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.

Results

“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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