Swine respiratory disease (SRD) remains the most economically significant and biologically complex challenge in modern pork production. While the clinical presentation of SRD may be challenging to spot at the onset, early identification is crucial because of the devastation it causes pig’s well-being as well as a producer’s profitability. Further complicating matters and making SRD more challenging to control is the frequent risk of co-infections that stem from a myriad of factors. 1
This article will explore SRD and one of the key contributing pathogens, Porcine Reproductive and Respiratory Syndrome (PRRS) – a widespread viral disease in the US. It will dive into the pathology and the havoc it can create amongst herds. Understanding these dynamics is important to addressing the problem SRD and PRRS pose to today’s pork producers.
Unpacking SRD
SRD refers to a plethora of clinical signs affecting a pig’s respiratory system. SRD can be caused by primary pathogens, which are bacteria and viruses considered the definitive cause of disease in healthy swine. It is also common for secondary pathogens, disease agents that normally don’t cause disease in healthy swine, to become more fatal when the immune system is weakened by concurrent infections.2
Noninfectious causes, including herd management and environmental factors, can also be a primary or secondary cause of SRD. Suboptimal conditions such as inadequate air, water, feed, overstocking, etc., can result in increased stress and damage to the animal’s respiratory tract.3
SRD can create devastation amongst herds, particularly in lactation, nursery and grow-finish phases of production with a continuous flow of pigs and/or mixing of multiple pig sources. This, combined with other stressors involved in transitional moves, can contribute to their vulnerability to, and the spread of, SRD.1
The SRD Elephant in the Room: PRRS
As the name suggests, Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) has traditionally caused reproductive failure in breeding animals and respiratory disease in all stages of growing pigs. Once pigs become infected, PRRS replicates in pulmonary alveolar and intravascular macrophages, the only indigenous cell types known to support PRRSV replication.4
Once animals become viremic, the virus can be shed and transmitted in nasal secretions, urine, semen, mammary secretions and feces.6 Another notable feature of PRRS is its ability to survive in long-term carrier pigs, lasting more than 200 days. While shedding carriers are likely the most common cause for introduction to herds, contaminated fomites (vehicles, equipment, clothing, needles, etc.) and aerosolization should also be a primary consideration when sourcing an infection. 5,6
While strain-dependent, PRRSV induces intense inflammation primarily via the pro-inflammatory cytokines TNF-α and IL-1β which causes a dysfunctional endothelial barrier in the lungs, but also allows macrophages to be more susceptible to the virus. This severely compromises the protective lining and the ability to mount an adequate immune response in the lungs resulting in inevitable secondary bacterial infections.7
PRRS virus is also known to cross the placenta and cause inflammation of the umbilical vessels. Depending on the stage of pregnancy at the time of the infection, this may result in reproductive failure including abortion, increased mummified fetuses, stillborn piglets, as well as weak, persistently infected piglets at birth. Additionally, because of pregnancy complications, sows can exhibit a plethora of clinical signs including fever, loss of appetite, failure to produce milk, and death.5
The PRRS virus is an enveloped RNA virus in the Arteriviridae family. Single-stranded RNA viruses are generally considered more “unstable,” compared to the double-stranded helix structure found in DNA viruses. This allows PRRS to mutate and recombine at a higher rate. Therefore, the PRRS virus’ ability to rapidly evolve and become highly diverse makes establishing lasting immunity – and thus controlling and eliminating the disease – very challenging.8 Controlling this disease becomes exceptionally challenging when we consider continuous transmission situations from dam-to-pig, pig-to-dam, and from pig-to-pig, particularly when multiple viruses and/or varying ages and immune statuses are involved. For this reason, all-in / all-out management practices have been a critical practice adopted by veterinarians and producers.
The Economic Disruptions of SRD and PRRS Outbreaks
PRRS is the most economically damaging illness for swine producers across the United States. A recent study published shows PRRS costs about $1.2 billion in losses each year in the US due to reproductive failure and high mortality in nursing pigs, an 80% increase from the previous decade. That same study breaks down to $192.23 / sow.9 Another study recently published estimated the impact of PRRS ranging from $117 to $378 / sow each year.1 We must remember these are averages calculated using all of the females in the study – affected and unaffected. Most strains of PRRS will end up costing a producer exponentially more on a per sow basis.
Like water, PRRS flows downhill. Not only is the economic impact on sows a hard pill to swallow, but PRRS will go on to negatively affect sow’s offspring in the nursery and finisher stages. This creates a resonating economic impact throughout a swine operation, which is eventually felt at the packer stage. The impact is expansive as pigs that beat mortality odds can still battle secondary diseases as well as have tissue damage to the lungs that affect overall growth performance. It is estimated that for every 10% of lung area permanently damaged within a pig, the average daily gain decreases by 0.08 pound.10
The cost of primary pathogens to producers can certainly be impactful on their own. When multiple challenges are in play, the cost to the producer can be exponential. It is well known and documented by now that the effect of multiple infections on mortality and performance is greater than the additive cost of single infections.11 Not only will the economic impact show up as higher mortality and lower average daily gain, but also through an increased number of discounted culls and light-weight pigs.
An individual Mycoplasma hyopneumoniae (M. hyo) infection incurs a performance cost of $0.63. When the M. hyo infection is coupled with PRRS infection, the performance cost can drastically increase to $9.69, based on a 2012 study.
SRD, primarily due to PRRS, remains the most opportunistic and economically damaging health challenge facing US pork producers today. PRRS’ ability to adapt and exploit immunological and environmental vulnerabilities makes it an ongoing threat to herd health and productivity. Effective management begins with prevention through a comprehensive approach to biosecurity, which mitigates the impact of PRRS and improves long-term economic outcomes for producers.
Sources
1 Boeters, M., Garcia-Morante, B., et al. 2023. “The economic impact of endemic respiratory disease in pigs and related interventions – a systematic review.” Porcine Health Management. https://porcinehealthmanagement.biomedcentral.com/articles/10.1186/s40813-023-00342-w
2 Zimmerman, J., Karriker, L., et al. 2019. “Diseases of Swine.” John Wiley & Sons, Inc. https://onlinelibrary.wiley.com/doi/book/10.1002/9781119350927
3 Brockmeier, S., Halbur, P., et al. “Porcine Respiratory Disease Complex.” In: “Polymicrobial Diseases.” ASM Press. https://www.ncbi.nlm.nih.gov/books/NBK2481/
4 Rossow, K. 1998. “Porcine Reproductive and Respiratory Syndrome.” In: “Veterinary Pathology.” American College of Veterinary Pathologists. https://journals.sagepub.com/doi/abs/10.1177/030098589803500101
5 “Swine Disease Manual” In: “Porcine Reproductive and Respiratory Syndrome (PRRS)” Iowa State University. https://webhost-dev.cvm.iastate.edu/swine-disease-manual/index-of-diseases/porcine-reproductive-and-respiratory-syndrome-prrs/#:~:text=An%20important%20feature%20of%20the,virions)%20but%20not%20highly%20contagious.
6 Arruda, A., Tousignant, S., et al. 2019. “Aerosol Detection and Transmission of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV): What Is the Evidence, and What Are the Knowledge Gaps?” In: “Porcine Viruses.” MDPI.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6723176/
7 Sun, Z., Chen, X., et al. 2023. “PRRSV-induced inflammation in pulmonary intravascular macrophages (PIMs) and pulmonary alveolar macrophages (PAMs) contributes to endothelial barrier function injury.” In: “Veterinary Microbiology.” Elsevier.
https://pubmed.ncbi.nlm.nih.gov/37068404/
8 Butler, J., Lager, K., et al. 2014. “Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic.” In: “Immunologic Research.” Springer. https://pmc.ncbi.nlm.nih.gov/articles/PMC7091131/
9 Osemeke, O., Silva, G., et al. 2025. “Economic impact of productivity losses attributable to porcine reproductive and respiratory syndrome virus in United States pork production, 2016-2020.” Prev. Vet. Med. https://pubmed.ncbi.nlm.nih.gov/40712375/
10 Straw, B., Tuovinen, V., et al. 1989. “Estimation of the cost of pneumonia in swine herds.” Journal of American Veterinary Medical Association. https://avmajournals.avma.org/view/journals/javma/195/12/javma.1989.195.12.1702.xml
11 Haden, C., Painter, T., et al. 2012. “Assessing production parameters and economic impact of swine influenza, PRRS and Mycoplasma hyopneumoniae on finishing pigs in a large production system.” American Association of Swine Veterinarians. https://docs.boehringer-ingelheim.com/13_Haden-et-al-Assessing-prdn-parameters-and-economic-impact-of-IAVS-PRRS-and-Mhp-on-finishing-pigs-in-a-large-prodn-system-2012.pdf
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