It has been almost three decades since the PPRS virus was first recognized as the infectious agent responsible for reproductive failure in sows and severe pneumonia in piglets.

During that time, the virus has proven to be a formidable nemesis. Progress in controlling the virus and the disease it causes has been frustratingly slow for producers, veterinarians and researchers. The disease remains the most costly disease facing the U.S. pork industry and a drain on its global competitiveness.

The American Association of Swine Veterinarians' (AASV) PRRS Risk Assessment for the Breeding Herd can help veterinarians tackle two questions that are fundamental to reducing the impact of the PRRS virus:

First, how do we reduce the frequency of outbreaks in PRRS-positive herds?

Second, how do we keep PRRS-negative herds negative?

Success in dealing with PRRS will essentially depend on how well we master these two challenges.

The risk assessment tool offers a standardized way of measuring and benchmarking biosecurity practices and estimated disease risks associated with alternative practices.

The pork industry is an increasingly high-risk industry. The PRRS virus is a significant source of that risk.

If U.S. pork producers want to be globally competitive, low-cost producers, they need to focus on getting better at biosecurity now and eventually eliminating this virus from the United States.

Reducing the frequency of outbreaks in PRRS-positive herds.

While swine previously vaccinated or infected with the PRRS virus are protected from related strains of the virus, they are less protected against unrelated or new strains. Outbreaks in PRRS-positive breeding herds can occur when unrelated strains of the virus are introduced.

In addition, recurring breaks may occur in endemically infected herds even if the introduction of new strains is prevented.

That's because not all animals become infected with the virus at the same time, which creates subpopulations of animals that may be in various stages of infection and immunity. And, multiple strains of the virus may coexist in the herd, setting the stage for periodic outbreaks.

John Waddell, DVM, Sutton (NE) Veterinary Clinic, used the PRRS Risk Assessment tool in a large production system to help identify and prioritize biosecurity improvements to reduce the frequency of PRRS outbreaks.

After the initial assessment in Year 1 was completed on all of the breeding herd sites in the production system, biosecurity changes were prioritized and written protocols were updated as follows:

  • Rendering pick-up sites for mortalities were moved to off-site locations.

  • Outside doors were locked at all times and signage was posted to warn visitors they were entering a biosecure area.

  • Keeping PRRS-negative herds negative.

    A separate truck wash was built for vehicles hauling pigs to market.

  • Off-site transfer stations were added for weaned pig movements, with dedicated trucks to move pigs from sow farm to transfer station.

  • Dedicated trucks were added for genetic transfers.

  • Showers were added to sites lacking them and instructions for entering facilities were posted.

  • Staff began using needle-free devices for some products and increased frequency of needle changes for products delivered with needle and syringe.

  • Scheduled feed deliveries to sites were adjusted according to their position in the pyramid and their PRRS status.

  • Rules were tightened to increase compliance with procedures restricting the use of semen before PRRS testing results from boar studs are known.

Waddell reassessed the sites annually to monitor the progress of biosecurity improvements (Table 1). He also tracked which sites experienced PRRS outbreaks and how many occurrences transpired over the same time period.

Over the three-year period studied, improvements in biosecurity decreased the average overall risk score for sites in the production system measured with the PRRS Risk Assessment. At the same time, the average number and percentage of herds reporting clinical PRRS outbreaks also declined (Table 1).

Recent innovations for eliminating the virus from swine breeding herds without completely depopulating the herd provide reason for optimism.

The basic approach, referred to as herd closure and rollover, involves eliminating the shedding of virus in the herd so that PRRS-negative gilts may be introduced as previously infected animals are rolled out of the herd using normal culling practices. To stop shedding of the virus, uniform immunity is established in the existing herd and the herd is closed to all introductions of new gilts.

Next Page: The role of risk assessment in regional and national elimination efforts.

Closing the herd to new gilt introductions eliminates a major source of potentially susceptible animals that can continue to shed and propagate the virus in the herd.

The other potentially susceptible subpopulation is the nursing piglets. Eliminating or significantly restricting crossfostering is an effective means to stop shedding of the virus in the suckling pigs.

When the herd is closed for 200 days or more and crossfostering restrictions are not violated, herd closure and rollovers are nearly always successful. Herd closure and rollovers can be completed with a relatively low investment, and there are various ways to fill shortfalls in breeding targets during the period when the herd is closed to gilt introductions.

The financial and productivity benefits that arise from raising pigs without PRRS virus are typically large.

So why isn't everyone doing herd closure and rollovers to eliminate PRRS?

The simple answer is that we struggle with keeping the virus out once it is eliminated. There is a general fear that the herd will not remain negative long enough to recover the cost of eliminating the virus, and that the outbreak that occurs when the virus is reintroduced may be worse because whatever protective immunity was present when the herd was positive is now gone.

The PRRS Risk Assessment tool can help producers and veterinarians decide whether a herd will stay negative long enough to get a positive return on the investment through elimination of the virus. The tool also identifies biosecurity measures that may be taken to increase the likelihood that the herd will stay negative longer.

Paul Yeske, DVM, Swine Veterinary Center at St. Peter, MN, routinely uses the PRRS Risk Assessment tool to predict if the herd will remain negative long enough to get a positive return.

In many cases, producers decide to give elimination a try when they understand that many of the biosecurity risks that decrease the likelihood of keeping the herd negative are within their control, and that a positive return on their investment is typically achieved if the herd stays negative for as little as two to six months, depending upon how the elimination is done.

Unfortunately, the major risk factor that is not in their control is their location relative to other swine farms.

The role of risk assessment in regional and national elimination efforts.

The PRRS virus status of your neighbor matters, especially if you are trying to maintain a negative herd. Consequently, several regional PRRS virus elimination projects have sprung up in an effort to increase the likelihood that all producers in the region succeed at raising PRRS-negative pigs.

Bob Morrison, DVM, at the University of Minnesota has coordinated a successful effort in Stevens County MN. Currently, all of the sites in the county are thought to be negative except one.

Another project in a three-county area of west central Illinois that began more recently is being coordinated by Dyneah Augsburger, DVM, Carthage (IL) Veterinary Services and Carly Dorazio, VMD, Tri-Oak Foods, based in Burlington, IA.

Barb Straw, DVM, at Michigan State University is coordinating a two-county project in western Michigan that is also just getting underway. Read more about these projects on pages 18 and 19.

For regional elimination projects, the PRRS Risk Assessment tool can help establish a baseline of biosecurity practices in the region and monitor the changes — hopefully improvements — in biosecurity practices over time. The risk assessment tool offers a standard set of questions related to biosecurity practices so it can be used to compare results from one regional project to another.

The program can also provide information for evaluating the feasibility of the project, as well as estimating the likelihood of success and expected timeline for completing the project.

There has also been recent movement toward national eradication of PRRS virus. Success in a national eradication program will depend upon keeping negative herds negative, and the AASV PRRS Risk Assessment will play an important role in that effort.

Utilizing the PRRS Risk Assessment for the breeding herd.

The PRRS Risk Assessment tool is one of the assessments available through the Production Animal Disease Risk Assessment Program (PADRAP). Iowa State University, College of Veterinary Medicine, coordinates PADRAP for AASV. The risk assessment tool was originally developed by Dale Polson, DVM, and others at Boehringer Ingelheim Vetmedica, Inc. (BIVI) and subsequently donated to the AASV.

Ongoing financial support for the program is provided by the AASV, National Pork Board, U.S. Department of Agriculture, PRRS-CAP (research consortium) and BIVI. More information may be found at www.padrap.org.

Veterinarians wanting to access the program may e-mail AASV at aasv@aasv.org for information about upcoming training sessions.