PRRS elimination, needs and feasability

By Camille Moore, DVM and published by Prairie Swine Center. PRRS, known as mystery disease at the beginning, was first noticed in 1987 and was a very important disease through the 90’s. It has been identified as a costly disease and reports have shown that a PRRS outbreak could cost up to $ 600 U.S. per sow.
calendar icon 20 April 2004
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PRRS elimination, needs and feasability - By Camille Moore, DVM and published by Prairie Swine Center. PRRS, known as mystery disease at the beginning, was first noticed in 1987 and was a very important disease through the 90’s. It has been identified as a costly disease and reports have shown that a PRRS outbreak could cost up to $ 600 U.S. per sow. Visit the Prairie Swine Centre


For many herds, after an acute outbreak, there is a long lasting chronic form for which it is much more difficult to evaluate the cost, but losses are important.

As of now, many means of control have been put in place but they are not always successful. Some people have looked at complete depop-repop but this is not always being an option therefore , since 2 or 3 years, many efforts have been put in place to develop elimination strategies.


    A definition for PRRS elimination was given in 2000 (Torremorell, 2000), “A PRRS virus-free herd produced by virus elimination shall exist when all vestiges of PRRS virus infection are undetectable in all stages of production, defined as part of the “herd” and all animals known to have been infected in their lifetime have been removed.”

    In reality, elimination starts with a positive herd that had the clinical symptoms in the past and become negative. This status is obtained without doing a complete depop-repop, but the herd is totally negative after a period of time.

    We need to understand that because all previously exposed animals need to be removed, it takes time to obtain this status. The need to remove all previous exposed animals is related to the current lack of knowledge on the exact status of those animals. We don’t know if they could be persistently infected and shed later on in their life or if animals could be intermittent shedders.

    The concept of disease elimination is not new and has been applied successfully in the past for bacterial disease like Actinobacillus pleuropneumoniae and swine dysentery; parasite like mange and virus like TGE, but because it is a virus and due to its nature, elimination for PRRS is much more difficult to achieve. Even if difficult, there are successful reports on PRRS elimination and we have to admit that today the technology is there and could be applied.


    As of now, more than one method has been applied to PRRS elimination. Even if one method seems to be used more, there is a niche for all of the others. A producer has to pick the method that has the greatest odds of success in his herd, with the lesser economic impact.


      A total herd depop-repop could probably not be called a standard elimination method, but this is certainly the approach that has the greatest chance of success. To be successful a good cleanup of the previously infected herd and access to a negative source of animals is needed. This is probably also the most expensive approach.


      This applies much more to farrow to finish operation where a part of the herd, (where instability has been detected) is completely emptied and remains empty for a given period of time. Prior to applying this approach, the sow herd needs to be as stable as possible and producing a negative population of animals on a consistent basis. This is much easier to apply if nursery and or growing barn are not on the same site as the sow herd.


      This approach is also called the Isowean. The goal is to produce negative young weaners from a positive stable sow herd. Piglets are weaned early ( 5 to 10 days of age) and moved to another site. This approach requires a lot of testing to ensure the newly derived population is truly negative. Usually, during the entire process, the main sow herd remains positive. This is a short term scenario but many success have been achieved, mainly by breeding stock companies.


      This approach applies to a stable sow herd producing negative offspring. In this type of herd we would go in and test all sows in an attempt to identify the positive ones. All of the positive sows would be removed in a given period of time and all new introduction would be negative. Here again, success has been reported but there are still many questions related to the status of the negative sows at testing time. Those sows might have been positive in the past and, as of today, with the current knowledge, we don’t know if those sows could become positive again or present a risk to the herd itself.


      This is like the old TGE eradication approach but applied to PRRS. Mass vaccination is used for total exposure of the entire population in the herd. When all animals have been exposed and with no further introduction in the herd, we believe we could stop virus circulation. Then flowing all vaccinated animals unidirectionally, at a point of time the herd becomes negative. This is certainly a high-risk approach, but again, success has been reported.


      This is currently the most used approach and the one that has probably the greatest odds for success. It does rely on PRRS stabilisation in the herd and normal attrition. This applies to the sow herd of a 3-site system. The main steps are as follows:


        The first step is to stop introduction of replacement animals for a prolonged period of time. It usually needs to be stopped for 6 months. To ensure normal replacement for the herd, a large amount of replacement of different ages needs to be introduced up front or an off site breeding project needs to be looked after.


        Prior to starting the process, a reliable source of negative replacements needs to be identified and monitored. This is the source of replacement that would be used in the future.


        If the herd has been tested and known to be stable (no virus transmission within the herd), the new negative breeding replacement stock is introduced. Those animals are used as sentinels and are followed serologically to make sure they are not seroconverting.


        When negative piglets are produced on a consistent basis and when we know that there is no virus circulation in the sow herd, (using the new breeding stock as sentinels) the other sites could be depoped and repoped with the newly negative offspring from the sow herd.


        Following the normal culling policy of the herd, all of the previously exposed sows are eliminated and the sow herd is called negative when all of them have been sent to slaughter (to fulfil the earlier definition given).


    Many success has been reported for the approach of herd closure but a lot of dedication is needed. Failures have been mainly related to the introduction of positive replacement, lateral introduction (transportation) or not waiting enough time before the introduction of the negative animals. Dedication and discipline are the main ingredients for success and remember, this is a long process and it is much more difficult to stay dedicated and motivated in those situations.

    Unfortunately, this approach does not apply (as of now) to farrow to finish herds on the same site. In those situations, partial depop is probably a must and it is not easy to achieve. Another option for those herds is probably mass vaccination + unidirectional pig flow, but this approach still needs to be fine tuned and adapted.

    Over the past 2 years we have heard about some large systems (mainly in the U.S.) attempting PRRS elimination on system basis. Those enterprises mainly use the depop-repop approach of a group of herds to pursue their goal. Even if protocols had been well prepared up front, so far, I am not aware of any success stories. I don’t want to be negative, but just illustrate how difficult it is to achieve.


    As mentioned earlier, we’ve heard a lot about PRRS elimination being a hot topic south of the border. I sincerely believe the Canadian PRRS situation is different than the U.S. one. As of today, with a few exceptions, PRRS has not created the big losses reported in the U.S. There are probably many explanations for this, but the main ones could be the following:

    • Perceive less virulent strain in our herds.
    • Usually smaller herds and smaller systems
    • Different climate and much more real confinement buildings.
    • Generally speaking, less density in production.
    • Overall better health status (less other diseases to add on PRRS)
    • Better understanding of biosecurity and better application.

    All of those differences need to be taken in consideration when evaluating the needs for elimination in a given herd.


    So far we have mentioned many times “stable herd” as something needed prior to elimination. You probably also have heard many times “PRRS stable” in an attempt to classify the PRRS status of a given herd. In theory, this means, that at a given point in time, in a given herd, there is no lateral virus transmission.

    In practice this means, assuming we would have enough precise tools to make the call, there is no virus transmission from one animal to another in the herd. Herd stability, even if difficult to assess, should be the number one goal for any PRRS positive herd. In many commercial situations, when real stability has been obtained, the need for true elimination might become questionable.

    The key factor to obtain a stable herd is replacement animal introduction. Even if this is not the topic of this talk, this should be the first thing to revise in your herd and you should be really open minded on how this could be achieved. There are many new options that could be used like closed herd, extend acclimatisation period, introduction of weaner or feeder pigs as replacements, strategic vaccination, to mention just a few. Again, this is the most important factor to obtain herd stability and you should discuss this in depth with your veterinarian.



      Location, location, location, this is always the key factor! Did your herd is located in a dense production area or it stands alone by itself? PRRS infection could be airborn and you need to assess risk of recontamination prior to attempting PRRS elimination.


      For breeding stock producers, the demand is there for a negative animals so you should be seriously looking at it. On the other hand, if you own a small farrow to finish operation with some section being continuous flow, odds of success are much lower and this probably not apply to you.

    3. HERD SIZE

      The herd size is always important in disease dynamic. Everything is easier to achieve in a 300 sow herd than it is in a 3000 sow herd. In those large herds we have to deal with what we call subpopulation. This mean that status variation could exist within the different sections of your herd. This needs to be assessed prior to any eradication attempt.


      The question is quite different depending if you are an independent producer or part of an integrated production system. If you are part of a system, you need to look at the status of the other herds within the system and risk of contamination between herds. If you are the sole positive one, you are the herd that is presenting a risk to the system, but if all other herds are positive and you would like to attempt elimination, your risk of recontamination down the road is much greater. In a system, PRRS strategies need to be built systematically for the system.


      Basic biosecurity rules need to be reviewed prior to the implementation of eradication protocols. This is common sense, we need to minimise risk of recontamination.


      Even if transportation is part of biosecurity, this should be looked at very meticulously. Cases of lateral introduction have been reported due to trucks in which positive animals were kept, came close a building to pick up culls animals. We also know that the PRRS virus lives in manure and manure contamination through truck exists.


      Assessing the PRRS status of your supplier is a must and you need to reinsure, on a current basis, that the animals supplied are truly PRRS negative.


      Prior to attempting any elimination process, you need to assess real losses due to PRRS in your herds over the past 5 years. This will dictate the real need for elimination.


      Herd stability being a critical factor for elimination success, you need to look in depth to assess any sign of unstability over the past year in all of the segments of your production unit. A close audit of all of the production data is needed for this.


      Many times in herds that seem to be stable and in which major losses and clinical signs are not seen, are still facing problems at gilt introduction and also with the progeny of those gilts. Here again, an in-depth look assessment needs to be done on those animals to make sure they are not facing any problems and are not non-perceived destabilising factor.


      Many times PRRS itself could be managed but if other diseases like Mycoplasma, APP, Glasser are added on top it becomes too difficult or too costly to manage. In those situations, strategies looking not only at PRRS elimination but also looking at other disease elimination and control need to be looked after.


      You need to look at the goals of the enterprise for the future. As an example, if a major expansion is planned, PRRS elimination could easily be part of it. We need to keep in mind that, even if not completely needed, every time it is easy to upgrade the health status of a herd, we should do it.


      As for any other decision in your herd, you need to look at the real economic impact of the process. More precisely, you need a total evaluation of the loss of production and cost related to it versus returns on your investment. The hog business is a dollar driven business and if there is no dollar advantage of being PRRS negative, why go through the process?


PRRS elimination is feasible and desirable for some herds and systems:

  • Replacement stock at the top of agiven production pyramid need to be PRRS negative.

  • If you are in a situation where it seems to be relatively easy to perform and with a low risk of recontamination, go for it!

  • If you are part of a system in which you are the sole positive herd, it is a must to go through elimination to protect the remaining part of the system.

  • If you are challenged by many other diseases and you are encountering big losses, it is an option but maybe you should look at a total depop-repop.

In all other situations I believe you should focus on keeping your herd stable and refine your breeding stock introduction and acclimatisation protocol.

I believe, with the current PRRS Canadian status, the current knowledge of the disease and current technology, PRRS elimination at a commercial level will not be the “focus of the future” healthwise in Canada.


Dee, SA (1998). Elimination of PRRS virus using a test and removal process in conjunction with serology and PCR diagnostics. Proc Am Assoc Swine Pract, Des Moines, IA pp 117-118.

DEE, SA, Morrison, RB, and Joo, HS (1993). Eradicating porcine reproductive and respiratory syndrome (RRRS) virus using two-site production and nursery depopulation. Swine Health and Prod. 5 :20-23.

Dee, Sa and Philips, R (1998). Using vaccination and unidirectional pig flow to control PRRSV transmission. Swine Health and Prod 6(1) : 21-25.

Gramer, ML, Christianson, WT and Harris, DL (1999). Producing PRRS negative pigs from PRRS positive sows. Proc Am Assoc Swine Pract. 413-416.

Torremorell, M, Henry, S and Moore C (2000). Producing PRRSv negative herds and systems from ORRSv positive animals : the Principles, the Process and the Achievement. Proc Am Assoc Swine Pract, Indianapolis, IN, pp 341-347.

Torremorrell, M, Christianson, WT (2002), PRRS Eradication by Herd Closure. Proc. Banff Pork Seminar (Advances in Pork Production), Banff AB, pp 169-176

Source - Prairie Swine Centre - March 2004

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