Diagnosing PRRS in Swine: A Practical Approach for Field Veterinarians

Porcine Reproductive and Respiratory Syndrome (PRRS) remains one of the most challenging diseases affecting swine production because of its diverse clinical presentation, prolonged viral persistence, and the difficulty of confirming infection based on clinical signs alone. Respiratory disease, reproductive failure, and secondary infections may raise suspicion, but laboratory confirmation is essential for an accurate diagnosis. Understanding the strengths and limitations of available diagnostic methods allows veterinarians to make informed decisions while planning herd investigations and control strategies. 

Recognizing When to Suspect PRRS 

PRRS should be considered whenever breeding or growing pigs present with compatible respiratory or reproductive problems. Clinical findings alone, however, are insufficient for a definitive diagnosis because several infectious diseases can produce similar signs. 

A thorough herd history, including recent animal introductions, breeding records, reproductive performance, and disease progression within different age groups, provides valuable context before laboratory testing is initiated. Since PRRS can persist within herds and subclinical infections are common, disease investigation should extend beyond visibly affected animals¹. 

Selecting the Right Diagnostic Samples 

The quality and timing of sample collection directly influence diagnostic success. During the acute phase of infection, the virus can be detected in blood and a variety of tissues. PRRSV has also been identified in urine and feces, although successful virus isolation from feces is uncommon². Experimentally infected boars have also demonstrated the presence of the virus in semen, highlighting the importance of considering breeding animals during herd investigations^2,3. 

Because viral shedding changes over the course of infection, selecting appropriate animals and collecting samples early in the disease process can improve the likelihood of laboratory confirmation. 

Laboratory Tests That Support Diagnosis 

No single diagnostic test is suitable for every clinical situation. Instead, laboratory findings should always be interpreted alongside clinical observations and herd history. 

Several laboratory techniques are available for confirming PRRS infection, including²: 

• Virus isolation 

• Enzyme-linked immunosorbent assay (ELISA) 

• Immunoperoxidase monolayer assay (IPMA) 

• Indirect fluorescent antibody (IFA) testing 

• Serum neutralization (SN) testing 

Serological testing plays an important role in determining herd exposure, particularly because many infected pigs may not exhibit obvious clinical signs immediately. Routine serological monitoring can therefore contribute to assessing herd status over time^2,4. 

Interpreting Results in the Herd Context 

Laboratory results should never be interpreted in isolation. Vaccination history, stage of infection, and herd status all influence diagnostic interpretation. 

Commercial PRRS vaccines are widely used, but they do not provide complete cross-protection because of the considerable genetic diversity of PRRSV⁵. This makes careful interpretation of serological findings particularly important in vaccinated herds. 

Similarly, the virus may persist in infected populations for prolonged periods. Infection commonly occurs in piglets after maternally derived colostral antibodies decline between three and six weeks of age, allowing continued viral circulation within apparently stable herds². Even after the acute phase has resolved, viral persistence may complicate diagnostic interpretation and disease monitoring. 

Differential Diagnosis Remains Essential 

Respiratory disease and reproductive failure in pigs can result from multiple infectious and management-related conditions. For this reason, PRRS should remain part of a broader differential diagnosis rather than being assumed solely on clinical presentation. 

Combining clinical findings with herd history, laboratory confirmation, and an understanding of herd dynamics enables veterinarians to reach a more reliable diagnosis while avoiding unnecessary interventions². 

Practical Clinical Insights 

Early recognition and appropriate laboratory testing remain the foundation of PRRS diagnosis. Careful sample selection, consideration of vaccination history, and interpretation of laboratory results within the context of herd management improve diagnostic confidence. Because PRRS can persist through subclinical infections and prolonged viral circulation, ongoing herd surveillance and systematic diagnostic evaluation are valuable components of effective disease management in commercial swine production. 

References 

1. Mulligan MK, Kleiman JE, Caldemeyer AC, Harding JC, Pasternak JA. Porcine reproductive and respiratory virus 2 infection of the fetus results in multi-organ cell cycle suppression. Veterinary Research. 2022 Feb 21;53(1):13. https://link.springer.com/content/pdf/10.1186/s13567-022-01030-3.pdf 

2. Rimayanti R, Khairullah AR, Lestari TD, Hernawati T, Mulyati S, Utama S, Damayanti R, Moses IB, Yanestria SM, Kusala MK, Raissa R. Porcine reproductive and respiratory syndrome developments: An in-depth review of recent findings. Open Veterinary Journal. 2024 Sep 30;14(9):2138. https://pmc.ncbi.nlm.nih.gov/articles/PMC11563630/ 

3. Wu J, Liu S, Zhou S, Wang Z, Li K, Zhang Y, Yu J, Cong X, Chi X, Li J, Xu S. Porcine reproductive and respiratory syndrome in hybrid wild boars, China. Emerging infectious diseases. 2011 Jun;17(6):1071. https://pmc.ncbi.nlm.nih.gov/articles/PMC3358207/pdf/10-1518_finalD.pdf 

4. 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 

5. 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