PRRS: Why Control Remains Challenging in Modern Pig Production

Controlling Porcine Reproductive and Respiratory Syndrome (PRRS) continues to be one of the greatest challenges in swine production. The virus spreads through multiple routes, persists within infected populations, and frequently establishes subclinical infections that complicate disease detection. While no single measure can eliminate the risk of transmission, combining biosecurity, herd management, surveillance, and vaccination can help reduce disease spread and improve herd stability. 

Why PRRS is Difficult to Control 

Several characteristics of PRRSV make disease control particularly challenging. Airborne transmission allows the virus to spread beyond direct animal contact, while infected pigs may continue to harbour the virus long after clinical signs have subsided. In addition, subclinical infections can maintain viral circulation within herds without obvious evidence of disease. 

These factors, together with the absence of simple diagnostic tests and the virus's persistence in infected populations, have limited the success of many control programmes¹. 

Strengthening Farm Biosecurity 

Preventing the introduction of PRRSV remains the first line of defence. 

Several biosecurity measures can reduce the likelihood of virus entry into a herd: 

• Test and quarantine newly introduced pigs before herd entry. 

• Restrict unnecessary visitor access to pig facilities. 

• Change footwear and clothing after pigs are marketed. 

• Prevent access by rodents and roaming animals. 

• Thoroughly clean and disinfect vehicles transporting pigs¹. 

Although these measures reduce risk, they cannot completely prevent infection because PRRSV may also spread through the air. 

Herd Management Strategies That Support Disease Control 

Management practices play an equally important role in limiting virus circulation within a farm. 

Routine serological testing can help monitor herd status, while production systems that separate pigs by age reduce opportunities for transmission between older and younger animals^2,3. Early off-site weaning, all-in/all-out production, varying weaning intervals, and early weaning combined with medication have also been used to interrupt the infection cycle in nursery pigs⁴. 

Replacement breeding stock should be obtained from herds with no history of PRRS, and their health status should be monitored continuously before introduction into the resident herd⁵. 

During reproductive outbreaks, relocating sows to the farrowing house approximately two weeks before farrowing has been recommended⁶. For sows that experience abortion or premature farrowing, delaying breeding until the normal weaning period may help maintain herd flow because fertility during the first post-abortion estrus is often poor⁷. 

Treatment and Vaccination: Important but Not Standalone Solutions 

There is no specific treatment that eliminates PRRSV infection. Supportive therapies may improve piglet survival, while antibiotics may help reduce the impact of secondary bacterial infections in some situations⁸. Additional management measures, including reducing herd size, removing clinically affected pigs, delaying rebreeding of affected sows, increasing the use of artificial insemination, and postponing iron administration and tail docking in neonates, may help reduce disease losses^9,10. 

Vaccination remains an important component of PRRS management and is widely used in many countries¹¹. However, considerable genetic diversity and rapid viral evolution limit cross-protection among different PRRSV genotypes¹². Current commercial vaccines include modified-live, inactivated, and subunit vaccines, but vaccine selection should consider not only genetic similarity but also viral gene regions associated with virus interaction and replication^13,14. 

Practical Clinical Insights 

Effective PRRS control requires a herd-level approach rather than reliance on a single intervention. Strong biosecurity, careful monitoring of replacement animals, age-segregated production, routine surveillance, and appropriate herd management together reduce opportunities for virus introduction and circulation. Vaccination can support these efforts but should be viewed as one component of an integrated control programme, particularly given the genetic diversity and persistence of PRRSV within commercial swine populations. 

References 

1. Alarcón LV, Allepuz A, Mateu E. Biosecurity in pig farms: a review. Porcine health management. 2021 Jan 4;7(1):5. https://link.springer.com/content/pdf/10.1186/s40813-020-00181-z.pdf 

2. Magalhães ES, Zimmerman JJ, Holtkamp DJ, Classen DM, Groth DD, Glowzenski L, Philips R, Silva GS, Linhares DC. Next generation of voluntary PRRS virus regional control programs. Frontiers in veterinary science. 2021 Nov 5;8:769312. https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2021.769312/full 

3. Butler JE, Lager KM, Golde W, Faaberg KS, Sinkora M, Loving C, Zhang YI. Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic. Immunologic research. 2014 Aug;59(1):81-108. https://pmc.ncbi.nlm.nih.gov/articles/PMC7091131/pdf/12026_2014_Article_8549.pdf 

4. 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 research. 2010 Dec 1;154(1-2):185-92. https://www.academia.edu/download/49546645/Control_y_Eliminacion_de_PRRS.pdf 

5. Fornyos K, Búza L, Makkai I, Polyák F, Pogácsás I, Savoia L, Szegedi L, Bálint Á, Jakab S, Bányai K, Szabó I. Sampling strategies in PRRS elimination in hungary: an observational study involving four farrow-to-finish swine herds. Veterinary sciences. 2023 Aug 30;10(9):546. https://www.mdpi.com/2306-7381/10/9/546 

6. Blavi L, Solà-Oriol D, Llonch P, López-Vergé S, Martín-Orúe SM, Pérez JF. Management and feeding strategies in early life to increase piglet performance and welfare around weaning: A review. Animals. 2021 Jan 25;11(2):302. https://www.mdpi.com/2076-2615/11/2/302 

7. Papakonstantinou GI, Psalla D, Pourlis A, Stylianaki I, Athanasiou LV, Tzika E, Meletis E, Kostoulas P, Maragkakis G, Christodoulopoulos G, Papaioannou N. Histopathological pulmonary lesions in 1st-day newborn piglets derived from PRRSV-1 MLV vaccinated sows at the last stage of gestation. Life. 2023 Jul 23;13(7):1609. https://www.mdpi.com/2075-1729/13/7/1609 

8. Odland CA, Edler R, Noyes NR, Dee SA, Nerem J, Davies PR. Evaluation of the impact of antimicrobial use protocols in porcine reproductive and respiratory syndrome virus-infected swine on phenotypic antimicrobial resistance patterns. Applied and environmental microbiology. 2022 Jan 11;88(1):e00970-21. https://journals.asm.org/doi/pdf/10.1128/aem.00970-21 

9. Fano E, Olea L, Pijoan C. Eradication of porcine reproductive and respiratory syndrome virus by serum inoculation of naive gilts. Canadian journal of veterinary research. 2005 Jan;69(1):71. https://pmc.ncbi.nlm.nih.gov/articles/PMC1142173/pdf/cjvr69pg071.pdf 

10. Pertich A, Barna Z, Makai O, Farkas J, Molnár T, Bálint Á, Szabó I, Albert M. Elimination of porcine reproductive and respiratory syndrome virus infection using an inactivated vaccine in combination with a roll-over method in a Hungarian large-scale pig herd. Acta Veterinaria Scandinavica. 2022 May 7;64(1):12. https://link.springer.com/content/pdf/10.1186/s13028-022-00630-5.pdf 

11. de Brito RC, Holtham K, Roser J, Saunders JE, Wezel Y, Henderson S, Mauch T, Sanz-Bernardo B, Frossard JP, Bernard M, Lean FZ. An attenuated herpesvirus vectored vaccine candidate induces T-cell responses against highly conserved porcine reproductive and respiratory syndrome virus M and NSP5 proteins that are unable to control infection. Frontiers in Immunology. 2023 Aug 3;14:1201973. https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2023.1201973/pdf 

12. Eclercy J, Renson P, Hirchaud E, Andraud M, Beven V, Paboeuf F, Rose N, Blanchard Y, Bourry O. Phenotypic and genetic evolutions of a porcine reproductive and respiratory syndrome modified live vaccine after limited passages in pigs. Vaccines. 2021 Apr 16;9(4):392. https://www.mdpi.com/2076-393X/9/4/392 

13. Zhou L, Yang Y, Xia Q, Guan Z, Zhang J, Li B, Qiu Y, Liu K, Shao D, Ma Z, Wang X. Genetic characterization of porcine reproductive and respiratory syndrome virus from Eastern China during 2017–2022. Frontiers in microbiology. 2022 Oct 13;13:971817. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.971817/pdf 

14. Kick AR, Grete AF, Crisci E, Almond GW, Käser T. Testable candidate immune correlates of protection for porcine reproductive and respiratory syndrome virus vaccination. Vaccines. 2023 Mar 5;11(3):594. https://www.mdpi.com/2076-393X/11/3/594