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Researchers in Canada chose herds that were PRRS stable, whether they were vaccinated or non-vaccinated. Stability was defined as 90% or more of the sows having an S/P ratio of less than 1.0 and no more than 10% of the herd with an S/P ratio of greater than 2.0 for vaccinated herds. Pigs were followed after weaning, either on site or into offsite SEW nurseries, to determine whether or not they were weaned and remained free of PRRS virus.

The vaccinated stable sow herds produced pigs with maternal antibodies that decayed over a course of 3 or 4 weeks. They never developed active antibodies thereafter. At the end of the nursery phase and beyond, they produced seronegative pigs. The non-vaccinated, apparently stable herds produced pigs whose titers very soon started to decay. They developed their own positive, active PRRS antibodies. They tended to shed virus to neighboring pigs.

There are several take-home messages from this study.

1. You do need stable herds to provide the opportunity to produce PRRS-negative pigs.
2. Vaccinated herds have a distinct ability to maintain stability and have seronegative PRRS pigs — a clear advantage over non-vaccinated herds.
3. Producers have the ability to produce PRRS-negative pigs — if they use multi-site production or off-site nursery production, remove the pigs from the sow herd and then maintain them in an off-site nursery as PRRS-negative pigs.

This study found that you can produce negative pigs from vaccinated herds, put them into off-site nurseries and have those pigs remain negative to satisfy whatever needs you have for PRRS-negative pigs. In fact, this study showed that stable vaccinated herds were better at producing negative pigs than the stable unvaccinated herds. That certainly can be applied to commercial production systems. If you are able to maintain PRRS-negative pigs in commercial production, you have a much greater chance of getting better production out of your pigs.

More information

Production of PRRS Negative Pigs Commingled from PRRS Positive Herds Using SEW. Rajic A, Dewey C, Deckert C, Frienship R, Martin W, Yoo Dongwan. Submitted for publication, Swine Health & Production.

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