Science Page: To filter or not to filter, that’s not the question anymore!

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, Drs. Torremorell and Janni explain what is new in the world of air filtration.

Key Points

  • There are multiple components that affect the effectiveness of filtration for a particular farm.
  • Virus concentrations, particle size, and prevalence of particle size impact virus concentration within the barn.
  • Filtration type and functionality over season can also affect virus concentration within the barn.

Long gone are the days when we debated whether it’s beneficial to install air filtration in a farm. If you are in a high dense area and you have new breaks often enough that filtration pays off, then filtrate! Whether it is the removal of virus from the air (which is what filters do), or the enhancement and enforcement of basic biosecurity measures which are part of the “filtration package”, air filtration has been shown to reduce the number of PRRSV (porcine reproductive and respiratory syndrome virus) breaks.

However, filtration has not always met farm owner expectations. Retrospective data analysis from the Morrison Swine Health Monitoring Project suggests that PRRSV incidence hasn’t been reduced as much as we had hoped for, although it’s still better than no filtration.

So, what can be going on?

To help producers and veterinarians to contrast and compare filtration options and to help understand the components that affect filtration, we created a model that estimates the theoretical number of airborne viruses that would enter a barn through either filters or through leakage given certain assumptions.

The model takes into account various inputs considered important to affect filtration and provides a look at the interaction between ventilation rate, building leakage, filter particle removal efficiency and particle size distribution where viruses may attach.

We have learned a few things already.

First, and the obvious one, is that filtering ventilation air does decrease virus concentration inside filtered barns. Second, ambient virus concentrations and their size distributions, which are largely unknown, have a large impact on virus concentration inside the barn. Although we have done some measurements of virus distribution based on particle size, the relative distribution is likely affected by various factors such as environmental conditions, type of virus, source of virus aerosol, etc.

In general we can say that if viruses are found mostly in the smallest particles (<1 microns), MERV 14 filters will do quite poorly since their lowest removal efficiency is for particles less than 1 micron (~ 76% removal efficiency). In contrast, if the largest amount of virus is found in medium and large particles (>1 micron), as our studies suggest, then MERV 14 filters should do quite well and the more viruses there are in the largest particles (>3 microns), the better MERV 14 filters do and in this case performance is similar to MERV 16 filters.

However, since virus particle size distribution is likely to change throughout the day and season, the only way to minimize the impact of virus particle size distribution is by using higher MERV rating filters with removal efficiencies above 95 % if not more. But, the overall importance of filter collection efficiencies is uncertain because the ambient virus concentrations and their size distributions are not really known.

The other point that is important to recognize is that there are higher barn virus concentrations with lower mechanical ventilating rates and higher barn leakage rates. In other words, during low ventilation rates (i.e. winter) the number of virus particles per cubic feet per minute is higher than during high ventilation rates (i.e. summer). However, when we consider total amount of virus particles that may enter the barn per minute, then higher ventilation rates result in higher in-barn virus concentration compared to lower ventilation rates. This observation is important also when considering positive ventilated barns since it is not uncommon that in the winter, they operate at higher flow rates than negative ventilated barns resulting in the potential introduction of more viruses through the filter, even though leakage is nearly eliminated.

Lastly, as it is well known reducing barn static pressure drop by increasing filter area helps reduce leakage and virus concentrations in the barn. So, it is not a question whether air filtration helps, but rather knowing which factors to consider when making the best of air filtration. Our model does not measure risk of PRRSV infection into a farm but it shows a fairly complex, not always obvious, interaction between ventilation rate, building leakage, filter particle removal efficiency and viral particle size distribution that knowing it, may be useful to producers and veterinarians when evaluating air filtration systems for sow farms.

For more information about the model, contact Kevin Janni ( or Montse Torremorell ( at the University of Minnesota.

UMN swine groups meet with the Minnesota Pork Board members

On December 17th, swine-focused faculty members from the College of Veterinary Medicine and the College of Food, Agricultural and Natural Resource Sciences met with the members of the Minnesota Pork Board research committee. 

At the Christensen Farms’ truck wash

This year was the third iteration of this meeting, hosted for the first time by Christensen Farms. Guests were invited to tour either the feed mill or the truck wash located in Sleepy Eye before sitting down for a day of productive exchange. 

After a short presentation by the host company, Deans Brian Buhr and Trevor Ames gave an update on the main initiatives from their respective Colleges. Dean Ames introduced the latest AGREETT faculty hires, Noelle Noyes and Declan Schroeder, as well as the anticipated arrival of Dr. Matheus Costa in the new year. The 2+2 program in collaboration with South Dakota State University was received with enthusiasm by the swine producers.

After lunch, attendees were divided into groups to brainstorm research ideas in four main areas: swine nutrition, swine health, manure management, and swine production and housing. After two hours of spirited discussion, 15 researchable questions were identified as the most pressing problematics faced by the Minnesotan pork industry. The meeting left the researchers inspired to keep offering science-driven solutions, and all agreed to meet again in 2019.

NHF: What does livestock-associated MRSA mean for the neighborhood?

Our monthly collaboration with the National Hog Farmer continues; this month Dr Peter Davies from the University of Minnesota, College of Veterinary Medicine,  explains what is livestock-associated MRSA and if it affects people living near pig farms.

“Livestock-associated” MRSA first isolated in 2004

Methicillin Resistant Staphylococcus aureus (MRSA) Source: Wikimedia Commons

Most people are likely familiar with MRSA (methicillin resistant Staphylococcus aureus), a flagship “superbug” that is a major concern to human medicine. And just about everybody in the pig industry has heard that certain variants of MRSA are very common in some livestock populations (including pigs), and these are referred to as “livestock-associated” MRSA (LA-MRSA).

A novel variant of MRSA (labelled ST398 using a DNA typing method) was first found in pigs in the Netherlands in 2004. Subsequently, ST398 MRSA and several other types (e.g., ST9 in Asia, ST5 in North America) were reported in pigs in numerous countries, and often in their caretakers as well. The discovery that pigs may be a large MRSA reservoir created some justifiable panic and confusion, raising questions about the implications for human health, particularly for industry workers (e.g., farmers, veterinarians, processing plant workers), pork consumers and, last but not least, people living in the neighborhood of pig farms.

Generally, LA-MRSA  lack most of the key “virulence factors” that enable the bacteria to cause clinical infections in people.

Human clinical infections by “livestock-associated” MRSA are rare

Although workers on MRSA-positive farms often harbor LA-MRSA in their nose, significant clinical infections in healthy workers have been rare. Human clinical infections with LA-MRSA do occur, but most cases tend to be of relatively mild disease (such as skin infections), with more severe infections typically limited to elderly and medically compromised patients.

Remembering that about 2% of healthy U.S. citizens carry human adapted variants of MRSA, the relative clinical importance of LA-MRSA appears to be minimal in most countries. Globally since 2004, there have been around 10 fatal cases of LA-MRSA infections reported, compared with about 50 fatal MRSA cases per day (18,650 per year in 2005) in the United States alone.

2016 study of Iowa hospitals found probable livestock variants in only 0.24% of MRSA cases, and 1% of S. aureus infections. In North Carolina, another leading swine-producing state, there were no LA-MRSA variants among more than 1,200 MRSA isolated from human bloodstream infections between 1995 and 2015 (Dr. Vance Fowler, Duke University, personal communication).

Living next to pig farms does not increase the risk of exposure

Although MRSA can be isolated from meat products, there is little evidence to suggest cause for concern about food-borne transmission. In contrast, conclusions of studies looking at the neighborhood risk of exposure to LA-MRSA from pig farms are conflicting. We will focus on the findings of studies that have compared pig workers and neighbors directly, measured the distance from pig barns to residences directly; and used laboratory testing to confirm the presence of LA-MRSA in the study populations. 

Across three early studies in Europe, LA-MRSA prevalence (nasal carriage) was greater than 180 times higher in 352 pig industry workers (44%) than in 2,094 rural residents without farm exposure (0.24%).

  • A similar study in Holland in 2017 showed similar prevalence of nasal colonization (0.56%) in people without livestock contact, but also found the positive people on average lived closer to farms (of any type). Importantly, the authors noted that routes of transmission underlying this were not known.
  • A very detailed study of cases of MRSA infection in Denmark showed that overall MRSA risk did not differ between pig-dense regions versus other regions. However, the likelihood that a MRSA infection would be a LA-MRSA type was higher in the pig-dense regions, confirming some “spillover” from the industry to the community.
  • Notably, a follow-up study published in 2018 found that, within pig-dense areas, the patients with LA-MRSA infections did not live closer to pig farms than population controls. The authors conclude that direct environmental spread from neighboring pig farms was unlikely and suggested that community spread through contact with people working with livestock,  might be the predominant mechanism.

In summary, the overall impact of LA-MRSA relative to human variants remains very small in most countries including the United States. There is no evidence that residence in rural areas increases overall MRSA risk.