Our goal today is to understand why this may be, and to review control strategies we can employ to minimize the impact of this virus on our own herds.
About the virus
Porcine respiratory and reproductive syndrome virus, known well to the industry as “PRRS,” has been a real and present threat to pork production for more than 30 years. This virus is responsible for shaping the pork industry of today, moving pigs inside, pushing robust hygiene practices, installing air filtration systems on barns, and even adding mitigants to the feed to reduce exposure to the disease.
It has pushed producers large and small out of the market, and has reduced the genetic diversity of our national swine herd through smaller breeders exiting the industry because of disease pressure. Symptoms from the virus, combined with secondary infections, have pushed antibiotic usage to new heights in areas where the virus is endemic.
This year, many producers have observed that the disease has seemed worse and that we have lost our summer “off season” that we typically get to enjoy. Our goal today is to understand why this may be, and to review control strategies we can employ to minimize the impact of this virus on our own herds.
How does the PRRS virus work?
To begin, let’s take a moment to review how the virus works, to get a better understanding of what we are working against.
The PRRS virus is enveloped, which means it has a protective coat that it makes using the cell membranes from host cells. This gives it a sort of camouflage to evade host defenses. The envelope also contains surface antigens called glycoproteins, which have the ability to rapidly mutate and further evade the immune system. The nature of the virus allows it to “bud” out from a host cell rather than causing the complete rupture and death of infected cells. This allows for ongoing production, utilizing host body resources from infected cells.
On the flip side, an enveloped virus is generally fragile outside the host and is sensitive to heat, drying, UV light, and a number of soaps and disinfectants. Also, enveloped viruses are typically not easily transmitted outside direct contact. However, the PRRS virus is well documented for hitching rides on fomites (contaminated surfaces) and through aerosol spread up to nine miles in the right conditions.
In a cruel twist of irony, the host cells that the virus targets are the pig’s pulmonary alveolar macrophages (PAM cells), which are the white blood cells that reside in the lungs. Their main role is clearing infections from the respiratory system. Concurrent infection with pig diseases such as Mycoplasma hyopneumoniae can increase PAM cell populations in the lungs due to increased disease pressure in the lungs, giving the PRRS virus an all-you-can-eat buffet when it enters the host. This results in higher virus shedding and, subsequently, higher infectivity to neighborhood pigs.
On top of that, it is currently theorized that over 50% of market trailers become contaminated with the virus, so if they are not adequately cleaned and disinfected between loads, they can become a very effective vehicle for transmission across large areas of market-ready pigs. Couple this with our industry’s production philosophy of sending pigs to concentrated areas to take advantage of lower feed prices, and we have formed an ideal breeding ground for the virus to thrive.
Diagnostic review
Now that we have reviewed the virus, let’s move on to understand if it is actually worse this year. We will begin by looking at data presented by the Domestic Swine Disease Surveillance Program. This collaborative project, funded by the Swine Health Information Center (SHIC), involves multiple veterinary diagnostic labs, with the goal of aggregating swine diagnostic data and reporting it in an intuitive format, describing pathogen dynamics over time, specimen, age group and geographical area. Data is from the Iowa State University VDL, South Dakota State University ADRDL, University of Minnesota VDL, Kansas State VDL, Ohio ADDL and Purdue ADDL.
This first graph outlines the typical trendline of percent positive cases seen across these diagnostic laboratories, lending to the correct observation that there is some seasonality to the virus. The blue line is the predicted percent positive cases by submission, while the light blue zone is the expected range of variation.
2025 started very hot, rising above the expected range from May 5 to June 22. This is typically when the virus should be on the decline, so this contrary trendline concerned many producers and veterinarians. However, there was a sharp decline in early July, and positive cases coming into the collective labs are now running below the trendline.
The Morrison Swine Health Monitoring Project (SHMP) Report, which is a voluntary reporting system for producers that reports PRRS cases by infected farm, lends some further clarification to this disease trend. Participants in this reporting system represent over 50% of U.S. pork production, so it carries good power in understanding PRRS trends across the industry.
There is a lot going on in the graph:
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The green dots are plot points for the number of newly infected farms for any given week.
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The blue line is the estimated weighted moving average of these plot points, showing a trend if cases are moving in a positive or negative direction.
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The red line is the threshold past which we consider the virus to be in “outbreak” status; over the red line would be “PRRS season,” and under the red line is when we can collectively take a quick breath.
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The stated date along the red line on the graph is the time point when the cases officially cross into “PRRS season.”
One can observe that there was predictable seasonality to PRRS cases in the early days (when only Iowa and Minnesota operations were part of the report), but over time the percentage of new cases has dropped, while negative periods of time have shortened. This is likely due to the addition of operations across the U.S., demonstrating the interconnectivity of our industry and how pig movements across the country can impact virus transmission dynamics.
Finally, going back to the SHIC report, we can further break down transmission dynamics between sow farms and wean-to-finish productions. Focus on the red and blue lines, red being wean-to-finish and blue being sow farms. Red vertical lines have been placed at the time points where wean-to-finish sites move from trending downwards in percent positivity to trending upwards. By this graph, this appears to have occurred in July to August over the past three years. Approximately one to two months later, the same transition occurred in the sow farm data each year. Is this a function of increased virus exposure through high numbers of infected market pigs?
Discussion
So was PRRS worse this year? By the data, we can observe that the season persisted longer and hit higher heights than previous years. We could also be seeing the emergence of a new dominant strain of the virus, which could contribute to increased infectivity across our herds due to reduced virus recognition of a new strain.
What environmental effects could be contributing to the changing disease dynamics? We know that the virus likes cool, moist, and low-UV-light environments. However, it seems that time periods that carry increased particulate matter give the virus an opportunity to catch a ride between operations. This could be through soot in the air from increased wildfire activity, dust in the air due to the dry spring we experienced, or even humid and foggy conditions like we experience during peak corn growing season. These are all theories, but ones worth considering nonetheless.
In the end, the proof is in the pudding. The market reflects our supply and demand dynamics, and despite a static to slight decrease in demand for pork products, we continue to see strong prices. Could this be partly because of the fallout of the disease pressure we saw in the early summer?
Our ongoing experiences with diseases such as PRRS really need to be a call to action to our industry. Are we employing practices that allow the virus to thrive, or are we being vigilant to the transmission dynamics of the virus? What will happen to our industry if a new virus emerges? What changes will need to be made?
Conclusion
It is good to end on a positive note. We have learned a great deal over the past three decades about disease transmission dynamics in pigs.
Hog operations commonly have showers to enter, a far cry from the operational design of our grandparents. We are filtering the air coming into barns to ensure clean, pathogen-free environments, and we have learned that the feed can become a potential fomite for disease transmission as well, opening the field of feed mitigant technologies. We have also developed cooperative regional control programs in which neighbors work together to try to keep the virus out of the area.
Vaccination technologies continue to grow as well. There is great promise in our ability to produce a relevant and safe modified live virus to help the body prepare for infection, and also in great adjuvants that help to improve the efficacy of our vaccines, such as the PRRS vaccine for pigs.
We are even exploring the unique role antibiotics and nutritional interventions can have in altering the pH of the body enough to impact viral replication in PAM cells in the lungs.
All of these innovations not only help in our combat against the PRRS virus, but will most certainly prove valuable tools in the event of any new virus introductions to our industry as well.
Credit for information goes to the Swine Health Information Center (SHIC) and the Morrison Swine Health Monitoring Project (SHMP). If you find this information valuable, please be sure to contribute to and support these projects that continue to drive further understanding of disease dynamics across our industry.
