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Filtering Out Viral Pathogens

Air filtration systems get high marks for keeping porcine reproductive and respiratory syndrome (PRRS) virus out of farms. Controlling aerosol spread

Air filtration systems get high marks for keeping porcine reproductive and respiratory syndrome (PRRS) virus out of farms.

Controlling aerosol spread of PRRS virus seemed nearly impossible a few years back, before the advent of air filtration systems for hog barns.

Boar stud expert Darwin Reicks, DVM, Swine Vet Center at St. Peter, MN, addressing a packed crowd at the Leman Swine Conference in St. Paul, MN, in mid-September, provided evidence that the use of air filtration has been a resounding success.

In 2½ years of experience with various types of air filtration systems on 15 client farms in hog-dense areas of south central Minnesota, just three farms have had PRRS breaks, he reports. Those three breaks occurred during the summer when the farms were not filtering outside air at all, relying instead on cool cell systems for ventilation. About 75% of the filtered farms had a prior history of PRRS breaks.

“We have not had any PRRS breaks on the sites during 100% filtration, nor have there been any breaks on the two bailout sites (discussed below). Of the 15 farms that are filtered, 13 are at boar studs, one is at a research farm and one is on a sow farm. Two more sow farms and a boar stud are installing filters this fall.

“Air filtration seems to be the technology that will be necessary for regional eradication,” stresses Reicks.

However, only a handful of farms have adopted the technology because of its high cost, he says. He estimates a PRRS outbreak costs an operation about $200/sow, virtually the same initial cost to install an air filtration system on a hog farm using 95 DOP filters.

Those filters provide 95% efficiency at screening out virus particles that are 0.3 microns in size. The smaller PRRS virus adheres to the particles of the filter as it attempts to pass through the openings in a process called interception.

Replacing DOP filters every two years adds an ongoing cost of $30-$40/sow/year. DOP filters are preferred because they are about half the cost of HEPA filters, and there is much less air restriction.

The cost of filtration can be balanced against the threat of a PRRS break, which increases when hog units are built in pig-dense areas, says Reicks.

There have been two breaks of swine influenza virus (SIV) in 100% filtered farms in the last three years. One of those breaks occurred last fall when the virus apparently survived the boar isolation period, according to diagnostic test results. The source of the other flu break was unknown.

Filtration Challenges

One of the main challenges with air filtration has been the restriction of the filter on airflow at the low static pressures typically operated in hog barns, says Reicks.

“The typical fans used to generate negative pressure are not very effective at pulling against the large static pressure created by the restriction of the filter,” he adds.

Implementation of a partial filtration system is simple and very low cost, says Reicks. “Many sites simply put a filter above each of the ceiling inlets (pictured on page 13). If one filter was placed above the inlet, the airflow is adequate until the outside temperature reaches 60-65° F.” For farms that initially just put filters on the ceiling inlets, they need to allow air to come into the barns unfiltered as the temperature heats up in the summer.” The problem is that the virus could easily travel 1-2 miles at that temperature, and without air filtration, the farm could become infected.

At higher temperatures, air filtration simply produces too much restriction, and producers must either remove the filters or utilize air coming through a cool cell in tunnel ventilation mode (summertime ventilation).

“One of the challenges with this system is that during the late spring and early fall in the Midwest, the temperature often gets hot enough during the day where tunnel mode is necessary. The problem is at night when the temperature drops down to 50-60°F when survivability of the PRRS virus is favorable,” observes Reicks. If the curtains don't go up and seal tight you have a big opportunity for virus to enter.

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Complete Filtration

There are three basic ways to do 100% filtration: negative pressure with a large number of filters, positive pressure using the squirrel cage-type fans to push air through a smaller number of filters but at a higher pressure or air conditioning. A number of facilities with 100% filtration using the 95% DOP filters initially were smaller units with excess ventilation or boar studs that are air-conditioned.

But now the notion of 100% filtration is being applied to larger units by use of a filter bank placed in front of a cool cell. Typically, to handle the large volume of air needed to pass through a cool cell during summer ventilation, a filter bank needs to be about three times the size of the cool cell pad, says Reicks.

“Air conditioning offers a distinct advantage in that it allows you to greatly reduce the amount of airflow into the facility, and thus, the number of filters that are required,” says Reicks.

For example, without air conditioning, barns generally run at 300-400 cfm (cu. ft./min.) per animal, maximum, in the summer through an evaporative cool cell pad. With air conditioning, this number can be reduced to as low as 6-20 cfm/animal, year 'round.

Bail Out Option

Because 100% filtration can be a costly option, Reicks reports it may be possible to bail out of the filtration system at a certain temperature for farms that have a relatively low risk of aerosol transmission of the PRRS virus.

“For these farms, there is really low risk when the outside temperature is above 80-85°F. First, PRRS doesn't survive as long at high temperatures. Second, if the temperature is that warm outside, normally it means the sun is shining, so you have some added benefit of ultraviolet light working against the virus,” explains Reicks.

With the bail out system, air is 100% filtered up until the last couple of stages of ventilation turn on. Then a door opens near the filters, allowing air to bypass the filtration system. When the air temperature cools down, the trap door closes. The door can be operated by an actuator or curtain controller and tied to the thermostat.

“The advantage is that it reduces the number of filters you need at a time when, if you are several miles away from other pigs and there are not many pigs in the area, the risk is really low anyway,” he reports.

Another option for the bail out system is in farms where all of the air comes through ceiling inlets. Reicks says an extra row of inlets can be installed at a fairly low cost. When actuated, these inlets can also be tied into the ventilation system to provide a bail out option during periods of higher temperatures. These unfiltered inlets would open up when the outside temperature reaches the 80s, for example, and then close as the temperature drops again.

Positive vs. Negative Pressure

Overall, for producers with existing, conventional hog barns considering air filtration, Reicks says the 95% DOP filters can still be used with some building modifications. For these negative- pressure systems, producers need a filter bank that is about three times the size of their existing ventilation system in order to “suck the air through the system,” he explains.

A positive-pressure system requires fewer filters, but larger fans are needed to force the air through the system. “If we can push air through filters at higher static pressure, then airflow can be maintained at its current level. However, in order to push the air through the filters at high static pressure, a different type of fan is needed,” says Reicks.

He suggests a squirrel cage-type fan as being much more efficient at moving large amounts of air against a high static pressure. These are the types of fans used with grain drying equipment.

Reicks says these fans can be placed in front of a filter bank to drive air through the filter bank at high static pressures and prepare it for entry through an evaporative cool cell pad into the building.

Although negative-pressure systems cost less than positive-pressure systems, they do create some added challenges:

  • Sealing up everything is a big concern, including around doors and windows. Duct tape and plastic can be used to seal doors.

  • Doors must not lead directly into the rooms. “We have a rule on filtered sites that no single doors can be opened. Culls and deads have to be taken through isolation or through a hallway where doors of the barn are closed,” he says. This means that some type of double-door system must be installed so no animals are exposed directly to outside air.

  • Back-drafting of fans should be avoided. Make sure fans don't stick open and air doesn't blow back into the rooms. Delay opening of wall fans in winter. Check wall fans at least once a week to ensure that they are in good operating order and are not stuck in the open position.

  • Close off fans not in operation. When some fans are not in use (i.e. winter), use garbage bags or other types of covers to prevent back-drafting.

The take-home message for producers is if they live in a hog-dense area, they need to have their hog farms 100% filtered if they want to stay PRRS negative, Reicks comments.

Regardless of the size of the farm, adding filter banks in front of the cool cells is the easiest way to go.