Since porcine reproductive and respiratory syndrome (PRRS) has been a major health concern, several companies have developed programs for producers to close their herds and raise gilts internally.

The challenge for producers raising their own gilts is to accomplish PRRS exposure without causing more problems to the main sow herd.

Case Study No. 1

A producer with about 250 sows farrows and finishes on one site. Sows are hand-mated in groups to facilitate all-in, all-out group management through the nursery stage.

The herd has produced its own gilts for several years but the producer does bring in boars each year.

The herd has a few grandparent females that are bred by artificial insemination (AI) using semen from a PRRS-negative boar stud.

PRRS was diagnosed in the herd several years ago, producing reproductive and respiratory symptoms. Production was stymied for several groups, and then recovered very quickly.

The producer was convinced that the PRRS virus entered the herd with some replacement gilts that were not isolated nor tested. The producer didn't know the status of his herd before the PRRS break, but assumed it was PRRS positive because all of his neighbors' herds were.

The producer implemented an internal gilt production system over two years ago when the main sow herd tested positive for PRRS.

Tests indicate the herd is producing pigs that are ELISA (enzyme-linked immunosorbent assay) positive and PCR (polymerase chain reaction) negative for PRRS at weaning. They become ELISA negative by the time they leave the nursery and are transferred to the gilt development unit.

In the older, on-site gilt developer building, gilts are penned next to cull sows. The cull sows are frequently switched to provide gilts with adequate exposure to urine, feces and saliva. Growing gilts are also given feedback material from the breeding herd.

Gilts become PRRS positive but “cool off” before they are introduced to the breeding herd.

The key to PRRS immunity is that it will be fairly strong and long lasting provided all animals on a site have been exposed to the herd's virus.

Case Study No. 2

A 1,200-sow, farrow-to-wean operation breeds sows naturally, and has several boar lines for its internal gilt-breeding program.

The sow farm is PRRS positive with only one strain of the virus identified. Feedback of material from the breeding area to pregnant sows and from the farrowing area to late pregnant sows is done routinely.

At weaning, pigs test ELISA positive but PCR negative. Pigs go to several nursery sites that hold 1-2 weeks of production. When pigs leave the nurseries, they test ELISA and PCR negative.

When the nurseries are emptied, maternal line gilts are selected and taken to a continuous-flow, grow-finish barn near the sow herd. The barn is purposely left dirty to expose grower gilts to the PRRS virus.

Several months later, these gilts have “cooled down” and test PRRS negative when they go into isolation at the sow farm. There they receive direct feedback from sows. These gilts continue to stay non-infectious. The tagged maternal gilts within the sow herd are regularly tested to maintain a long-term record of PRRS status.

Case Analyses

These two cases demonstrate the complexity of production and procedures. No two farms are alike and the need for intense investigation and cooperation is high.

Single-site farms may not have the same level of risk for virus introduction because there are fewer people to be compromised.

There is still a lot we don't know about the PRRS virus. Each strain appears to have different levels of virulence or ability to cause disease. Many farms have more than one strain of the virus.

Summary

Veterinarians must fully evaluate the entire production system to develop a reasonable method for gilt introduction. Monitoring and risk assessment are crucial. Too much focus on PRRS may allow some other infectious agent to work its way into the herd.

There are still big questions about PRRS virus spread within an area by aerosol, insects and other means.

Producers and their farm staffs must be totally uncompromising with procedures for sanitation, biosecurity and transport vehicles.

They also need to be certain their suppliers of semen or live animals are doing routine testing for PRRS and other disease agents. Routine testing should be done within the herd on known-status animals so that changes can be identified.

Producers should develop a PRRS plan of action that best meets their situation.