Porcine reproductive and respiratory syndrome (PRRS) causes clinical disease in many pig farms. Numerous procedures are being attempted to alleviate the effects of this disease.

Case Study No. 1

An 1,800-sow, farrow-to-finish operation has one sow site, two nursery sites and six finishing sites. The farm buys PRRS-negative adult gilts every two months from a gilt multiplier and isolates them on the sow site.

The sow herd had been chronically infected with PRRS — sick sows, abortions, poor quality pigs and poor production. These episodes commonly coincided with the timing of herd additions. It was known that the herd was infected with at least one PRRS strain. But neither consistent exposure to gilt additions nor blood testing to monitor herd status was done.

Tests identified one PRRS virus strain in the sow herd, very similar to a commercial modified-live vaccine. All sows were mass-vaccinated with that vaccine. The herd was revaccinated 30 days later, and vaccination was repeated every 90 days for two years.

The gilt isolation/acclimation area was used to expose incoming gilts to the virus from the herd by feedback of lungs from affected suckling piglets. Gilts were tracked by monthly blood tests. Gilts were also vaccinated with the commercial modified-live PRRS product.

Gilts entered the breeding area once they tested negative twice at four-week intervals by polymerase chain reaction (PCR) test.

Since the herd was stabilized, it has produced PCR-negative weaned pigs. Those same pigs are also PRRS-negative by ELISA (enzyme-linked immunosorbent assay) upon leaving the nurseries.

The farm continues to receive PRRS-negative animals from its supplier, but has added isolation space, as it takes 10-12 weeks to get the animals to become PCR negative for PRRS after exposure by feedback to the farm's virus.

Internal biosecurity procedures have been intensified to reduce the risk of introducing another virus strain from the hog-dense area. The nursery and finisher sites do not share personnel nor equipment, and enhanced sanitation of transport vehicles and loading procedures has been implemented.

Case Study No. 2

A 1,200-sow, farrow-to-finish operation produces replacement gilts internally. Gilts go through contract nurseries and return to a finisher site near the sow farm to be acclimated and grown to selection weight and age.

The sow farm had periodic clinical PRRS outbreaks, but no plan for managing gilt introductions. The producer assumed that since they raised their own gilts, the health status would automatically be “equal” to the sow farm's because that's where the gilts were born. Gilts were added to the sow herd without being tested.

After a severe outbreak produced 80 abortions and 30 sow deaths, a more organized plan was developed. It appeared that gilts were PRRS negative when they entered the sow herd, but seroconverted during feedback exposure prior to farrowing. When the population of susceptible animals reached a certain level, clinical disease returned, causing virus shedding and sick pigs.

Testing identified a PRRS strain quite different from any commercial vaccine.

Due to the severity of the virus and the impact on production, it was decided to produce an autogenous PRRS vaccine. During vaccine production (8-12 weeks), the herd was purposely infected throughout using lungs from viremic pigs.

When gilts reached selection age and size, some were rechecked for virus. Groups that tested ELISA positive but PCR negative (no virus in the blood) were approved for breeding.

Weaned pigs had maternal antibodies to PRRS, but have stayed PCR-negative through all-in, all-out nurseries. Some finishers have stayed negative to market.


These two cases demonstrate the complexity of both production and procedures in dealing with PRRS. No two farms are alike, and the need for intense investigation and cooperation is great. Single-site farms may not have the same risks for another introduction of virus, because there are fewer people to be compromised.

Producers and their veterinarians need to:

  • Develop a reasonable method of gilt introduction.

  • Agree on a plan of action.

  • Exercise extreme diligence in implementing biosecurity procedures, including routine testing of live animals and semen supplies for PRRS and other diseases.

The risk of introducing another PRRS virus into a herd is real, as we still don't know all the ways this virus can be carried or transmitted.

Producers should visit with their veterinarian or health advisor to develop a plan for their operation that best meets their needs.