Biosecurity is not Swiss Cheese

Why ventilation, biosecurity, and animal resilience all matter in the fight against PRRS 

Porcine reproductive and respiratory syndrome, or PRRS, continues to be one of the most expensive and frustrating diseases in modern swine production. It affects sow farms, nurseries, grow-finish systems, veterinarians, nutritionists, production managers, employees, and ultimately the entire pork supply chain. 

For years, the industry has used the “Swiss cheese model” to describe biosecurity: every layer has holes, but if enough layers are stacked together, the holes are less likely to line up. The analogy is useful at a basic level, but it also risks oversimplifying the reality of PRRS. 

As Dr. Casey Bradley would say, PRRS is not a simple “add another slice of cheese” problem. It is complex. It needs to be taken seriously on every front. Biosecurity matters. Ventilation matters. Filtration matters. Pig movement matters. People movement matters. Transport matters. Nutrition and immune resilience matter. And even when we do many things right, PRRS can still find a way through. 

That is not a reason to give up. It is a reason to be more disciplined. 

The economic weight of PRRS breaks

The economics of PRRS are no longer theoretical. A recent Iowa State University update estimated that PRRS costs the U.S. pork industry approximately $1.2 billion per year in lost production, based on 2016–2020 data. That estimate represents an approximately 80% increase compared with the earlier $664 million annual loss estimate from data collected between 2006 and 2010. 

Those losses are not just numbers on a spreadsheet. A PRRS break can mean increased sow mortality, reproductive losses, abortions, weak-born pigs, pre-weaning mortality, increased nursery and finishing mortality, reduced average daily gain, poorer feed efficiency, increased veterinary intervention, disrupted pig flow, and emotional fatigue for caretakers. 

The financial damage also differs by phase of production. Historically, PRRS was often discussed primarily as a sow farm issue because of its dramatic reproductive impact. However, recent economic analyses indicate that a growing share of PRRS-related losses now occurs in growing pigs. This matters because PRRS is not simply a breeding-herd stability problem. It is a system-wide production and health problem. 

In Rob Langenhorst’s words from the World Pork Expo conversation, producers are trying to “keep the doors open” while dealing with rapidly changing strains and repeated breaks. That is the practical reality behind the economics. 

A virus that keeps changing 

PRRSV is an RNA virus, and its ability to change is part of what makes it so difficult to control. In the interview, Rob described the post-2021 situation with lineage 1C strains as “kind of like we’re chasing a ghost.” That phrase captures the frustration many veterinarians and producers feel. 

The industry builds control strategies, adapts vaccination protocols, strengthens biosecurity, and changes management practices—then the virus shifts. New strains emerge. Regional pressure changes. Breaks occur in farms that thought they were relatively protected. 

This is why PRRS control must be approached as a dynamic system. A written biosecurity plan sitting in a binder is not enough. Protocols must be implemented, audited, updated, and understood by the people doing the work. 

Airborne risk: what ventilation and filtration can—and cannot—do

Airborne PRRS transmission has been studied for years. The science is nuanced. PRRSV does not always spread long distances through the air, and not every outbreak is an aerosol event. But aerosol transmission is possible, especially under the right combination of virus strain, environmental conditions, pig density, wind patterns, and farm proximity. 

Rob made an important point in the interview: viruses are small, but they typically do not move through the air as naked virus particles. They are commonly associated with transport vectors such as dust particles, droplets, dander, and other airborne material. That is where filtration and ventilation management become relevant. The goal is not to catch a free-floating virus molecule in isolation. The goal is to reduce the entry of virus-associated airborne particles into the barn. 

Peer-reviewed studies have shown that filtration systems can reduce aerosol transmission risk. Early experimental work demonstrated that high-efficiency filtration and other air-treatment systems could reduce PRRSV transmission under controlled conditions. Longer-term field evaluations also found lower rates of new PRRSV infections in filtered herds compared with non-filtered herds in high-density swine regions. 

More recently, a University of Minnesota/Morrison Swine Health Monitoring Project analysis evaluated more than 400 sow farms over approximately 16 years. The study reported that negative-pressure filtered farms had a 51% lower PRRS outbreak rate and positive-pressure filtered farms had a 58% lower outbreak rate compared with unfiltered farms. Importantly, the study did not find a meaningful difference between positive and negative pressure filtration in that analysis. 

That aligns with Rob’s practical comment: the decision is not simply “which system is better?” Positive-pressure and negative-pressure systems both have advantages and disadvantages. The right choice depends on the barn, the age of the facility, existing ventilation, air leakage points, construction materials, roof and ceiling integrity, filter bank space, and the producer’s management system. 

A filter is not a filter 

One of the most important practical takeaways from Rob’s interview is that filtration for PRRS control is not the same as putting a household filter in a barn. 

A household-style filter may function as a pre-filter, removing larger dust particles, but it is not the primary filter responsible for reducing virus-associated aerosols. In swine systems, filter selection must consider airflow, static pressure, MERV rating, filter life, ventilation demand, seasonal ventilation changes, and whether the facility can be sealed well enough for the system to work as intended. 

Rob discussed how the industry has evolved from combinations such as MERV 14 and MERV 16 filters to broader use of MERV 15 systems and newer high-flow MERV 16 options. That evolution matters because filtration is always a balancing act. Producers need protection, but they also need enough airflow to maintain pig comfort, air quality, temperature control, and worker conditions. 

Poorly designed filtration can create problems. A system that reduces airflow too much may compromise ventilation. A system installed in a leaky barn may allow air to bypass the filters. A system that is not maintained may create a false sense of security. 

The science and the field experience point in the same direction: filtration can be powerful, but only when designed, installed, monitored, and managed correctly. 

The barn itself matters 

Air follows the path of least resistance. That simple principle can make or break a filtration strategy. 

Rob noted that when evaluating farms, especially negative-pressure systems, producers must look for leaks: holes, batten strips, sill plates, windows, doors, and other points where unfiltered air may enter. Older barns may have more leak points. Newer barns with concrete walls may be easier to seal, but positive-pressure systems may require a sealed roof and ceiling. In other words, the barn often dictates the filtration strategy. 

This is where producers need more than a product recommendation. They need a facility-level evaluation. The questions should include: 

• Where is air entering the barn? 
• Where is air bypassing the intended inlet path? 
• Can the building be sealed adequately? 
• What are the summer and winter ventilation needs? 
• Can the system maintain pig comfort during high-demand ventilation periods? 
• What maintenance schedule is realistic for the farm team? 
• Who is responsible for monitoring filter condition and pressure changes?

Filtration should not be treated as a one-time capital project. It is an ongoing management system. 

Biosecurity is not Swiss cheese—it is a pressure-tested system

The “Swiss cheese model” is easy to explain, but PRRS deserves a stronger mental model. 

Biosecurity is not just stacked layers. It is a pressure-tested system. Every part of the system is under pressure from virus movement, people movement, pig movement, feed delivery, mortality handling, transport vehicles, weather, wildlife, employee fatigue, and human shortcuts. 

Rob used a different analogy in the interview: biosecurity is like an onion, full of layers. Air filtration is one layer. But he was also clear that filtration changes the process. Once a farm installs filtration, everyone must follow specific procedures to prevent the virus from entering through other routes. 

That is the key point. Filtered barns can still break. When they do, the cause may not be incoming air. It may be a loadout event, an employee entry error, a lunch cooler, a feed bin line, a transport risk, a pressure-washer mistake, a rendering event, an equipment transfer, or a gap in downtime compliance. 

A serious PRRS control plan must include: 

• Air entry management
• People entry protocols 
• Supply entry protocols 
• Transport sanitation 
• Loadout design 
• Mortality management 
• Feed and ingredient biosecurity 
• Semen and gilt-source biosecurity 
• Pig flow discipline 
• Downtime and shower-in/shower-out expectations 
• Employee training and accountability 
• Ongoing audits 
• Rapid detection and response plans

This is why the Swiss cheese analogy can feel too soft. PRRS control is not passive. It is not simply hoping holes do not align. It is active pressure management. 

Ventilation also affects people

Ventilation and filtration are often discussed from the pig-health perspective, but Rob raised another important point: worker health and comfort. 

Barn air can be difficult to breathe, particularly in winter when ventilation rates are lower and dust levels can feel high. Filtered systems, when designed and managed correctly, can improve incoming air quality and create a more comfortable work environment. Rob noted that clean, adequate airflow can make the barn more enjoyable for workers—and without workers, we cannot raise pigs. 

This matters. The industry often discusses labor retention, employee training, and caretaker responsibility, but air quality is part of the working environment. A system that supports pig health and improves the daily experience for employees has broader value than disease prevention alone. 

The role of nutrition and animal resilience

As an animal nutritionist, it is important to be honest: nutrition does not “fix” PRRS. 

Dr. Bradley made that point directly in the interview: “I can’t fix PRRS with nutrition. I can help support the animal, but I can’t fix it.” 

That distinction matters. Nutrition should not be oversold as a replacement for vaccination, biosecurity, filtration, or veterinary oversight. However, nutrition does influence the pig’s ability to respond to stress and disease. Robust animals are better prepared to handle immune challenges, maintain intake, repair tissue, and recover from health insults. 

A PRRS-resilience strategy may include attention to: 

• Adequate energy and amino acid supply during immune activation 
• Mineral and vitamin adequacy, especially nutrients involved in immune function and antioxidant defense 
• Gut health and barrier function 
• Mycotoxin risk management 
• Water quality and intake 
• Feed access and diet transitions 
• Body condition management in sows 
• Reducing non-disease stressors that compound immune challenge 

This is not about pretending feed can stop PRRS. It cannot. It is about recognizing that when disease pressure occurs, the animal’s baseline resilience affects how hard the system falls and how quickly it recovers. 

Biosecurity aims to keep the virus out. Nutrition and management help prepare the pig if the virus gets in. 

Moving from fear to disciplined prevention

PRRS is emotionally exhausting because it punishes complacency. A farm can do many things right and still experience a break. That reality can create frustration, fatigue, and fatalism. 

But the recent filtration data are encouraging because they suggest measurable risk reduction is possible. A 51–58% reduction in outbreak rates does not mean elimination. It does mean that disciplined investments in air management can reduce risk in meaningful ways. 

The larger message is not “install filters and relax.” The message is “use every serious tool available, and manage each one with discipline.” 

PRRS control requires a comprehensive approach: 

Reduce pathogen entry through air, animals, people, supplies, and vehicles. 

Detect problems early through monitoring and diagnostics. 

Respond quickly when instability occurs. 

Build animals that are robust enough to withstand health pressure. 

Train teams so protocols are understood, not just posted. 

Audit the system before the virus does. 

Conclusion

PRRS remains one of the most serious disease challenges in swine production. The economics are too large, the animal welfare consequences too real, and the operational disruption too significant to treat it casually. 

Air filtration and ventilation control are important tools, especially in high-risk regions and high-value breeding herds. Recent data strengthen the business case for filtration and show that both positive- and negative-pressure systems can reduce PRRS outbreak risk when designed and managed properly. 

But filtration is not a silver bullet. Neither is vaccination. Neither is nutrition. Neither is a shower. Neither is a UV box. PRRS control requires seriousness on all fronts. 

The industry does not need another simplified analogy as much as it needs disciplined systems thinking. Biosecurity is not just Swiss cheese. It is a living, pressure-tested system that must be designed, maintained, audited, and respected every day. 

And at the center of that system is still the pig. 

Our goal is not only to keep disease out. It is also to raise animals with the resilience to survive, recover, and thrive when the system is challenged. 

References

Alonso, C., Davies, P. R., Polson, D. D., Dee, S. A. 2012. Evaluation of the long-term effect of air filtration on the occurrence of new PRRSV infections in large breeding herds in swine-dense regions. Viruses. 

Dee, S. A., et al. 2006. Evaluation of systems for reducing the transmission of porcine reproductive and respiratory syndrome virus by aerosol. Canadian Journal of Veterinary Research. 

Dee, S. A., et al. 2005. Evaluation of an air-filtration system for preventing aerosol transmission of porcine reproductive and respiratory syndrome virus. Canadian Journal of Veterinary Research. 

Holtkamp, D. J., Osemeke, O. H., and colleagues. 2024. Updated economic impact estimates of porcine reproductive and respiratory syndrome virus in U.S. pork production. Presented at the International Pig Veterinary Society Congress and summarized by Iowa State University. 

Morrison Swine Health Monitoring Project / University of Minnesota. 2025. Sixteen-year analysis of air filtration and PRRSV outbreak rates in U.S. breeding herds. 

Otake, S., Dee, S. A., et al. 2010. Aerosol detection and transmission of porcine reproductive and respiratory syndrome virus: review of current knowledge and knowledge gaps. Viruses. 

Sykes, A. L., Silva, G. S., Holtkamp, D. J., Linhares, D. C. L., Machado, G., et al. 2021. Interpretable machine learning applied to on-farm biosecurity and porcine reproductive and respiratory syndrome virus. 

Rob Langenhorst interview transcript. World Pork Expo conversation with The Real P3.