Owners of a 600-sow gilt multiplier placed strict limitations on their veterinarian's proposed five-year cleanup plan for PRRS. The plan, which emphasizes testing and vaccination, has kept production steady without costing much extra money.

In some ways, 1998 was a good year for the Pumphrey family and staff at Ag Production Enterprises, based in Greensburg, IN.

Sure, hog profits were nonexistent. But that made it a good time to roll over much of the old, commercial sow herd to put the finishing touches on eradication of pseudorabies (PRV). And the operation caught a break because the State of Indiana paid for all of the blood testing.

Robert Pumphrey and production manager Bill Shobe were pleased with the PRV eradication outcome, and had been successful at managing around other diseases such as Actinobacillus pleuropneumonia, salmonella and transmissible gastroenteritis (TGE).

Five-Year PRRS Plan

After all the blood test work and production hits endured with PRV, it took some strong urging from swine consulting veterinarian Matt Ackerman to convince the Pumphreys to take on another challenge — namely a five-year plan to eliminate porcine reproductive and respiratory syndrome (PRRS).

“The Pumphreys had already switched to buying PRRS-negative animals in 1998 (Newsham Genetics), and we were already using PRRS killed virus vaccine in their herds, so why not go the extra step?” asks the Greensburg, IN, swine practitioner.

Pumphrey and Shobe finally agreed to the plan, but with some restrictions. They told Ackerman: “You are welcome to eradicate PRRS here as long as it doesn't cost us anything extra, it doesn't require a lot of extra blood testing, it doesn't interfere with production, and there is no risk of total disaster!”

Ackerman smiled and agreed to all but the last requirement. He explains, “So for PRRS cleanup, the operation was already paying for monthly blood testing, since the 600-sow operation to be cleaned up is a user group gilt multiplier. We just reallocated to bleed more sows and less finishing pigs.”

Hagerstown Farm is a closed-herd gilt multiplier with PRRS-negative semen supplied by one of the Newsham AI Centers in Indiana.

“Downstream, more gilts were being bled again as they came into the commercial farm, so we had additional testing down at that end,” adds Ackerman.

To avoid interfering with production, Ackerman has followed a sow test-and-remove plan at weaning (30 head), which is much less invasive than a herd closure, depopulation-repopulation or a whole-herd test-and-remove plan. The plan also gives the Hagerstown production team the right to refuse to cull PRRS-positive animals if, for instance, 100% of the weaned sows in any given week should turn up PRRS-positive.

Farm History of PRRS

Pumphrey readily admits his restrictions on the PRRS cleanup plan have been severe.

“Part of the reason we were able to put these restrictions on Matt's (Ackerman) plan was that we weren't having a big problem with clinical signs of PRRS. We didn't want to take a big risk and screw up something that really wasn't too bad in the first place.”

Performance Monitor data recorded by PigCHAMP shows that from 1999 through 2003, the Hagerstown Farm has averaged an 85% farrowing rate, 11-plus pigs born live/litter, over 2.4 litters/mated female/year and over 21.5 pigs weaned/mated female/year.

When PRRS was first noticed in the Hagerstown operation in 1995, few reproductive problems occurred, recalls Shobe. PRRS caused a flu-like condition in finishing and later caused a myriad of problems in the continuous-flow nursery.

“At the start of the eradication project (in December 1999), the sow herd was serologically PRRS positive and a modified-live-virus (MLV) vaccine had been used for approximately one year. Due to lack of commercial MLV PRRS vaccine options, the herd transitioned to a commercial, killed PRRS vaccine” (PRRomiSe, then sold by Bayer and later by Intervet Inc.), explains Ackerman.

In January 2000, the entire sow herd was mass-vaccinated with the killed vaccine. Then for four months, all sows were double-vaccinated with killed vaccine in mid-gestation.

Currently, all gilts are double-vaccinated in isolation on arrival at about 6 months of age, then are given a second dose three weeks later, says Ackerman. Both sows and gilts still receive one dose of killed PRRS vaccine in mid-gestation.

About 50 head of female breeding stock are blood tested monthly to maintain PRV- and brucellosis-free validation.

Following vaccination in January 2000, blood testing indicated that the sow herd was fairly stable with a low prevalence of PRRS, with about 40% of the sow herd PRRS positive, says Ackerman.

Ackerman decided on a partial depopulation of the finisher to block transmission of PRRS to sows. The farm is a modified three-site operation, with 600 sows on the main farm and a second site with a 2,000-head finisher and a 1,000-head nursery about a half-mile away.

The Indiana veterinarian decided to wait a few months after the partial depopulation to do the next round of blood testing, so the finisher could be filled up again. By May 2000, positive sample-to-positive (s/p) ratios were seen in pigs 4-7 weeks of age. These s/p ratios lasted until 8-10 weeks of age, and were determined to be normal PRRS maternal antibodies.

An s/p ratio, sometimes referred to as a titer, reflects a measure of the amount of antibody in a serum sample. If the s/p ratio is 0.4 or above on any individual animal, then the herd is considered positive for the PRRS virus, he explains.

Twelve different age groups were tested in the nursery and finisher to determine their PRRS serologic status.

Producing Negative Pigs

From May 10, 2000 to the present, the gilt multiplication herd has consistently produced PRRS-negative animals from PRRS-positive sows, stresses Ackerman.

He declares: “If those animals stay negative, then you can feel pretty confident about the stability of your sow herd. This tells us that the sow herd is PRRS positive, but they are producing PRRS-negative offspring.”

Those figures contrast with data for 2000, when an estimated 40% of the herd's females were PRRS positive. The production team wasn't excited about culling large numbers of females, so positives were culled based on age at weaning and replaced with negatives, explains Ackerman. Eventually, all PRRS-positive females will be rolled out of the herd by normal attrition (50-60% annual replacement rate).

The progression of rolling in PRRS-negative animals and rolling out PRRS-positive females is dramatized when the results of Figure 1 (Dec. 31, 2000) are compared with results depicted in Figure 2 (July 1, 2003).

“We are rolling in negative animals and rolling out the positives with very little seroconversion (development of antibodies which reflect exposure to a disease pathogen),” says Ackerman. As of Sept. 1, 2003, the herd tested 95% PRRS negative.

With permission from the management team, Ackerman conducted a whole-herd bleed on Oct. 22, 2003. Out of 647 females tested, 32 came back PRRS positive, or just under 5%. These animals were retested using immunofluorescent antibody (IFA) as a confirming test; all sows tested PRRS-negative.

Pumphrey promised if around 30 head turned up positive, they could be culled, and that was done. Blood testing of nursery-to-finish animals was also repeated to confirm earlier findings that PRRS was not present in the growing pig population.

Conclusions

In January 2004, test results took a sudden turn when about 10% of random samples tested PRRS positive. Sows that came back positive were marked, and that factor will be used as culling criteria, remarks Shobe.

Ackerman believes his control program, combining testing, vaccination and proper management, still appears to have PRRS on the run.

Monthly test results through April 2004 continued to confirm there was no clinical evidence of PRRS virus. Only a small percentage of the sow herd tested positive for PRRS by ELISA (enzyme-linked immunosorbent assay) and were IFA negative.

In short, Ackerman and the Ag Production team remain optimistic that the highly unpredictable PRRS virus, if it is indeed still present in the herd, will be eliminated in another year or so.

For insurance, they agreed to vaccinate with the killed PRRS virus product for at least another year.

“Just remember that vaccines are designed to minimize clinical signs, and won't necessarily prevent seroconversion,” says Ackerman. Depopulation-repopulation is the surest way to eliminate the PRRS virus.

“But if you want to consistently produce PRRS-negative pigs, it would be my recommendation that you utilize PRRS-negative replacements, PRRS-negative semen and a killed PRRS vaccine. We are continuing to vaccinate gilts twice in isolation, and sows and gilts once in mid-gestation (or whole herd quarterly) and continuing to produce PRRS-negative offspring out of PRRS-positive sows,” comments Ackerman.

Effect of Repeated PRRS Vaccination

Many sow herds are repeatedly vaccinated for porcine reproductive and respiratory syndrome (PRRS), despite evidence that a single vaccination provides long-acting protection, says Eileen Thacker, DVM, Iowa State University immunologist.

Thacker compared immune responses to both modified-live-virus (MLV) and killed-virus (KV) PRRS vaccines in sows that had received multiple vaccinations during their lifetimes.

She found that “repeated exposure of the immune system to the same PRRS virus antigens induces limited recall responses.”

But Thacker adds that the use of both types of PRRS vaccines “appeared to enhance the immune response.

“Thus, based on the results of these preliminary studies, use of combined MLV and KV immunization strategies may be worth further consideration,” she concludes.