Porcine reproductive and respiratory syndrome remains the most problematic disease confronting pork production today and one of your biggest headaches. Even if the disease hasn’t visited your farm, you are aware of the economic consequences it can wreak. Of course, co-infections can complicate control and add to the losses.

A conservative estimate puts PRRS losses for the industry at $600 million annually. Although eradication is receiving increased attention from veterinarians, diagnosticians and producers, until the disease is eradicated, prevention is the place to concentrate efforts.

Because the PRRS virus is easily spread, biosecurity protocols are crucial to preventing the disease. “PRRS virus can be transmitted between production facilities by several routes including infected pigs, contaminated boar semen, hands of barn personnel, transport vehicles, boots or coveralls, and insects,” says Scott Dee, DVM, with the University of Minnesota veterinary population medicine department.

Biosecurity protocols such as requiring personnel to shower-in/shower-out and wash hands thoroughly; boot disinfection; and washing, disinfecting and drying transport trailers all play important roles in reducing the risk of PRRS virus transmission, according to Dee.

“Disinfecting incoming supplies is a good way to eliminate PRRS virus from the surfaces of these items,” Dee notes. He recommends using validated disinfectant products at their recommended dilution rates and contact periods specified on the label.

Aerosol virus transmission from a group of infected pigs is another transmission route and perhaps the most common infection prospect in swine-dense regions. Dee and others, including Jeff Zimmerman, DVM, Iowa State University, have conducted extensive research on this transmission route that produced insights and methods to improve farm biosecurity.

For example, Dee’s research shows the PRRS virus can move over long distances through the air. “There’s no more mystery about how the virus can move from farm to farm,” he adds. “It’s not if the virus is transmitted via aerosol spread; the question is how far can it travel.”

So far, Dee has found that the distance is at least 5.7 miles. “That’s as far as we’ve looked,” he adds.  “We don’t know for certain how far the virus can be transported, but we know for sure that it can go almost 6 miles.” His research also looked at Mycoplasma hyopneumo-niae and found it to move the same distance.

Most recently Dee has investigated how disease is spread in a pig-dense production area and which biosecurity measures are most effective to prevent transmission. “We checked high-, medium- and low-level biosecurity facilities to determine the differences.”

That research showed that air filtration can be highly effective when used in conjunction with other more commonly used biosecurity measures to block aerosol transmission of pathogens including PRRS virus and Mycoplasma hyopneumoniae. He is quick to emphasize that ventilation filters alone are not magic and that you still must have a complete biosecurity program in place.

The study focused on airborne transmission of PRRS and Mycoplasma hyopneumoniae in facilities protected by MERV-14 or MERV-16 air-filtration systems. “We found no infection from either organism occurring in susceptible pigs within these systems,” Dee says. “However, in unfiltered systems, infection continued to occur.”

Dee also reviewed the effectiveness of antimicrobial and electrostatic filters. “We have not found considerable differences between the types of filters,” he notes. “All filters performed admirably.”

In most of the filtered systems studied, air was brought in through the facility’s soffit vents. It passed through the air filters via negative-pressure ventilation and into the pig space. Unfiltered air entering through idle fans or cracks is another concern. But a double-shutter arrangement, which prevents unfiltered air from entering through stationary fans, was tested and “we found it worked very well,” Dee says. Caulking cracks and gaps is important in keeping unfiltered air out.

Certain weather conditions, such as high humidity and prevailing winds, also facilitated virus aerosol transmission. “We found overcast, cool, foggy, high-humidity days presented a greater risk,” Dee explains. Warm, sunny days without wind present less transmission threat. “When filters need to be changed, or pigs need to be moved, pick a warm, sunny day if at all possible,” he adds.

While boar-stud facilities have adopted air-filtration systems extensively, the technology faces more hurdles on sow farms and other settings.

Cost is certainly an issue but it’s difficult to pinpoint an exact figure as it varies greatly depending on the filters, the ventilation system in place and other possible modifications for filter installation. Dee estimates the cost to add filters to a 3,200-sow barn at $1 to $2 per pig when amortized over time. While that’s significant, “preventing one PRRS break makes the investment seem more reasonable,” Dee emphasizes. 

Do a Pre-Filter Evaluation

For anyone considering installing air-filtration systems to reduce the airborne transmission of porcine reproductive and respiratory syndrome virus, it’s important to keep the facility’s overall ventilation performance in mind.

Steve Pohl, agricultural and biosystems engineer at South Dakota State University, lists several  questions to ask:

  • Does the present ventilation system meet  industry standards?
  • What will happen to overall ventilation-system performance when filters are in place?
  • What filter types, sizes and quality are needed?
  • How much will it cost to install a complete  filtration system?
  • What is the potential longevity of the filters under the environmental and usage conditions?

Start by assessing your ventilation system. “Key factors to evaluate include air-distribution patterns, ventilation fans, air-exchange rates and requirements per animal, attic-air intake, pit transitions and annexes, fan staging and understanding controller function and supplemental cooling,” Pohl says.

Ventilation fans will run at higher static pressure rates during hot weather when all fans are required. For typical ventilation fans used in swine operations, Pohl recommends the maximum static pressure should be around 0.20 inches of water column but preferably less. 

“Fan performance can drop off considerably when operated at static pressures greater than 0.20 inches,” he notes. This is especially true for some large diameter fans.”

The Bioenvironmental and Structural Systems Laboratory has established an air-flow ratio [air flow at 0.2 inches s.p. divided by air flow at 0.05 inches s.p.] for most agricultural ventilation fans. For example, air-flow ratios for tested 50- to 52-inch, single-phase fans ranged from a low of 0.09 to a high of 0.87. The higher the air-flow ratio the better the fan performs as static pressure increases. For existing facilities, the ratio will provide a guide on whether or not fans should be replaced when adding a filter system.

“Using air filtration with negative-pressure systems requires sealing all building air leaks,” Pohl adds. "These include fans not in use, cracks, pit annexes, doors or any other unplanned openings.” 

As for the filters, Pohl reminds that this is a new area for most companies and several issues need to be addressed including:

  • Has the filter been tested according to the ASHARE Standard 52.2?
  • What are the test results at low airflow rates?
  • Has it been tested against the PRRS virus?
  • Will the filter stand up during installation?
  • All MERV-16 or MERV-14 filters are not equal in efficacy.

Careful consideration is needed when determining the filter area required for a ceiling air inlet, tunnel-ventilation opening or cool cell. “When determining the filter quantity, the combination of pre-filter and MERV filter should create no more than 0.15 inches of water column and preferably 0.10 inches,” Pohl notes.

The static pressure that filters generate will be added to the other static pressure points (attic-air intake, air inlet, cool cell). “Filters need to be designed so that there is no bypass of incoming air due to low air flow rates,” Pohl says. “Many filters rely on a minimal amount of air pressure from air flow to seal the filter edge in the frame or mounting device. This is an important consideration during cold-weather ventilation when rates are often restricted.” 

After installation, Pohl recommends monitoring static pressures as “this will provide insight into when the filters become plugged, which reduces ventilation performance.” Filters also should be inspected periodically for rodent and other damage. “Inspect filter installations for proper seals and damage. No filter can stop particles that bypass the media,” he adds.