Dee says he expected aerosol transmission would be influenced by season, with more evidence of spread during cold months when the virus survives.

But that wasn't what the research showed. There was just one break linked to aerosol spread each month during the October-March period. During the April-September period, there was just one case of PRRS recorded in June, and when things really warmed up, there was no evidence of virus transmission.

However, what Dee found more exciting was what happened in weather transition periods. When the weather transitioned in late September, more activity was noted. The same situation occurred when the weather transitioned from winter to spring in late April to early May. “We see a flurry of aerosol activity during those periods of time,” he reports.

There are two types of high-risk days: overcast or wet, humid days with little wind; and days that are cloudy, with rising barometric pressure and low-velocity winds.

Three examples of low-risk days are:clear skies, bright sun and calm wind; humid, cool and windy; and low humidity, falling pressure and gusty winds.

Key Findings

Dee says the landmark study showed:

• Aerosol transmission of the PRRS virus has now been established out to 396 ft., the longest reported distance to date.

• The evidence for virus transmission on high-risk vs. low-risk days may influence decisions on pig movement, use of air filtration systems and even siting hog barns in locations known for a prevalence of low-risk days.

• Wind direction, humidity, ultraviolet light and barometric pressure were four weather factors that were repeatedly recognized as potential risk factors for aerosol transmission of PRRS virus.

• PRRS virus infection occurred more often in high-risk weather periods, but sometimes in low-risk weather periods, supporting Dee's contention that the industry needs to design production systems for year-round aerosol protection, possibly combining air filtration with alternative technologies.

The PRRS research project confirmed to Dee that the industry's medium-level biosecurity standard won't survive for long against PRRS in a hog-dense region.

“These farms will break at some point in time, and we showed that it could happen 30% of the time in one year in our research,” he explains. “But remember, this is still better than those farms that are not adopting any biosecurity measures at all.”

Future Work

This month, Dee and graduate students at the research farm embark on the third year of the five-year PRRS study, focusing on the impact on biosecurity when Mycoplasmal pneumonia is also present.

Years 4 and 5 will study PRRS virus transmission when hog barns at the research farm are spaced farther apart. More tests on area spread, transmission factors and weather events will also be conducted.

PRRS Regional Projects Falter

Four years after two pilot eradication projects for PRRS (porcine reproductive and respiratory syndrome) began, the project coordinator addresses critics who question the plan's chance of success.

“We have been working on this project for four years, and a skeptic would say that we are no further ahead in eradicating the virus from our two regions (Rice County and Stevens County,” says Robert Morrison, DVM, University of Minnesota.

“Several of the producers who started the program with positive herds still have positive herds, and show no inclination to eliminate the virus in the near future.

“Being an optimist, I say we have made progress, but the challenges for a voluntary program to eradicate PRRS are huge. Our producers in the two regions are more informed about PRRS, and most want us to succeed - but not all are involved. And until all are on board, we won't accomplish regional, let alone national eradication,” stresses Morrison.

The study involves 127 sites owned by 80 producers. Rice County was selected because of its low prevalence of PRRS and natural geographic barriers. Stevens County presents a different picture because of more breeding stock operations and larger and newer sow systems.

Accomplishments include 90% participation by producers in allowing their herds to be tested for PRRS and sharing their data with other producers, he says.

And while the PRRS virus has been detected at many sites, there have been very few cases pointing to area spread between farms since the project began.

However, ongoing challenges include:

• Unknown status of some herds;

• Identifying all sites on which hogs may be produced; and

• Failure of a few producers to invest in eliminating PRRS infection.

Morrison says the tools are available to clean up PRRS in these two counties - and he remains optimistic that all producers will participate fully in the process.