Herd Health: Can we finally keep PRRS out of our pig barns?

Monday, April 5, 2010

The answer is that we're close, but only time will tell if we've really closed all the holes

by S. ERNEST SANFORD

At the most recent Leman Swine Conference held in St. Paul, Minn., last September, University of Minnesota PRRS researcher Dr. Scott Dee gave an overview of just how far we have come in keeping PRRS virus (PRRSV) from re-infecting our sow herds.

It was about 10 years ago that we finally became confident that we could successfully eliminate PRRSV from individual herds. We had developed the skills, the know-how and the overall technical ability to systematically eliminate the PRRSV from virtually any swine herd.

Test and removal was one of the first successful methods available. A little later, herd "closure and rollover" replaced "test and removal." Closing the herd for extended periods (about six months) with no introduction of outside animals, including no replacement gilts, essentially shut down virus circulation and set the stage for complete elimination of the virus.

With this knowledge firmly fixed, there was a rush in 2000 and 2001 to eliminate PRRSV from as many swine herds as possible, especially in the U.S. Midwest. But PRRS was waiting!

With the presence of a large number of PRRS-negative herds to choose from, a new strain of PRRSV emerged and infected hundreds of herds. Area spread was identified as the single most important factor contributing to renewed PRRS outbreaks in these herds. 

In 2001, a research plan was formulated by Dr. Dee to determine each component contributing to indirect area spread of PRRSV. Between 2001 and 2005, Dr. Dee, along with a succession of graduate students and other colleagues, systematically identified the following individual components or factors which were contributing to PRRSV area spread:

Fomites
In a series of studies, the research team showed that inanimate objects, such as needles, boots, coveralls, clothing, tools and instruments, that came in contact with the PRRS virus could mechanically carry it from one premises to another. This occurs more readily in fall and winter than in the hot summer months.

Solution: washing and disinfecting. Once these objects were identified, the solution was washing and disinfecting with any of several disinfectants – for example, glutaraldehyde, quaternary ammonium compounds and peroxygen-based compounds – to stop this method of spread. Changing injection needles frequently (after each litter) cut down spread via needles.

Personnel
Extensive research demonstrated that people do not harbour the PRRSV in their nostrils or upper respiratory tracts. However, they can carry the virus on their hands after handling PRRSV-infected pigs. 

Solution: Personnel biosecurity entry protocols. The research also showed that all personnel biosecurity entry protocols into barns that were tested were equally effective at eliminating PRRSV from personnel.  These included: shower in and out after 12-hour downtime; shower in and out with no downtime; Danish entry system consisting of washing hands and changing into barn-specific clothing and boots after crossing a clearly defined barrier. 

Transport
Contaminated transport was identified as a definite source for infecting PRRSV-naïve pigs.

Solution: Drying trucks and trailers after washing. Drying trucks and trailers after washing is the primary component of transport biosecurity.

Insects
Houseflies and mosquitoes are mechanical, but not biological, vectors of PRRSV.  Houseflies can transport infectious PRRSV out to at least 2.4 kilometres.

Solution: Insect screens. Use of screens on sidewall inlets significantly reduced insect entry into barns.

Aerosols
Aerosol transmission has been a contentious issue in the swine community for many years. Many swine veterinarians and producers working in the field long ago decided that the PRRS virus was moving from herd to herd via aerosol. Try as they might, however, researchers could not document the spread of PRRSV under controlled laboratory or field conditions by aerosol means. 

Finally, after many years and failed experiments, the researchers were able to demonstrate that aerosol was a significant means of spread of the PRRSV. It was further determined that the frequency and ease of spread of PRRSV by aerosol might be strain-dependent. Pigs infected with a highly pathogenic PRRSV strain (MN-184) had significantly higher than normal titres of virus in blood, tissues, nasal and oral secretions and produced aerosols.

Solution: Air filtration. Long before PRRSV appeared on the scene, filtration of incoming air had been used successfully at a swine research farm in France to prevent airborne pathogens, such as Mycoplasma hyopneumoniae, from entering the barn.

That research farm is located in one of France's most pig-dense areas and its air filtration system has successfully kept the farm's pigs mycoplasma-free for more than 25 years. It has also kept PRRSV out for the nearly 20 years that PRRS has been present in France.  This has been achieved by using high-efficiency particulate air (HEPA) filters, which are designed to prevent particles as small as the smallest viruses from getting through.

HEPA filters are excellent for preventing entry of viruses like PRRSV, but they are very expensive and in France they were operating under a positive-pressure ventilation system.     A search evolved to identify low-cost alternatives that would operate under our North American negative-pressure ventilation barns. This led the researchers to minimum efficiency reporting value (MERV) filters.

The Camfill MERV-16 filter is 95 per cent efficient at removing particles equal or greater than 0.3 microns in diameter. The PRRSV is about twice the size of the lowest limits of the MERV-16 filter.

Testing the MERV-16 filter in scaled modal barn settings showed it to be as effective as HEPA filters in preventing PRRSV entry. Pursuing even less expensive filters led to testing the Camfill MERV-14 filters, which came at a lower cost than the MERV-16s. The MERV-14s proved to be effective in keeping out not only PRRSV but Mycoplasma hyopneumoniae as well.

From MERV filters, the researchers went on to investigate antimicrobial filters and, most recently, electrostatic filters. Work with the electrostatic filters is only just beginning, so is too new to comment on at this time. The Noveko antimicrobial filter from Quebec works on a "contact kill" principle, whereby viruses or any living microorganisms are killed on contact with chemicals impregnated in the filter.

The results of a study comparing Camfill MERV-16, Camfill MERV-14, a Chinese MERV-16, a Turkish MERV-16 and the Noveko antimicrobial filters, tested against various concentrations of PRRSV, showed that the Camfill MERV-16 and MERV-14 and the Noveko filters protected against the introduction of viable PRRSV under field conditions.

The MERV-14 and Noveko did let in small amounts of virus at very high concentrations of virus, much higher than what would be experienced under field conditions. Even so, the small amounts of virus getting through the Noveko filters were killed by the chemical impregnated in the filter. The Chinese and Turkish MERV-16s turned out to be much less effective than the other filters tested.

So, are we now able to keep PRRSV out of our barns?

We have come a long way over the last 20 years in our battle against PRRSV. We now seem to be very close to plugging every conceivable entry point that the virus uses to enter pig barns. Our biosecurity measures have been greatly increased and have become a key component in management of hog barns. We are tantalizingly close to finally having all the tools to keep PRRSV out of barns.

The final plug is the installation of air filtration systems to filter incoming air into barns.  Boar studs across the United States have rapidly retrofitted their studs with air filters, and several studs in Quebec and Ontario have done the same. The rest of the industry has been slower in adopting air filtration for sow barns and other swine buildings, which is not surprising given today's economic conditions and the lack of a 100 per cent guarantee at this stage.

That is slowly changing as the early adopters have been reporting complete absence of PRRS outbreaks in filtered sow barns compared with previous years, when there were was usually at least one PRRS outbreak every year or two.

One cannot expect that filtration will completely stop every PRRS outbreak. Human nature being what it is, there will be mistakes, short cuts and failures along the way. 

Even as I write this article, there are reports from the U.S. Midwest of the first PRRS outbreak in a filtered barn. Preliminary investigations have revealed that there was no break in the filtration system. Instead, PRRSV was identified by polymerase chain reaction(PCR) at the truck wash and in trailers that service the sow barn in question, again emphasizing the need to keep up biosecurity against all avenues through which the virus is known to get into barns. So we're close, but only time will tell if we've really closed all the holes. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: ernest.sanford@ boehringer-ingelheim.com