Which of vaccination protocols (mass vaccination or 6/60, or others) will be more effective in PRRS elimination from a herd? Can we introduce naïve replacement gilts, or rather vaccinated? 

Answered by: Tomasz Stadejek
Published on: July 14, 2016
Having in mind that PRRSV can persist in tonsils for a year, and that following elimination procedure, after 8 months the diagnostic results are negative, can we introduce naïve replacement gilts, or rather vaccinated? If vaccinated, with what vaccine, modified live or inactivated?

The question should be more specific, are we talking about elimination of PRRSV circulation in sows and production of PRRSV free piglets (stable herd), or the plan is to obtain complete elimination of PRRSV and specific antibodies (negative herd)? At present the most often used criteria are those developed by Holtkamp and co-workers.

Sow farms with animals reacting positive in ELISA and PCR, and showing clinical symptoms of PRRS are termed positive unstable (category I). Sow farms positive stable (category II) are ELISA positive but in piglets at weaning PRRSV was not detected in the last 90 days, in at least 4 consecutive tests, and no clinical symptoms of PRRS are observed.

In provisionally negative farms (category III) naïve replacement gilts remain seronegative after 60 days from introduction to a seropositive herd. Lack of seroconversion after direct contact with seropositive sows proves that older sows do not shed PRRSV.

Negative sow farms (category IV) are PCR and ELISA negative. Herd stabilization (category II) can be achieved following herd closure and/or mass vaccination program with modified live vaccine.

If the herd is open, replacement gilts have to be vaccinated at quarantine. Next steps depend on the final goal the producer wants to achieve. If the goal is category IV, no vaccines can be used. Maintaining category II requires gilts vaccination with modified live vaccine. If the risk of PRRSV re-infection is high (low external biosecurity, high pig farm density) it makes more sense to continue mass vaccination in sow herd, and keeping category II. Application of inactivated vaccines can be only considered as an additional booster following vaccination with attenuated vaccine.

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