Porcine reproductive and respiratory syndrome (PRRS) is an endemic and devastating disease in most swine-producing regions worldwide.¹ The PRRS virus (PRRSV) can persist in individuals and pig populations for several months.² Acute disease outbreaks are common and associated with new virus introduction and lack of appropriate herd immunity.³ Porcine reproductive and respiratory syndrome is among the diseases with the highest economic impact in modern pig production,⁴ with generalized estimated annual production losses of $664 million in the United States and $150 million in Canada.^5,6 Different immunologic solutions, including live virus inoculation (LVI) and modified-live virus (MLV) vaccines, or a combination of both, have been used to reduce the impact of PRRSV on production in breeding herds.^7,8

The time to stability (TTS), defined as the number of weeks to produce negative pigs at weaning, and the total production losses (TL), defined as number of pigs weaned below the herd-specific baseline, normalized by 1000 sows, may be correlated with different PRRS management practices and virus characteristics.^7-9 The use of LVI as part of a whole-herd exposure program to control and eliminate PRRSV contributed to shortening TTS compared to using an MLV vaccine.⁸ Intervention with MLV vaccine has been demonstrated to reduce the duration of viral shedding.¹⁰ In addition, breeding herds detected with three or more PRRSV strains or the presence of recombinant variants were associated with increased TTS and TL.⁹ Although different interventions have been reported in response to PRRS outbreaks, the results of management practices vary and have been inconsistent across studies.^7,10-12

Among different biosecurity and management strategies, herd closure, with or without whole-herd exposure (eg, MLV or LVI), is a common practice in North America to manage PRRSV infection in breeding herds.¹³ Herd closure is the interruption of animal introduction (eg, replacement gilts) for a determined period (usually until the herd achieves stability); the combined implementation of herd closure and whole-herd exposure using MLV vaccination or LVI is often referred to as load-close-expose.^8,11,14 The concept of load-close-expose is that pig introduction into a breeding herd is interrupted until the pathogen’s infection cycle ends; most often when PRRSV is no longer detected in pigs at weaning age,^8,15 The principle is to prevent the introduction of susceptible pigs that, when in contact with PRRSV, become infected and disseminate the virus within the herd, thus perpetuating the within-farm infection.¹⁵

Despite significant progress in understanding interventions to manage PRRSV infection, achieving consistent results in endemically infected herds varies with no unique or completely effective intervention identified.^1,16 An understanding of practices implemented in the field may help veterinarians and producers standardize PRRS management and control strategies. This study aimed to describe and benchmark strategies and practices used in the field across the United States to control and eliminate PRRSV in response to a PRRS outbreak.

Animal care and use

This study was approved by the Iowa State University Institutional Animal Care and Use Committee under protocol number 19-118. The data was shared anonymously, and disclosure of responses outside the research will not place participants at risk of harm.

Materials and methods

Overview

A voluntary survey was used to collect information on practices implemented in response to a PRRS outbreak in different herds. Herd veterinarians were contacted between 2019 and 2021 and were asked to voluntarily share herd demographic characteristics, biomanagement practices, diagnostic test and testing results, and production data. All collected, collated, and standardized data were described according to the herd’s outbreak characteristics.

Eligibility and exclusion criteria

The eligibility criteria included swine breeding herds reporting a PRRS outbreak, working on a plan to manage the infection, and tracking the recovery from the outbreak using diagnostic testing and productivity data monitoring. A PRRS outbreak was characterized by RNA detection using reverse transcription-quantitative polymerase chain reaction (RT-PCR) and clinical signs of PRRS observed by the veterinarian (eg, increase in abortions, increase in sow mortality, increase in the number of stillborn piglets). The respective herd veterinarians were asked to complete a survey with information on herd demographic characteristics and the interventions implemented in response to the PRRS outbreak. The veterinarians were encouraged to revise the survey quarterly until the herd achieved the desired status according to the American Association of Swine Veterinarians’ (AASV) recommendations for PRRSV herd classification.^17,18

The exclusion criteria included events that would impact the study’s outcomes, such as outbreaks of other diseases (eg, porcine epidemic diarrhea), a second PRRS outbreak before achieving stability, or a ≥ 20% change in sow inventory due to factors unrelated to the PRRS outbreak.

Survey and data collection

An Excel-based survey was developed to collect data on the practices implemented in response to PRRS outbreaks (Table 1). Herd demographic characteristics, veterinarians’ contact information, and immunologic solutions for gilts and sows were collected. Biomanagement practices adopted, eg, management changes to reduce exposure to bacteria to eliminate losses (McREBEL) like practices,¹⁹ were collected from the herds seeking PRRSV stability. Veterinarians received the survey via email, and follow-up emails and phone calls were used to keep in touch about the initial information provided and interventions applied in the herds until the desired AASV classification status was achieved. All herd-specific information regarding the survey and interventions applied was confirmed after achieving the desired status. An Iowa State University consent form of participation, data handling, and confidentiality was signed to assure agreement and data protection for all parties. Data were collected from farms located in Iowa, Nebraska, Oklahoma, Minnesota, Illinois, Indiana, Texas, Ohio, Colorado, and Kansas.

Monitoring, PRRSV classification, and diagnostic data

Herds were monitored for PRRSV weekly using PRRSV RNA detection by RT-PCR from processing fluid (PF) samples. Processing fluids are obtained from the serosanguinous fluid recovered from piglet castration and tail docking.²⁰ The veterinarian submitted one pool of PF per week to Iowa State University Veterinary Diagnostic Laboratory (ISU VDL) or the University of Minnesota Veterinary Diagnostic Laboratory (UMN VDL) until the desired AASV herd classification status was achieved. The herds were classified following recommendations from the AASV PRRS classification^17,18: positive unstable (I), positive stable (II-A), positive stable (II-B [undergoing elimination]), provisional negative (III), and negative (IV). Diagnostic data were shared through the ISU VDL or UMN VDL client web interface applications, and combined data was accessed through the Animal Health Monitoring and Evaluation System (www.vdl.iastate.edu) using a standardized and consistent methodology.

TTS, TL, and time to baseline productivity analysis

Three recovery metrics were used in this study: TTS, time to baseline productivity (TTBP), and TL. For each PRRS outbreak, TTS was declared when the herd reached 8 consecutive weeks without PRRSV RNA detection by RT-PCR in weekly PF samples. Time to baseline productivity was defined as the number of weeks the herd took to recover to the number of pigs weaned per week prior to the PRRS outbreak and was calculated using an exponentially weighted moving average with 3 sigmas, 0.4 lambda, and a baseline of 21 weeks prior to the outbreak following a previously reported methodology.⁸ The severity of the PRRS outbreak was defined by TL and calculated as the number of pigs weaned below the herd-specific baseline, normalized by 1000 sows from the initial PRRS outbreak to when the herd returned to TTBP.

At 1 to 4 weeks after the PRRS outbreak, the virus was classified according to restriction fragment length polymorphisms and lineages, both based on the open reading frame-5 gene as previously described.^21-23

Results

Overview

Eighty-six herds experiencing a PRRS outbreak were enrolled in this study, with each herd followed until the desired PRRS herd status was achieved. All the herds provided information about intervention characteristics (Table 2), and 35 herds (40.7%) reported biomanagement practices (Figures 1, 2, and 3). The mean herd size was 3902 sows (range, 765-12,694 sows). Different interventions used to control and reduce losses were identified and described in the survey responses. There was great variation in the interquartile ranges for TTS, TTBP, and TL (Table 3) among herds. No herds met the exclusion criteria defined for this study.

Descriptive results

The states represented by participating herds are presented in Figure 4. Descriptive results of herd characteristics, response levels, and the number of herds in each of the categories are presented in Table 2.

Biomanagement strategies

Forty percent of the respondents (35 of 86) reported recommending and implementing biomanagement strategies to minimize PRRSV transmission among sows and piglets. Figures 1, 2, and 3 demonstrate the level of biomanagement practice implementation within each herd according to the veterinarian respondents, with none being considered as no practice implemented at all, 25%, 50%, and 75% as a percentage of practice implemented over the period of the outbreak, and fully implemented as implemented until achieving PRRSV stability.

Discussion

The median TTS (38.0 weeks), TTBP (22.0 weeks), and TL (3675 pigs/1000 sows) were higher than previously reported in 2014 (26.6 weeks, 16.5 weeks, and 2217 pigs/1000 sows, respectively).⁸ The longer TTS may be related to more representation of sampling methods used for PRRSV monitoring (eg, PF) within the herd population in this study compared to ten years ago, where serum from a finite number of animals was used.^20,24,25 In addition, the longer TTS and TTBP and higher TL in this study might be associated with changes in herd size, production flow, PRRSV variants, and other variables not assessed in this study. The number of PRRSV strains and recombination events have been reported to be associated with longer TTS and higher TL.⁹ The numerical range of TTS, TTBP, and TL and the variability of practices implemented in the field to control or control and eliminate PRRSV reported in this study emphasize the need to better understand best practices to minimize the PRRSV impact in breeding herds.

Responders who reported seeking elimination and herd closure implementation as part of the PRRSV control and elimination plan varied among the herds. The implementation of herd closure has been reported to control and eliminate PRRSV at the farm level.¹⁴ Beyond herd closure implementation, the PRRSV control and elimination program has been associated with closed-herd internal multiplication, negative gilts introduced into a negative herd, focus on biosecurity methods, use of PRRSV-negative semen, and single-source pig flow.²⁶ Despite reported rules of success for PRRSV control and elimination, this descriptive study has shown that PRRS management is complex, including desired AASV herd PRRSV classification status and strategies to achieve TTS.

One participant reported using a two-week batch flow, and 3 participants reported using a four-week batch flow as part of a strategy to improve biocontainment and reduce PRRSV transmission through better all-in/all-out management and farrowing room disinfection between batches. Batch farrowing management allows fixed-interval mating groups of sows of equal size, leading to all-in/all-out pig management in which animals in different batches have no contact,²⁷ and may help to control herd health status.^28-30 The reported median TTS of herds operating in a four-week batch system was 27 weeks.³¹ The use of a batch system may be an opportunity to shorten TTS and reduce TL in breeding herds facing a PRRS outbreak.

The interventions used with sows and gilts reported in this study were inconsistent across different herd outbreaks. The use of LVI, MLV, or a combination of LVI and MLV in sows was similarly reported. Different management procedures for PRRSV control at the farm level have been previously reported,^{7,8,10,14,15,26} and the use of PRRS MLV vaccines has been predominant in the US breeding herd.^11,32,33 The use of LVI, preparation and administration of LVI, the timing of interventions, and timing of MLV use are practices and interventions that might change according to the control and elimination strategy adopted by the veterinarian. Still, there are limitations regarding intervention assessment and a better understanding of all these factor combinations is needed.

The survey used in this study included various questions regarding biomanagement strategies to reduce virus transmission between sows and piglets. The results were inconsistent among participants. Studies have highlighted the importance of biomanagement practices to avoid PRRSV transmission^1,14,34 and practices, such as limiting cross fostering and avoiding mixing animals from different litters, on PRRSV-positive farms to optimize production have been reported.³⁵ Biomanagement protocols based on the McREBEL pig flow management implementation system have been reported as an important piece of PRRSV control and elimination.^19,36

The reported biomanagement strategies adopted following PRRS outbreaks were variable. Biomanagement refers to management practices to mitigate the transmission of pathogens between animals within the same population.²⁵ In addition, identifying a farm’s weak points, prioritizing the items to be improved first, and constantly revising and auditing the implemented biosecurity and biomanagement strategies were essential to prevent and control virus transmission within and among large herds.³² The variety of biomanagement practices reported in this study demonstrated the need for more consistency among the herds after a PRRS outbreak.

Implications

Under the conditions of this descriptive study:

• Management practices used during PRRS outbreaks were highly diverse among herds.
• Herd closure, interventions, and biomanagement strategies were inconsistent.
• The TTS and TTBP were longer and TL higher than reported 10 years ago.

Acknowledgments

The authors acknowledge the veterinarians and producers for their support and cooperative data provided for this study. This study was supported by the Iowa Pork Producers Association grant No. 20-109 IPPA and by Boehringer Ingelheim Animal Health.

Conflict of interest

The authors declare that this research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Disclaimer

Dr Linhares, a member of this journal’s editorial board, was not involved in the editorial review of or decision to publish this article.

Scientific manuscripts published in the Journal of Swine Health and Production are peer reviewed. However, information on medications, feed, and management techniques may be specific to the research or commercial situation presented in the manuscript. It is the responsibility of the reader to use information responsibly and in accordance with the rules and regulations governing research or the practice of veterinary medicine in their country or region.

References

1. Zimmerman JJ, Dee SA, Holtkamp DJ, Murtaugh MP, Stadejek T, Stevenson GW, Torremorell M, Yang H, Zhang J. Porcine reproductive and respiratory syndrome viruses (Porcine Arteriviruses). In: Zimmerman JJ, Karriker LA, Ramirez A, Schwartz KJ, Stevenson GW, Zhang J, eds. Diseases of Swine. 11^th ed. Wiley & Sons; 2019:685-708. https://doi.org/10.1002/9781119350927.ch41

2. Arruda AG, Friendship R, Carpenter J, Hand K, Ojkic D, Poljak Z. Investigation of the occurrence of porcine reproductive and respiratory virus in swine herds participating in an area regional control and elimination project in Ontario, Canada. Transbound Emerg Dis. 2017;64:89-100. https://doi.org/10.1111/tbed.12343

3. Rowland RRR, Morrison RB. Challenges and opportunities for the control and elimination of porcine reproductive and respiratory syndrome virus. Transbound Emerg Dis. 2012;59:55-59. https://doi.org/10.1111/ j.1865-1682.2011.01306.x

4. Lunney JK, Benfield DA, Rowland RRR. Porcine reproductive and respiratory syndrome virus: An update on an emerging and re-emerging viral disease of swine. Virus Res. 2010;154:1-6. https://doi.org/10.1016/j.virusres.2010.10.009

5. Holtkamp DJ, Kliebenstein JB, Neumann E, Zimmerman JJ, Rotto H, Yoder TK, Wang C, Yeske P, Mowrer CL, Haley C. 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. https://doi.org/10.54846/jshap/754

*6. Mussell A, Oginskyy A, Grier K, Morin M, Lachance M, Whittington L, Friendship R. A risk, benefit, strength, weakness, opportunity and threat analysis for the control and possible eradication of porcine reproductive and respiratory syndrome (PRRS) virus within the Canadian swine herd. Canadian Swine Health Board Report. 2011:1–122.

7. Linhares DCL, Betlach C, Morrison RB. Effect of immunologic solutions on sows and gilts on time to stability, and production losses in breeding herds infected with 1-7-4 PRRSV. Prev Vet Med. 2017;144:112-116. https://doi.org/10.1016/j.prevetmed.2017.05.024

8. Linhares DCL, Cano JP, Torremorell M, Morrison RB. Comparison of time to PRRSV-stability and production losses between two exposure programs to control PRRSV in sow herds. Prev Vet Med. 2014;116:111-119. https://doi.org/10.1016/j.prevetmed.2014.05.010

9. Trevisan G, Zeller M, Li G, Zhang J, Gauger P, Linhares DCL. Implementing a user-friendly format to analyze PRRSV next-generation sequencing results and associating breeding herd production performance with number of PRRSV strains and recombination events. Transbound Emerg Dis. 2022;69:e2214-e2229. https://doi.org/10.1111/tbed.14560

10. 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;25:4382-4391. https://doi.org/10.1016/j.vaccine.2007.03.031

11. Arruda AG, Friendship R, Carpenter J, Greer A, Poljak Z. Evaluation of control strategies for porcine reproductive and respiratory syndrome (PRRS) in swine breeding herds using a discrete event agent-based model. PLoS One. 2016;11:e0166596. https://doi.org/10.1371/journal.pone.0166596

12. Sanhueza JM, Vilalta C, Corzo C, Arruda AG. Factors affecting porcine reproductive and respiratory syndrome virus time-to-stability in breeding herds in the Midwestern United States. Transbound Emerg Dis. 2019;66:823-830. https://doi.org/10.1111/tbed.13091

*13. Yeske P. Winter of 2007-2008: A practitioner’s experience with PRRS. In: Proceedings of the 40th AASV Annual Meeting. American Association of Swine Veterinarians; 2009:455-458.

14. 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;154:185-192. https://doi.org/10.1016/j.virusres.2010.08.016

*15. Torremorell M, Henry S, Christianson WT. Eradication using herd closure. PRRS Compendium. National Pork Board; 2003:157-162.

16. Morrison RB. Control and elimination of PRRS virus. Adv Pork Prod. 2011;22:137-145. https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=01731b1c7147f821c5f91e7094f6c15213af4beb

17. Holtkamp DJ, Torremorell M, Corzo CA, Linhares DCL, Almeida MN, Yeske P, Polson DD, Becton L, Snelson H, Donovan T, Pittman J, Johnson C, Vilalta C, Silva GS, Sanhueza J. Proposed modifications to porcine reproductive and respiratory syndrome virus herd classification. J Swine Health Prod. 2021;29:261-270. https://doi.org/10.54846/jshap/1218

18. Holtkamp DJ, Polson DD, Torremorell M, Morrison B, Classen DM, Becton L, Henry S, Rodibaugh MT, Rowland RR, Snelson H, Straw B, Yeske P, Zimmerman JJ. Terminology for classifying swine herds by porcine reproductive and respiratory syndrome virus status. J Swine Health Prod. 2011;19:44-56. https://doi.org/10.54846/jshap/669

*19. Alonso C. Internal Biosecurity: McREBEL and beyond!! Pig 333. Published September 29, 2020. Accessed November 28, 2023. https://www.pig333.com/articles/internal-biosecurity-mcrebel-and-beyond_15979/

20. Lopez W, Angulo J, Zimmerman J, Linhares D. Porcine reproductive and respiratory syndrome monitoring in breeding herds using processing fluids. J Swine Health Prod. 2018;26:146-150. https://doi.org/10.54846/jshap/1055

21. Paploski IAD, Pamornchainavakul N, Makau DN, Rovira A, Corzo CA, Schroeder DC, Cheeran MC-J, Doeschl-Wilson A, Kao RR, Lycett S, VanderWaal K. Phylogenetic structure and sequential dominance of sub-lineages of PRRSV Type-2 Lineage 1 in the United States. Vaccines (Basel). 2021;9:608. https://doi.org/10.3390/vaccines9060608

22. Shi M, Lam TT-Y, Hon C-C, Murtaugh MP, Davies PR, Hui RK-H, Li J, Wong LT-W, Yip C-W, Jiang J-W, Leung FC-C. Phylogeny-based evolutionary, demographical, and geographical dissection of North American type 2 porcine reproductive and respiratory syndrome viruses. J Virol. 2010;84:8700-8711. https://doi.org/10.1128/jvi.02551-09

23. Wesley RD, Mengeling WL, Lager KM, Clouser DF, Landgraf JG, Frey ML. Differentiation of a porcine reproductive and respiratory syndrome virus vaccine strain from North American field strains by restriction fragment length polymorphism analysis of ORF 5. J Vet Diagn Invest. 1998;10:140-144. https://doi.org/10.1177/104063879801000204

24. Almeida MN, Zhang M, Lopez WAL, Vilalta C, Sanhueza J, Corzo CA, Zimmerman JJ, Linhares DCL. A comparison of three sampling approaches for detecting PRRSV in suckling piglets. Prev Vet Med. 2021;194:105427. https://doi.org/10.1016/j.prevetmed.2021.105427

25. Trevisan G, Linhares LCM, Crim B, Dubey P, Schwartz KJ, Burrough ER, Main RG, Sundberg P, Thurn M, Lages PTF, Corzo CA, Torrison J, Henningson J, Herrman E, Hanzlicek GA, Raghavan R, Marthaler D, Greseth J, Clement T, Christopher-Hennings J, Linhares DCL. Macroepidemiological aspects of porcine reproductive and respiratory syndrome virus detection by major United States veterinary diagnostic laboratories over time, age group, and specimen. PLoS One. 2019;14:e0223544. https://doi.org/10.1371/journal.pone.0223544

*26. Donovan T. PRRS control and eradication in a production system. In: Proceedings of the Allen D. Leman Swine Conference. University of Minnesota; 2011:151-155.

27. Lurette A, Belloc C, Touzeau S, Hoch T, Seegers H, Fourichon C. Modelling batch farrowing management within a farrow-to-finish pig herd: Influence of management on contact structure and pig delivery to the slaughterhouse. Animal. 2008;2:105-116. https://doi.org/10.1017/S1751731107000997

28. Berends BR, Urlings HA, Snijders JM, Van Knapen F. Identification and quantification of risk factors in animal management and transport regarding Salmonella spp. in pigs. Int J Food Microbiol. 1996;30:37-53. https://doi.org/10.1016/0168-1605(96)00990-7

29. Madec F, Bridoux N, Bounaix S, Jestin A. Measurement of digestive disorders in the piglet at weaning and related risk factors. Prev Vet Med. 1998;35:53-72. https://doi.org/10.1016/S0167-5877(97)00057-3

30. Rose N, Larour G, Le Diguerher G, Eveno E, Jolly JP, Blanchard P, Oger A, Le Dimna M, Jestin A, Madec F. Risk factors for porcine post-weaning multisystemic wasting syndrome (PMWS) in 149 French farrow-to-finish herds. Prev Vet Med. 2003;61:209-225. https://doi.org/10.1016/j.prevetmed.2003.07.003

31. Trevisan G, Jablonski E, Angulo J, Lopez WA, Linhares DCL. Use of processing fluid samples for longitudinal monitoring of PRRS virus in herds undergoing virus elimination. Porcine Health Manag. 2019;5:18. https://doi.org/10.1186/s40813-019-0125-x

32. Murtaugh MP, Genzow M. Immunological solutions for treatment and prevention of porcine reproductive and respiratory syndrome (PRRS). Vaccine. 2011;29:8192-8204. https://doi.org/10.1016/j.vaccine.2011.09.013

33. Tousignant SJP, Perez AM, Lowe JF, Yeske PE, Morrison RB. Temporal and spatial dynamics of porcine reproductive and respiratory syndrome virus infection in the United States. Am J Vet Res. 2015;76:70-76. https://doi.org/10.2460/ajvr.76.1.70

*34. Pitkin A, Otake S, Dee S. Biosecurity protocols for the prevention of spread of porcine reproductive and respiratory syndrome virus. University of Minnesota College of Veterinary Medicine Swine Disease Eradication Center; 2009. Accessed November 28, 2023. http://www.biosecureairinc.com/files/protocols.pdf

35. Colomer MÀ, Margalida A, Fraile L. Improving the management procedures in farms infected with the porcine reproductive and respiratory syndrome virus using PDP models. Sci Rep. 2019;9:9959. https://doi.org/10.1038/s41598-019-46339-w

36. Rathkjen PH, Dall J. Control and eradication of porcine reproductive and respiratory syndrome virus type 2 using a modified-live type 2 vaccine in combination with a load, close, homogenise model: An area elimination study. Acta Vet Scand. 2017;59:4. https://doi.org/10.1186/s13028-016-0270-z

* Non-refereed references.