: Modulation of host cell responses and evasion strategies for porcine reproductive and respiratory syndrome virus. Virus Res. 154, 48–60. Zimmerman, J., Jacobs, A., J. Hermann, C. Mun˜oz-Zanzi, J. Prickett, M. Roof, and K. Yoon, 2010: Stability of porcine reproductive and respiratory syndrome virus at ambient temperatures. J. Vet. Diagn. Invest. 22, 257–260.
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REVIEW ARTICLE
Challenges and Opportunities for the Control and Elimination of Porcine Reproductive and Respiratory Syndrome Virus R. R. R. Rowland1 and R. B. Morrison2 1 2
Department of Diagnostic Medicine and Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS, USA Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN, USA
Keywords: porcine reproductive and respiratory syndrome virus; PPRS; regional control; elimination Correspondence: R. R. R. Rowland. Department of Diagnostic Medicine and Pathobiology, College of Veterinary Medicine, 1800 Denison Ave, Kansas State University, Manhattan, KS 66506, USA. Tel.: +785 532 4631; Fax: +785 532 4481; E-mail: [email protected]
Summary The control and elimination of porcine reproductive and respiratory syndrome virus (PRRSV) represent two of the most challenging tasks facing the pig industry worldwide. Several factors related to the biology of the virus make disease detection and elimination difficult. Efforts are further hampered by the lack of vaccines that can protect naı¨ve herds from infection. With this in mind, elimination efforts are being initiated which incorporate existing tools and knowledge. A new approach extends herd control strategies to the level of a region. One example of success in PRRSV regional elimination is the Stevens County project in Minnesota.
Received for publication July 28, 2011 doi:10.1111/j.1865-1682.2011.01306.x
Biology epidemiology and ecology Porcine reproductive and respiratory syndrome (PRRS), initially described in the late 1980s as ‘Mystery Swine Disease’, is associated with reproductive failure in sows, respiratory distress in nursing pigs, and poor growth performance during finishing. Porcine reproductive and respiratory syndrome has become endemic worldwide, with only the continents of Australia and Antarctica free from the virus. The causative agent of PRRS, the PRRS virus (PRRSV), was first isolated and identified by investigators in the Netherlands in 1991 (Wensvoort et al., 1991) and shortly thereafter in the United States (Benfield et al., 1992). North American and European viruses share only about 67% identity at the nucleotide level; therefore, European isolates are designated as Type 1 genotype viruses and North American isolates as Type 2. Type 1 viruses of European origin were first identified in US herds in 1999. The presence of two distinct genotypes
with diverse antigenic properties further complicates efforts to control PRRS (Fang et al., 2007). The properties of the virus that present the greatest challenges to PRRS control and elimination include (i) a complex virion surface topology and composition, (ii) the ability of the virus to remain persistent in a population, (iii) avoidance and subversion of innate and host immune responses, and (iv) the capacity to generate a large degree of genetic diversity in both structural and non-structural proteins (Lunney et al., 2010). Porcine reproductive and respiratory syndrome virus is an enveloped, positive polarity, non-segmented, single-stranded RNA virus belonging to the family Arteriviridae, within the order Nidovirales. The 15–15.5 kb PRRSV genome contains at least 11 ORFs and two untranslated regions flanking the 5¢ and 3¢ ends of the genome (Wootton et al., 2000; Wu et al., 2005; Johnson et al., 2011). The principal non-structural proteins, encoded by ORF1a and ORF1ab, have protease and replicase functions. The 3¢
ª 2012 Blackwell Verlag GmbH • Transboundary and Emerging Diseases. 59 (Suppl. 1) (2012) 55–59
55
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end of the genome codes for at least eight structural proteins. The virion structural protein composition and organization is reviewed in Dokland (2010). The major structural proteins, GP5, matrix (M) and nucleocapsid (N), are derived from ORFs 5, 6 and 7, respectively. GP2, GP3 and GP4 are minor structural proteins critical for virus replication, that represent important targets for neutralization by antibody. The most recently identified proteins are 2b and 5a, which are translated from ORF2 and ORF5, respectively (Wu et al., 2005; Johnson et al., 2011). Resistance to antibody is conferred by the presence of sugar residues on surface proteins, such as GP2, GP3, GP4 and GP5, combined with peptide sequence hypervariability (Fang et al., 2007; Shi et al., 2010; Vu H. et al., 2011). Viral proteins, such as nsp1, participate in the blocking of early innate responses, such as the induction of type 1 interferon (Yoo et al., 2010). At the level of a population, PRRSV is efficiently transmitted both horizontally and vertically. After the initial acute phase of infection, pigs may become subclinical carriers for several months, thus further perpetuating the virus (Rowland et al., 2003). The continued maintenance of the virus as a subclinical infection circulating within a production system is periodically interrupted by outbreaks of acute disease accompanied by high morbidity and mortality. Outbreaks are often linked to the introduction of a new virus combined with a lack of sufficient protection by existing herd immunity. Another possibility is antibody-dependent enhancement of infection, first described in 1996 (Tirado and Yoon, 2003). The most prevalent forms of viral entry into a production system are through the introduction of infected pigs, PRRSV-contaminated semen, and contaminated equipment, such as transportation vehicles. The risk of transmission by aerosols, also known as area spread, remains poorly understood; however, a recent report indicates that certain virus isolates can travel up to 9.8 km under the right experimental and environmental conditions (Otake et al., 2010). Modified live virus (MLV) vaccines became available in the United States in 1994. In general, MLV vaccines have been shown to reduce clinical signs and can aid in recovery following outbreaks of acute PRRS. However, several deficiencies have been noted, including incomplete protection, inability to distinguish infected from vaccinated pigs, and the potential for reversion to virulence (Botner et al., 1997; Mengeling et al., 1999; Darwich et al., 2010). An alternative to vaccination is the intentional infection of naive animals with wild-type live PRRSV either through contact with infected animals or exposure to infectious material. Acclimation with a live virus is an attempt to induce herd immunity against farm-specific strains. However, infection with a virulent virus presents several potential unintended consequences, such as the 56
continuous spread of the virus within a production system and the risk of introducing other pathogens. Environmental instability: a PRRSV Achilles’ Heel A complex biology, immunology and ecology combined with the absence of a vaccine sufficient to meet the needs of virus elimination make control and elimination of PRRSV seemingly impossible tasks. However, there are properties of the virus that can be exploited. For example, PRRSV is relatively unstable under normal environmental conditions and is especially sensitive to UV radiation, changes in pH, and increased temperature (Cutler et al., 2011). Jacobs et al. (2010) calculated T1/2 values of 1.6, 27.4, 84.8, and 155.5 h for temperatures of 30, 20, 10, and 4C, respectively. The virus is completely inactivated after a short incubation at temperatures >56C (Bloemraad et al., 1994). The application of common virucides, disinfectants, or steam is sufficient to completely inactivate PRRSV on surfaces. Furthermore, the virus does not survive in properly cured or cooked pork products. Even though PRRSV has been reported to travel for up to 9.8 km under experimental conditions, aerial transmission of virus over long distances appears to be a low risk and dependent on a set of ‘ideal’ environmental conditions (Otake et al., 2010). And finally, even though PRRSV is considered to be a persistent virus, the level of infection in individual animals eventually decays over time to the point of extinction (Rowland et al., 2003). Control of PRRSV at the herd level Over the years, several approaches have been employed for the control and elimination of PRRSV in single herds (see Corzo et al., 2010 for review). Highly effective approaches include depopulation-repopulation and all-in-all-out methods. Both depend on the placement of PRRSV-free pigs into a facility that is ‘free’ of virus. The application of time and common disinfection methods are sufficient to remove PRRSV from buildings contaminated with virus. Herd closure and rollover is the most common method for eliminating virus from sow farms. The approach is based on the gradual extinction of PRRSV in herds that are closed for approximately 200 days. Sows that remain seropositive are removed and replaced with negative pigs. The most recent tool for pig-dense regions is the use of whole barn filtration to block aerosol entry (Dee et al., 2010). A regional approach to PRRSV control and elimination Using the methods described above, it is relatively easy to eliminate PRRSV from a single herd; however, an
ª 2012 Blackwell Verlag GmbH • Transboundary and Emerging Diseases. 59 (Suppl. 1) (2012) 55–59
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outbreak with a new virus is all but inevitable. A new approach for the stable control and elimination of PRRSV is the implementation of herd control methods that simultaneously target multiple farms within a relatively large region. The rationale for a regional approach is to lessen the risk of re-introduction of virus into any single farm. Current regional elimination projects in the United States are supported by private companies and the USDA-funded PRRS Coordinated Agricultural Project (PRRS CAP). The steps for regional elimination are summarized below. Detailed descriptions of useful tools and biosecurity protocols can be downloaded at the PRRS CAP website (http://www.prrs.org). 1 Define the boundaries of a ‘region’ suitable for conducting PRRSV elimination and determine the level of producer participation. A region for conducting PRRSV elimination can be defined by a set of boundaries consisting of natural and/or man-made barriers, such as lakes, cities, mountains, or areas where a cluster of farms is spatially separated from other pig production sites. The ultimate boundary lines of a regional project are often defined by the group of farms that have a high interest in conducting a regional elimination project. Unlike traditional disease eradication programmes, which involve some level of government enforcement, regional elimination efforts are strictly voluntary. At any time during an elimination project, participants may withdraw. Therefore, ongoing communication and producer engagement are important elements for success. The sociology and psychology of producer participation is an area in need of further study. 2 Record premises characteristics and herd density. Premises can consist of sow farms, boar studs, nurseries, grow-finish farms, etc. Location, population size of each site, and the overall farm density within a region are recorded. Porcine reproductive and respiratory syndrome virus elimination is less problematic in regions dominated by fairly homogenous farm types at relatively low densities as compared to densely populated regions with mixed farm types. 3 Determine PRRSV status at each site. Porcine reproductive and respiratory syndrome virus RT-PCR and serology, common diagnostic tests, are used as to determine the infection status of individual herds. The amount and frequency of testing are determined based on the farm type and level of confidence needed to obtain an accurate result. Holtkamp et al. (2011) describe herd status designations, ranging from PRRSV Positive Unstable (Category 1) to PRRSV Negative (Category 4). This common set of terminologies is useful for communicating information within a region and for developing standardized herd status reporting methods.
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4
5
6
7
Assess overall herd biosecurity and risk for introduction of PRRSV. The web-based tool, Production Animal Disease Risk Assessment Program (PADRAP), is useful for assessing overall PRRS biosecurity at the herd level and can be used for estimating the success of a PRRSV elimination program (http://www.padrap.org). When applied at several time points, PADRAP can be used to measure changes in biosecurity over time. Map pig movements between farms within the region and from sources entering from outside the region. As discussed above, a major biosecurity risk for the entry of PRRSV is through the introduction of PRRSV-infected pigs and equipment, such as contaminated trucks. Therefore, a good prospect for PRRSV elimination is a situation where the principal sources of pigs and pig transport are confined to sites within the region (intraregional movement). Pigs that enter from sources outside of the region and possess an unknown PRRSV status represent one the largest threats to success. Implement herd control strategies and report progress. Starting with the herd elimination methods described above (Corzo et al., 2010), a combination of herd control strategies can be initiated that best fit the type and density of pig farms within the region. Regular status reports are important for updating participating producers on progress towards elimination. Open lines of communication, obtainable goals, and clear criteria related to progress are critical to keeping producers engaged in the process. Reported data include the number of pigs and the PRRSV status (Category 1–4) for each herd, as well as a general description of progress, including the identification of obstacles. Publicized progress provides an incentive for PRRSVpositive farms to make progress towards a negative status. Surveillance. Once Category 4 (PRRSV Negative) status is achieved, continued monitoring is important to ensure that farms remain PRRSV negative. The most common method is to monitor for the presence of PRRSV by standard diagnostic serology. The frequency of sampling is variable, but should be conducted at least twice a year. Even though syndromic monitoring is not always an accurate indicator of PRRS, herds are monitored for the appearance of PRRS-associated clinical signs.
Current progress and future technologies At this time, the PRRS CAP supports seven regional elimination projects in the United States, which enroll approximately 2.5 million pigs. Progress and updates for each project can be found at the PRRS CAP website (http://www.prrs.org). The Stevens County project, in
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Minnesota, is the longest running regional elimination project (Corzo et al., 2010). The county is 1490 km sq and contains 87 pig sites (164 000 pigs), including sow farms, boar studs, nurseries and grow-finish sites. Only four farms declined to participate in the project. As a region, Stevens County is relatively isolated from other pig-associated sites. At the beginning of the project in 2004, 29 sites were PRRSV positive, 19 sites PRRSV negative, and the remaining sites were of unknown status. As of 2010, all sites were negative for PRRSV, with only sporadic outbreaks in sow farms. The source of virus in all cases was linked to the import of PRRSV-positive pigs from outside the region. Recently, the project was expanded to include all of Minnesota north of Highway 212. At this time, approximately one million pigs are enrolled in the project. New technologies and methodologies are being employed to improve the effectiveness of PRRSV elimination and lessen the costs. One example is oral fluid testing, which can be used as a substitute for serum in the detection of PRRSV infection (Kittawornrat et al., 2010). Oral fluid contains about 1% serum. The oral fluid samples are collected by allowing pigs in a single pen to chew on a rope. Oral fluid is extracted by squeezing the contents of the rope into a collection container. With only a few modifications, the oral fluid sample is treated and processed in the same manner as a routine diagnostic serum sample, including the incorporation into standard molecular and serologic diagnostic tests. Advantages of oral fluid collection include the ease of collection, decreased stress on pigs, and the ability to easily survey an entire population. Another advancement in support of regional elimination is in the area of risk-based testing and surveillance. Current sampling methods include the application of the same sampling protocol for every herd in a regional project. In a risk-based approach, the historical biosecurity status of a farm and surrounding farms, combined with other information, can be used to create a herd-specific sampling regimen that can maintain accurate surveillance while minimizing sample collection and associated costs. The application of genomic and genetic approaches to identify genes associated with PRRS resistance, susceptibility, or tolerance has far-reaching implications in the control and elimination of PRRSV. One goal of a genetic approach is to perform marker-assisted selection to develop pig breeds with improved PRRS resistance, and to avoid the unintended selection of genes linked to disease susceptibility. The approach is to conduct deep phenotyping on large numbers of pigs following experimental infection with a single PRRSV isolate (Lunney et al., 2011). The experimental model incorporates the infection of nursery pigs. Serum, total RNA, and weights are collected over a 58
42-day period. Progress to date includes the experimental infection of 1600 pigs from five commercial genetic sources. Genome-wide association studies are underway to identify genes and markers linked to two phenotypic traits: growth performance and PRRS virus load. A successful regional elimination project can be defined on three levels. The first is the installation of a process that fosters communication, education and improved biosecurity awareness among producers who seek a common goal. The second level is the development and implementation of new technologies that lessen costs and improve efficiency. And finally, the demonstration that PRRSV has been eliminated, a process that can be expected to require a much longer-term commitment. Acknowledgement This work is supported by USDA NIFA Award 200855620-19132. Conflict of interest statement In conducting this work, the authors have no conflict of interest, including no financial interests. References Benfield, D., E. Nelson, J. Collins, L. Harris, S. Goyal, D. Robison, W. Christianson, R. Morrison, D. Gorcyca, and D. Chladek, 1992: Characterization of swine infertility and respiratory syndrome (SIRS) virus (isolate ATCC VR-2332). J. Vet. Diagn. Invest. 4, 127–133. Bloemraad, M., E. de Kluijver, A. Petersen, G. Burkhardt, and G. Wensvoort, 1994: Porcine reproductive and respiratory syndrome: temperature and pH stability of Lelystad virus and its survival in tissue specimens from viraemic pigs. Vet. Microbiol. 42, 361–371. Botner, A., B. Strandbygaard, K. Sorensen, P. Have, K. Madsen, E. Madsen, and S. Alexandersen, 1997: Appearance of acute PRRS-like symptoms in sow herds after vaccination with a modified live PRRS vaccine. Vet. Rec. 141, 497–499. Corzo, C., E. Mondaca, S. Wayne, M. Torremorell, S. Dee, P. Davies, and R. Morrison, 2010: Control and elimination of porcine reproductive and respiratory syndrome virus. Virus Res. 154, 185–192. Cutler, T., C. Wang, Q. Qin, F. Zhou, K. Warren, K. Yoon, S. Hoff, J. Ridpath, and J. Zimmerman, 2011: Kinetics of UV(254) inactivation of selected viral pathogens in a static system. J. Appl. Microbiol. 111, 389–395. Darwich, L., I. Dı´az, and E. Mateu, 2010: Certainties, doubts and hypotheses in porcine reproductive and respiratory syndrome virus immunobiology. Virus Res. 154, 123–132. Dee, S., S. Otake, and J. Deen, 2010: Use of a production region model to assess the efficacy of various air filtration
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systems for preventing airborne transmission of porcine reproductive and respiratory syndrome virus and Mycoplasma hyopneumoniae: results from a 2-year study. Virus Res. 154, 177–184. Dokland, T., 2010: The structural biology of PRRSV. Virus Res. 154, 86–97. Fang, Y., P. Schneider, W. Zhang, K. Faaberg, E. Nelson, and R. Rowland, 2007: Diversity and evolution of a newly emerged North American Type 1 porcine arterivirus: analysis of isolates collected between 1999 and 2004. Arch. Virol. 152, 1009–1017. Holtkamp, D., D. Polson, M. Torremorell, R. Morrison, D. Classen, L. Becton, S. Henry, M. Rodibaugh, R. Rowland, H. Snelson, B. Straw, P. Yeske, and J. Zimmerman, 2011: Terminology for classifying swine herds by porcine reproductive and respiratory syndrome virus status. J. Swine Health Prod. 19, 44–56. Johnson, C., T. Griggs, J. Gnanandarajah, and M. Murtaugh, 2011: Novel structural protein in porcine reproductive and respiratory syndrome virus encoded by an alternative ORF5 present in all arteriviruses. J. Gen. Virol. 92, 1107–1116. Kittawornrat, A., J. Prickett, W. Chittick, C. Wang, M. Engle, J. Johnson, D. Patnayak, T. Schwartz, D. Whitney, C. Olson, K. Schwartz, and J. Zimmerman, 2010: Porcine reproductive and respiratory syndrome virus (PRRSV) in serum and oral fluid samples from individual boars: will oral fluid replace serum for PRRSV surveillance? Virus Res. 154, 170–176. Lunney, J., D. Benfield, and R. Rowland, 2010: Porcine reproductive and respiratory syndrome virus: an update on an emerging and re-emerging viral disease of swine. Virus Res. 154, 1–6. Lunney, J., J. Steibel, J. Reecy, E. Fritz, M. Rothschild, M. Kerrigan, B. Trible, and R. Rowland, 2011: Probing genetic control of swine responses to PRRSV infection: current progress of the PRRS host genetics consortium. BMC Proc. 5, (Suppl. 4) S30. Mengeling, W., K. Lager, R. Wesley, D. Clouser, A. Vorwald, and M. Roof, 1999: Diagnostic Implications of concurrent infections with attenuated and virulent strains of porcine reproductive and respiratory syndrome virus. Am. J. Vet. Res. 60, 119. Otake, S., S. Dee, C. Corzo, S. Oliveira, and J. Deen, 2010: Long-distance airborne transport of infectious PRRSV and
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Mycoplasma hyopneumoniae from a swine population infected with multiple viral variants. Vet. Microbiol. 145, 198–208. Rowland, R., S. Lawson, K. Rossow, and D. Benfield:, 2003: Lymphotropism of porcine reproductive and respiratory syndrome virus replication during persistent infection of pigs originally exposed to virus in utero. Vet. Microbiol. 96, 219–235. Shi, M., T. Lam, C. Hon, R. Hui, K. Faaberg, T. Wennblom, M. Murtaugh, T. Stadejek, and F. Leung, 2010: Molecular epidemiology of PRRSV: a phylogenetic perspective. Virus Res. 154, 7–17. Tirado, S., and K. Yoon, 2003: Antibody dependent enhancement of virus infection and disease. Viral Immunol. 16, 69– 86. Vu, H., B. Kwon, K. Yoon, W. Laegreid, A. Pattnaik, and F. Osorio, 2011: Immune evasion of porcine reproductive and respiratory syndrome virus through glycan shielding involves both glycoprotein 5 as well as glycoprotein 3. J. Virol. 85, 5555–5564. Wensvoort, G., C. Terpstra, J. Pol, E. ter Laak, M. Bloemraad, E. de Kluyver, C. Kragten, L. van Buiten, A. den Besten, F. Wagenaar, J. Broekhuijsen, P. Moonen, T. Zetstra, E. de Boer, H. Tibben, M. de Jong, P. van’t Veld, G. Groenland, J. Gennep, M. Voets, J. Verheijden, and J. Braamskamp, 1991: Mystery swine disease in The Netherlands: the isolation of Lelystad virus. Vet. Q. 13, 121–130. Wootton, S., D. Yoo, and D. Rogan, 2000: Full-length sequence of a Canadian porcine reproductive and respiratory syndrome virus (PRRSV) isolate. Arch. Virol. 145, 2297–2323. Wu, W., Y. Fang, R. Rowland, S. Lawson, J. Christopher-Hennings, K. Yoon, and E. Nelson, 2005: The 2b protein as a minor structural component of PRRSV. Virus Res. 114, 177– 181. Yoo, D., C. Song, Y. Sun, Y. Du, O. Kim, and H. Liu, 2010: Modulation of host cell responses and evasion strategies for porcine reproductive and respiratory syndrome virus. Virus Res. 154, 48–60. Zimmerman, J., Jacobs, A., J. Hermann, C. Mun˜oz-Zanzi, J. Prickett, M. Roof, and K. Yoon, 2010: Stability of porcine reproductive and respiratory syndrome virus at ambient temperatures. J. Vet. Diagn. Invest. 22, 257–260.
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