Filtering Out PRRS

MINNESOTA - Researchers at the University of Minnesota have found a way to keep Porcine Reproductive and Respiratory Syndrome (PRRS) away from a swineherd.
calendar icon 3 March 2008
clock icon 7 minute read
The virus can be filtered before it enters the barn using Minimum Efficiency Reporting Value (MERV) filters.

These filters have a 95 percent DOP at greater than or equal to 0.3 microns and they are being used to keep some boar stud and sow farrowing operations free of PRRS, writes Andrea Johnson, in Minnesota's Farm Guide. The cost of the filter and installation runs about $250/sow or $150/boar stud. Annual maintenance costs are $37/head

“The filter system definitely has the ability to withstand more virus than we'll ever see,” said Scott Dee, D.V.M. Ph.D., University of Minnesota professor in the Veterinary Population Medicine Department.

The department is part of a research programme funded by The National Pork Board, Minnesota Rapid Agricultural Response Fund and the Minnesota Pork Board, to evaluating ways of reducing PRRS infection and spread of the disease.

Dee and Andrea Pitkin, Master of Science candidate, conducted a yearlong study at the University of Minnesota Swine Disease Eradication Center (SDEC) Research Farm in west central Minnesota. They set up a swine production model representing four neighboring farms.

Their goal was to provide a regular source of PRRS via one unsecured barn. They wanted to find out how the virus spread to pigs on neighboring “farms,” located about 120 yards southeast of the source farm.

Populations in all four barns started out PRRS-free. Then the scientists inoculated pigs, in a traditional barn, with a PRRS virus that spreads very easily by aerosol. The three other buildings were set up with different levels of biosecurity.

  • A low-level security barn had no protocols or biosecurity to show that spread occurs in the absence no defence.

  • A medium-level facility reflected the commercial biosecurity typical of many Minnesota farms. The building was sealed to keep out potential vectors.

  • A high-level biosecurity facility, identical to the medium-level facility was situated close by (12 feet) and fitted with the MERV filtration system.

The Filter System

A metal hood was placed over the air inlets for both facilities. The medium-level facility featured a cool cell pad, while the high-level facility had the filtration system.

The filtration system included inexpensive pre-filters that collected dust and debris as well as botanical material. The filters helped keep clean the expensive 95 percent DOP at .3 microns filter.

“This is a commercial filter. It is cheaper than a HEPA filter,” Dee said. “It is very commonly made and used in a lot of commercial structures around the country. This is nothing we invented, we just packaged this all together.”

In both barns, double door entries allowed for the use of negative pressure ventilation that reduced the risk of air-borne entry of the PRRS virus.

Daily for one year, staff followed a protocol for completing chores. Showers were taken at the farmhouse. Chores were completed first in the high-level biosecurity barn. The staff also took swabs of their hands, coveralls and boots to be certain they were not carrying the virus into the barns on these items. The staff changed boots and washed their hands between the barns.


Results from the study were very significant. Over an entire year, the high-level biosecurity facility remained free of virus.

“We were sampling air, people, insects, trucks and trailers,” said Dee. “Everything that could possibly bring virus into the farm was blocked through that high level biosecurity protocol.”

The scientists found they could pinpoint the cause of PRRS virus infection every time there was a break.

“The cause was always fomites, personnel, aerosols or insects,” said Dee. “We know how this virus can be spread. There is no more mystery about how this virus moves around.”

The medium-level facility - the current industry standard - broke with PRRS virus 31 percent of the time.

“We found entering air, from the not-filtered inlet, that carried the virus that we used in our source population,” he said. “All the other routes were blocked by the bioscreen protocol, including insect, transport, fomites and personnel.”

While a 31 percent level of outbreak may bother producers, the medium-level facility did offer some protection. Pigs in the low-level facility broke with PRRS 66 percent of the time.

“You can make improvements by working on your trucks, working on your facilities, working on your personnel flow and controlling insects,” said Dee. “You can make a significant improvement with very limited costs. The big cost is jumping from medium to high biosecurity.”

The scientists are using this information to help Minnesota veterinarians and producers.

“We have a number of farms filtered that are remaining negative for PRRS,” said Dee. “Practitioners are using this information. It's working very well in farms in very dense areas of hog production. We're starting to get some good feedback from the field that our observations were accurate, and the technology can be applied in the real world.”

Other Factors

The scientists collected weather data along with PRRS outbreak information.

They found PRRS was most likely to spread when there was gentle air movement, overcast conditions, rain or fog, and above freezing conditions. PRRS also spread on days when the weather was changeable. PRRS was less likely to spread on hot days with clear skies, a high UV index and a rising barometer. It was also less likely to spread on days with clear skies, strong winds and very dry conditions.

The scientists also determined that aerosol transmission could occur 120 yards from the source barn and PRRS infections did occur throughout the year and so keeping barns filtered could be good management to prevent transmission.

“In the past, all the work had been done over one yard. We stretched that out and showed PRRS can be spread almost the length of a football field,” said Dee.

Further Investigations

Dee is now working with Satoshi Otake, post-doctoral candidate, on a follow-up study. The scientists want to see what is the impact of combining PRSS virus and Mycoplasma pneumonia at the SDEC research farm. The study will take place over two years.

A weather station has also been added to the farm to help them further understand the role of weather conditions in promoting the spread of virus over time.

Speaking at the recent Minnesota Pork Congress, Dee said that air filtration systems on hog barns represent a double-edged sword.

High-end air filtrations systems certainly look as though they can prevent the PRRS virus from entering hog barns.

However, there is no information available on how long the filters will hold out. Some producers have kept filters in place for three years.

Dee and others are also looking for filtration alternatives. Work is also underway to evaluate less expensive filtration methods.

“We have the solution. It's just a matter of getting it done. We have the answer. We have to maintain the optimism. We have to keep doing the research, and producers have to take the chance that we can make this work on their farms,"Dee said.

To read the report click here.
© 2000 - 2024 - Global Ag Media. All Rights Reserved | No part of this site may be reproduced without permission.