Rotating vaccines for Porcine Reproductive and Respiratory Syndrome (PRRS) adds a level of protection against various strains of the disease.

In field trials at a commercial hog farm in North Carolina, gilts and sows of various parities were first vaccinated against PRRS with RespPRRS/Repro (NOBL Laboratories).Researchers conducted serum neutralization (SN) tests to measure antibody responses to the four PRRS strains involved in the study.

Then the animals were boostered with Prime Pac PRRS (Schering-Plough Animal Health).

Three weeks after the second vaccination, researchers saw big increases in antibody titers to all four strains of PRRS, lasting for at least five weeks after vaccination.

"We succeeded in broadening the immune system's recognition of more different PRRS viruses, which in theory increases the probability that you'll protect against field viruses and the diseases they cause," says Terri Wasmoen, a research director with Schering-Plough.

The percentage of animals showing antibody response to the four PRRS strains greatly increased after rotating vaccines, adds Wasmoen.

Doug Hutchinson, DVM, technical service veterinarian for Schering-Plough, was involved in the field trial. He says the increase in immune response came as somewhat of a surprise.

"It was originally intended to be a safety trial," he recalls. "Some people had asked whether putting another strain of a modified-live PRRS vaccine into a herd would be detrimental and create problems, perhaps recombine with the other vaccine or a field strain and create a new mutant strain.

"Not only did the study show no danger to rotating the two PRRS vaccines, it actually showed an increase in immune response to four different and very distinct isolates of the PRRS virus," Hutchinson notes.

Rotating the two PRRS vaccines should be considered to protect incoming breeding stock, says Wasmoen. The decision whether to continue rotating PRRS vaccines in the breeding herd will depend on the perceived level of risk of PRRS entering the herd.

For Mike Mohr, DVM, Swine Vet Center, St. Peter, MN, rotating PRRS vaccines was a chance worth taking. "Initially, we had some concerns about rotating two, modified-live vaccines," he says. "But, when a vaccinated herd is experiencing 40% abortions and 25% of the production is lost, there is nothing to lose by trying."

According to the Minnesota veterinarian, rotating PRRS vaccines has sharply reduced clinical signs of PRRS. "It stopped the abortions and the preweaning mortality," he says. "The farms are seeing better birth weights. Fewer squeaker pigs are being born."

And Kinston, NC, veterinarian Richard Conger thinks alternating vaccines has helped to stabilize several of his problem herds.

"A lot of the credit has to go to better management. Our producers are buckling down and following the programs we outline for them, without taking shortcuts," he observes. "But I think alternating vaccines is a good practice in some herds. That doesn't mean one product is better than the other, but I think there's something to be said for increasing a herd's exposure to different strains."

Conger explains herd history is important. "If the producer is doing everything he's supposed to do and still having problems, rotating vaccines may be a good option to consider."

Schering-Plough's Wasmoen sees vaccine rotation as good protection against major wrecks. "We had a lot of herds last year in southeastern Iowa that were ticking along just fine, when all of a sudden they crashed and burned with 200-400 abortions," she says. "To me, rotating vaccines seems like cheap insurance against this type of event."

A highly sensitive test developed with recombinant DNA technology has been automated for the detection of the PRRS (Porcine Reproductive and Respiratory Syndrome) virus in swine semen.

Tom Molitor and a team of researchers at the University of Minnesota worked with a test called TaqMan, proven to be reliable for detection of PRRS in semen at low levels and not produce false positives.

The automated TaqMan assay detected PRRS in 19 of 19 samples chosen to represent a wide range of PRRS isolates and the widest genetic variation.

The Lelystad (European) strain of PRRS was also analyzed in this test and could not be detected, researchers said. A second TaqMan assay is being developedfor the detection of European-type isolates of PRRS.

In short, there is a rapid explosion of the use of artificial insemination (AI) in the swine industry, and a growing concern that PRRS virus may be spreading to naive animals via semen.

"It is expected that the TaqMan-based PRRS test for semen will provide an effective monitoring tool for the AI industry," the Minnesota researchers say.

The TaqMan-based PCR (polymerase chain reaction) technology is very sensitive and specific for the detection of a number of disease pathogens. The PCR test, which can be done in 48 hours, will also detect PRRS virus in serum and tissues of infected pigs.

A TaqMan PCR test for the detection of salmonella is currently being marketed by a California company. The firm is also working on tests for detection of E. coli 0157:H7, listeria and campylobacter, which could be utilized by large food manufacturers in their HACCP (Hazard Analysis and Critical Control Point) programs.

A Lansing, MI-based biotechnology company, Origen, Inc., has developed a three-step diagnostic test based on PCR technology for detection of PRRS virus in serum and other bodily fluids. The company's Orisure-PRRS may have the most value in detecting persistently infected piglets normally masked by the presence of maternal antibodies and not showing clinical signs of the disease. The test detects virus at 1-8 days post-infection.