Understanding PRRSV Transmission and Farm-Level Risk Factors

Successful control of Porcine Reproductive and Respiratory Syndrome (PRRS) begins with understanding how the virus spreads within and between herds. PRRSV employs multiple routes of transmission, allowing infection to persist even in well-managed farms. For practicing veterinarians, recognizing these transmission pathways and identifying farm-level risk factors are essential for investigating outbreaks, advising producers, and implementing practical biosecurity measures that reduce disease spread. 

Multiple Routes of Virus Transmission 

PRRSV is capable of spreading through both direct and indirect routes, making disease control particularly challenging. 

Direct contact with infected pigs remains one of the most important modes of transmission, especially among breeding and finishing pigs¹. Acutely infected animals can readily transmit the virus through close contact for up to 14 weeks after infection, and isolated contact infections have been reported even after 99 days². 

The virus has also been detected in urine and feces, although successful isolation from feces is uncommon³. In experimentally infected boars, PRRSV has been recovered from semen, indicating that artificial insemination may contribute to virus transmission when semen is collected during the acute phase of infection³. 

Another important characteristic of PRRSV is its ability to persist within infected animals. Viral reappearance has been documented in the oropharynx up to 157 days after infection, demonstrating that recovered animals may continue to play a role in herd-level virus maintenance³. 

Herd Dynamics Influence Disease Spread 

The extent of virus transmission depends not only on the presence of infected animals but also on herd management practices. 

In closed herds, PRRSV may continue circulating among weaned pigs after maternally derived colostral antibodies decline between three and six weeks of age³. Large finishing units that receive pigs from multiple sources create favourable conditions for sustained virus transmission because animals often differ in their health and immune status. 

Although persistent or latently infected pigs are believed to contribute to virus circulation, field evidence demonstrating the exact impact of these animals remains limited⁴. 

Farm-Level Risk Factors 

Several management-related factors increase the likelihood of PRRS introduction and spread. 

Important risk factors include: 

• Purchase of replacement pigs 

• Close proximity to infected pig farms 

• Absence of quarantine for newly introduced animals 

• Large herd size⁵ 

In one investigation involving infected herds in Germany, most affected farms had either introduced new stock within four weeks before the outbreak or were located within 5 km of an infected herd⁶. These observations reinforce the importance of carefully evaluating animal movement and neighbouring herd status during outbreak investigations. 

Airborne dissemination also contributes significantly to local transmission, alongside the movement of infected pigs between farms³. Although other reservoir species have not been confirmed, preliminary observations suggest that migratory birds may carry the virus and potentially act as vectors⁷. 

Applying This Knowledge During Field Investigations 

When investigating suspected PRRS outbreaks, veterinarians should look beyond clinically affected pigs and assess the entire production system. 

Questions that can assist field investigations include: 

• Have replacement animals been introduced recently? 

• Were appropriate quarantine measures followed? 

• Are neighbouring pig farms experiencing respiratory or reproductive disease? 

• Does pig movement occur between multiple production sites? 

• Are different age groups adequately separated? 

Answering these questions helps identify possible sources of infection and supports more targeted control recommendations. 

Practical Clinical Insights 

PRRSV transmission is influenced by a combination of viral persistence, animal movement, management practices, and local epidemiological factors. Understanding these pathways enables veterinarians to identify weaknesses in farm biosecurity and investigate outbreaks more systematically. Careful assessment of replacement stock, quarantine procedures, herd structure, and neighbouring farms should form an integral part of every PRRS investigation, helping reduce opportunities for virus introduction and ongoing circulation within commercial swine herds. 

References 

1. Pileri E, Mateu E. Review on the transmission porcine reproductive and respiratory syndrome virus between pigs and farms and impact on vaccination. Veterinary research. 2016 Oct 28;47(1):108. https://link.springer.com/content/pdf/10.1186/s13567-016-0391-4.pdf 

2. Raymond P, Bellehumeur C, Nagarajan M, Longtin D, Ferland A, Müller P, Bissonnette R, Simard C. Porcine reproductive and respiratory syndrome virus (PRRSV) in pig meat. Canadian Journal of Veterinary Research. 2017 Jul 1;81(3):162-70. https://pmc.ncbi.nlm.nih.gov/articles/PMC5508380/pdf/cjvr_03_162.pdf 

3. 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/ 

4. Arruda AG, Tousignant S, Sanhueza J, Vilalta C, Poljak Z, Torremorell M, Alonso C, Corzo CA. Aerosol detection and transmission of porcine reproductive and respiratory syndrome virus (PRRSV): What is the evidence, and what are the knowledge gaps?. Viruses. 2019 Aug 3;11(8):712. https://www.mdpi.com/1999-4915/11/8/712 

5. Hasahya E, Thakur KK, Dione MM, Wieland B, Oba P, Kungu J, Lee HS. Modeling the spread of porcine reproductive and respiratory syndrome among pig farms in Lira district of northern Uganda. Frontiers in veterinary science. 2021 Aug 30;8:727895. https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2021.727895/pdf 

6. Hu R, Zhang T, Lai R, Ding Z, Zhuang Y, Liu H, Cao H, Gao X, Luo J, Chen Z, Zhang C. PRRSV elimination in a farrow-to-finish pig herd using herd closure and rollover approach. Viruses. 2023 May 25;15(6):1239. https://www.mdpi.com/1999-4915/15/6/1239 

7. Wang C, Huang B, Kong N, Li Q, Ma Y, Li Z, Gao J, Zhang C, Wang X, Liang C, Dang L. A novel porcine reproductive and respiratory syndrome virus vector system that stably expresses enhanced green fluorescent protein as a separate transcription unit. Veterinary research. 2013 Oct 31;44(1):104. https://link.springer.com/content/pdf/10.1186/1297-9716-44-104.pdf