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Reducing The Area Spread of PRRS

A leading expert on the PRRS virus at the University of Minnesota says his research team has pinpointed the essential biosecurity tools. The pork industry

A leading expert on the PRRS virus at the University of Minnesota says his research team has pinpointed the essential biosecurity tools.

The pork industry has known how to eliminate the PRRS (porcine reproductive and respiratory syndrome) virus from hog farms since 2000.

The catch has been in identifying ways to control area spread of the virus to protect negative farms from regularly becoming reinfected. That quest has become a big part of Minnesota veterinarian Scott Dee's research focus.

Dee spent much of his 12 years in private practice on developing control strategies for on-farm PRRS infections. Eight years ago, he joined the University of Minnesota, serving as co-director of the Swine Disease Eradication Center, and leading efforts toward unlocking the secrets to area spread of PRRS.

He has been a strong proponent that the pork industry needs to set a course of PRRS eradication (See: “The New War on PRRS,” National Hog Farmer, June 15, 2006, page 34.).

Speaking at the first Pijoan Lecture during the Leman Swine Conference (named after Carlos Pijoan, DVM, University of Minnesota, who passed away last December), Dee summarized his research evidence.

PRRS Components

Fomites, the mechanical means of transmission of the PRRS virus, are responsible for more frequent occurrences of virus spread in cold weather than in warm weather. Producers can reduce risk of disease spread by changing boots, coveralls, washing hands, showering and incorporating 12 hours of downtime between pig contact periods, according to Dee.

The use of disposable footwear and gloves and double-bagging products for entry into farms greatly lessens the level of PRRS contamination and mechanical spread of the virus, he says.

Contaminated transport vehicles have definitely been implicated as a source of PRRS virus infection for naïve pigs, and drying is a key element in maintaining biosecurity.

Insects have been shown to be mechanical, but not biological, carriers of the PRRS virus. Their control can be enhanced using a combination of screening of the air inlets of hog units along with the use of targeted insecticides and habitat management.

The role of aerosol spread in PRRS transmission has been debated for many years, and has proven to be “the toughest nut for me to crack,” says Dee. When his first research project failed to reproduce aerosol spread, a graduate student showed him that the frequency of shedding and transmission of the virus could be strain-dependent.

In later studies, using PRRS strain MN-184, documented for its high frequency of aerosol spread, Dee was able to show that the risk of aerosol transmission was reduced by air filtration.

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Novel Research Strategy

All of these intervention strategies were tested in a first-of-its-kind, year-long, PRRS research project to study how PRRS virus spreads between farms. The work was just completed at the University of Minnesota research farm in western Minnesota, located 10 miles from the nearest hog farm.

“Our objective was to develop a model of a swine production region that was endemically infected with PRRS virus, and identify ways of spread, and the effectiveness of biosecurity protocols that mimic what happens in the field,” says Dee.

The project was divided into two time periods - the high-risk period for PPRS virus spread (October-March) and the low-risk period (April-September).

Three small units housing 20 feeder pigs each were designated as high-level, medium-level and low-level types of biosecurity, flowed all-in, all-out. Trials included 26 replicates - each group was tested for two weeks and facilities were washed and disinfected between groups. All pigs were then moved into the continuous-flow, mechanically ventilated, 300-head, grow-finish “virus factory” where pigs were raised from 55 lb. to about 220 lb.

Assumptions were that the “gold standard,” high-level filtered facility (95% DOP filters) would not become contaminated. The medium-level facility was more of an industry standard where most hog operations are at today, which features no air filtration, but protection against all other routes of infection, observes Dee. The low-level facility had no biosecurity protocols.

The high- and medium-level bio-security units were placed 396 ft. from the source farm, in the direction of the prevailing northwest winds in winter. The low-level facility was also sited 396 ft. from the source farm, but facing in the opposite direction to take advantage of southeasterly winds during the temperate times of the year. The high- and medium-level facilities were located only 13 ft. apart, to provide a stiff test for biosecurity.

Sampling Regime

Testing for airborne PRRS virus particles at the three test facilities and the PRRS-source farm was done daily and samples were analyzed by PCR (polymerase chain reaction), says Dee.

In addition, daily measurements of weather patterns were recorded and analyzed by PCR.

In all, data was collected from 1,760 pigs on 361 days, with a total of 11,571 samples analyzed.


“In the high-level biosecurity facilities throughout the entire year, there was no evidence of transmission or virus transport into that barn. Every sample that we collected from that facility was negative for PRRS,” emphasizes Dee.

The medium-level biosecurity facility that was only 13 ft. away from the high-level facility broke with PRRS 31% of the time. “We sequenced the pig virus, the air sample virus and the source population virus and they showed homology (similarity), so we ruled in air and we ruled out all of the other potential routes of infection,” he explains.

To be expected, the low-level facility, which followed no biosecurity rules, had the potential to break with PRRS virus by many different routes, and showed an infection rate of 66% during the year, Dee says.

Data analysis, conducted by John Deen, DVM, University of Minnesota, confirmed there was a significant association amongst all three routes of transmission evaluated — air, fomites and personnel. He also estimated daily risk of infection:

  • The low-level facility had a 10% risk of breaking with PRRS, or roughly the equivalent of 10 breaks in 100 days, a very high level of risk;

  • The medium-level facility had a 3% risk; and

  • The high-level facility had virtually zero risk.

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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.