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In an unstable herd, PRRS virus regularly cycles from pockets of infected sows to susceptible sows or gilts. Those breeding animals then infect offspring, which spread the virus downstream as well as recycling the virus back to the breeding herd. The goal in stabilizing an unstable PRRS-positive herd is to first create a breeding herd in which the virus isn't being passed from sow to sow or from sow to pigs. Once that's accomplished, you can move on to maintaining that stability, attempting to stabilize the downstream pig flow, and/or trying to completely eliminate the virus from the operation.

Strategically applying a modified-live vaccine to that process — along with other management techniques — can help increase the odds of successful stabilization, even while reducing the interruptions in all-important production flow.

Such mass vaccination — also called whole-herd vaccination — vaccinates the entire sow herd simultaneously following diagnostic confirmation that the virus is circulating. That re-booting of the entire herd's PRRS immune status — in combination with temporary herd closure that doesn't introduce more susceptible animals or shedding animals — in effect gives the field strain of the virus circulating in the herd nowhere to turn. By removing the “tinder” of susceptible animals, as Minnesota veterinarian Tom Wetzell expresses it, the PRRS infection may still smolder, but it has no fuel to rekindle. Eventually, it will burn itself out.

Typically, the protocol involves herd closure, two rounds of vaccination, off-site gilt vaccination and acclimatization, and then possible maintenance vaccination along with careful herd-diagnostic monitoring to watch for a recurrence of the disease. With all that said, though, notes Indiana veterinarian Tom Gillespie, one of the pioneers of the mass-vaccination protocol, the most important point is to remember no single plan works for every operation. The first step, therefore, is to find and work out a custom action plan with your veterinarian. Then, a typical plan might be as follows:

Herd monitoring

Institute herd monitoring to understand your status. Is the herd:

• Negative and naïve?
• Positive but stable?
• Positive and unstable?

In what production phase or segment is the virus actively circulating? Working with those answers, you and your veterinarian can devise the best control or elimination plan. It may or may not ultimately involve vaccination.

Herd closure

Close the herd to new introductions. Herd closure minimizes the introduction of new PRRS virus strains, and it allows time for protective immunity to develop.

Whole-herd vaccination

Vaccination of every adult in the herd on the same day with modified-live vaccine “resets” the immunity of naïve, recovered and acute animals within the population, permitting you to control the timing of exposure and the consistent development of protective immunity across the population. Because cell-mediated immunity is necessary to control PRRS, modified-live vaccines are believed to be more effective than killed vaccines. Vaccinated animals can shed vaccine virus and there are some risks in vaccinating pregnant animals, so it's important to work out your action plan with a veterinarian familiar with your operation.

In the gilt developer — preferably off-site — gilts vaccinated at the same time will begin to develop immune response concurrent with the rest of the breeding herd.

Follow-up vaccination

Follow-up vaccination 30 days later maximizes immune response and begins to standardize immune status of the entire herd. Often it is recommended to repeat the mass vaccination to ensure uniform exposure and population immunity. Gillespie suggests herds that have routinely vaccinated gilts twice before entering the sow herd and then boostered them before each breeding may be able to skip the second dose. On the other hand, even animals that have been vaccinated twice before may still require the mass vaccination to achieve immune status — chronically open sows, for instance, often get missed on their booster.

Likewise, follow-up vaccination in the gilt pool will help bring those animals up to uniform immune status. Then, further gilt acclimatization to the specific herd strain(s) — whether through natural exposure or other tools — becomes more reliable.

Once uniform immune status has been established, the lack of non-immune animals locks out the field virus, giving it no place to hide and minimizing its ability to circulate within the population.

Quarterly maintenance

Move to quarterly maintenance mass vaccination. That interval can be extended or shortened under direction from your veterinarian, depending on the level of herd challenge. Replacement gilts routinely vaccinated should begin immunization at least 42 days before entering the herd.

Monthly herd monitoring

Institute monthly herd monitoring of all production stages to double-check the success of the program, ensure stabilization continues, and provide early detection in case of new exposure.

Control losses in growing pigs

After the breeding herd is stabilized, modified-live vaccination can also be used to control losses from the virus in the growing-pig flow. Serological profiling can help target vaccine placement to the best window of opportunity to achieve maximum immune response when it's needed to control the disease: four weeks prior to virus exposure.

More info

Gillespie, T.G.; Carroll, A.L. Methods of control and elimination of porcine reproductive and respiratory syndrome virus using modified live vaccine in a two-site production system. J Swine Health Prod. 2003 11(6):291-295.

Gillespie, T.G.; Carroll, A.L. Techniques for PRRSV elimination utilizing modified live virus vaccine on single-site swine farms. Proc. AASV 2003:549-552.