Clinical Manifestations and Pathological Findings of PRRS: Recognizing the Disease Beyond the Obvious

Porcine Reproductive and Respiratory Syndrome (PRRS) presents with a wide spectrum of clinical signs, making early recognition challenging. The disease affects pigs of all ages, with reproductive problems predominating in breeding animals and respiratory disease being more evident in growing pigs. Clinical expression may vary between herds and even among animals within the same production system. Recognizing the characteristic clinical picture, together with pathological findings, is essential for timely diagnosis and appropriate herd management. 

Clinical Presentation Across Different Age Groups 

PRRS is primarily characterized by reproductive failure and respiratory disease. In breeding herds, infection can lead to abortions, premature farrowing, stillbirths, weak-born piglets, and increased neonatal mortality. These reproductive losses often represent the first indication that PRRSV has entered a previously unaffected herd¹. 

Growing and finishing pigs typically exhibit respiratory disease of varying severity. Clinical signs may include laboured breathing, reduced growth performance, lethargy, and poor overall condition. The severity of respiratory involvement varies depending on the level of viral circulation within the herd and the presence of concurrent infections. 

Because PRRS can establish subclinical infections, not every infected animal develops obvious clinical signs. Serological evidence indicates that many infections remain unapparent despite active viral circulation within swine populations. This highlights the importance of evaluating herd-level performance rather than relying solely on clinically affected animals¹. 

Understanding the Role of Secondary Infections 

PRRSV alone can compromise herd health, but secondary bacterial infections often contribute significantly to disease severity and mortality. 

Vaccination trials demonstrated that pigs receiving a modified-live vaccine were partially protected against severe clinical disease, including prolonged fever, viremia, and nasal discharge. However, some animals still succumbed to secondary pulmonary infections caused by Trueperella pyogenes and Streptococcus suis¹. These observations illustrate how concurrent infections can influence clinical outcomes even when partial protection against PRRSV has been achieved. 

Supportive treatment and appropriate management of secondary bacterial infections may therefore influence overall herd recovery². 

Pathological Findings That Support Diagnosis 

Clinical findings should always be complemented by post-mortem examination whenever possible. 

PRRSV can be present in multiple tissues and body fluids during infection, including blood, urine, feces, and semen, although successful virus isolation from feces is uncommon¹. During reproductive outbreaks, aborted fetuses, placentas, and dead piglets may contain high concentrations of virus in the blood, lungs, and other organs, making these valuable diagnostic specimens³. 

Careful examination of these materials not only supports laboratory confirmation but also assists in understanding the extent of virus circulation within the herd. 

Looking Beyond Individual Animals 

One of the defining characteristics of PRRS is its ability to persist within affected herds. Infection commonly develops in piglets after maternally derived colostral antibodies decline between three and six weeks of age, allowing continued viral transmission among successive groups of pigs¹. 

Large production systems that introduce pigs from multiple sources may experience continuous virus circulation because animals differ in their immune status and previous disease exposure. As a result, disease patterns often extend beyond individual clinical cases and become a herd-level health issue. 

Practical Clinical Insights 

PRRS should be suspected whenever reproductive losses coincide with respiratory disease, particularly if multiple age groups are affected. Clinical signs alone are not sufficient for diagnosis, but they provide valuable direction for further investigation. Examining aborted materials, dead piglets, and clinically affected animals can strengthen diagnostic efforts, while awareness of secondary bacterial infections helps explain variations in disease severity. Evaluating the overall herd picture, rather than isolated clinical cases, remains essential for recognizing PRRS and guiding subsequent diagnostic and control measures. 

References 

1. 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/ 

2. 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 

3. Colomer MÀ, Margalida A, Fraile L. Improving the management procedures in farms infected with the Porcine Reproductive and Respiratory Syndrome virus using PDP models. Scientific reports. 2019 Jul 10;9(1):9959. https://www.nature.com/articles/s41598-019-46339-w.pdf