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Controversial PRRS Control Procedure Wins Advocates

A number of swine veterinarians are successfully using a relatively new scheme to put the clamps on difficult cases of porcine reproductive and respiratory syndrome (PRRS). Frustrated with the failure of vaccine and other control measures to stabilize sow herd immunity, some U.S. swine veterinarians are experimenting with a controversial procedure that they say has effectively stopped the PRRS virus

A number of swine veterinarians are successfully using a relatively new scheme to put the clamps on difficult cases of porcine reproductive and respiratory syndrome (PRRS).

Frustrated with the failure of vaccine and other control measures to stabilize sow herd immunity, some U.S. swine veterinarians are experimenting with a controversial procedure that they say has effectively stopped the PRRS virus in its tracks.

Supporters say serum inoculation (therapy) has achieved wide success when all other attempts have failed. It has reduced the lengths of PRRS outbreaks, stabilized sow herds and returned farms to production of PRRS-negative pigs.

Like other methods, however, it will not protect the herd from becoming infected with a new strain of PRRS virus, and the process doesn't work 100% of the time, reports Clark Huinker, DVM, Fairmont (MN) Veterinary Clinic.

“With serum therapy, we are seeing fewer PRRS outbreaks and fewer unstable sow herds, and the breaks that we do see seem to be less severe in terms of the percentage of sows that abort and poor quality pigs,” he says. This is due to a shorter duration of clinical signs.

Serum therapy is the process of extracting a known PRRS virus strain from infected hogs and injecting it back into animals on that farm only.

“What we've found is that one of the challenges with PRRS is that for many reasons, the virus doesn't appear to infect the population very completely,” says Mark Wagner, DVM, Fairmont Veterinary Clinic. Uniform exposure and immunity are necessary to provide herd stability, and that's what serum therapy has provided.

Roots in TGE Protection

Problems with control of transmissible gastroenteritis (TGE) 30 years ago prompted swine veterinarians to start using a somewhat similar but new approach, explain Huinker and Wagner.

The method involves using feedback of infected material from baby pig scours and intestines, ground up and fed to sows, to mount an immune response against that farm's TGE virus.

Other management attempts which have had variable results in the control of PRRS include exposing gilts to infected cull sows; feeding back infected placenta and other birth materials; and exposing sows and replacement gilts to infected, shedding pigs, recalls Huinker.

Likewise, vaccination and cleanup programs like depopulation-repopulation and test-and-removal have shown limited effectiveness in keeping out new PRRS strains, says Tim Loula, DVM, Swine Vet Center, St. Peter, MN.

Shortens PRRS Outbreaks

With serum therapy, the herd still becomes sick with the PRRS virus — sows die, abort and have mummified pigs, says Loula. But the duration of the “controlled” outbreak is much shorter, averaging from a few weeks to a few months vs. a whole turn of production with a typical field outbreak of PRRS.

“A lot of times, six weeks after using serum therapy, you go in the farrowing barns and the sows are producing some very nice pigs. It is a fast turnaround. Stillborns and mummy rates may still be higher after 4-6 weeks, but preweaning mortality may already be back at 10%,” says Loula.

“And you eliminate the in-farm subpopulations (groups of pigs of uneven immune status) that cause the farm to leak and continually send out some virus. In a multi-site system or a system that is selling or commingling pigs, this can send active PRRS virus all over the country,” he says.

In contrast, serum therapy provides more confidence that the sow herd has been well exposed, is more stable to PRRS and therefore, is leaking less virus and posing less of a disease threat to neighbors' farms, adds Huinker.

Serum Therapy Applications

The most common way to use serum therapy is by first blood-testing incoming replacement gilts in isolation to make sure that they are naïve to the PRRS virus, and that no new strains of the virus have been brought onto the farm, says Loula.

Then blood is collected from an infected sow or piglet and spun down by centrifuge to obtain PRRS-infected serum, says Loula. This process is all done on the farm; the serum never leaves the farm on which it was collected.

The collected serum is injected into the replacement gilts. The gilts are then retested to confirm that the virus that is in those gilts is the same as the virus that was extracted.

“We want to characterize and sequence out that virus to confirm that they do match up, and that we can be sure that what we put into that gilt is what we know to be in the sow herd, just in case a more recent strain has just arrived on that farm,” notes Wagner.

It's also important to wait until the replacement gilts cool down (test negative to PRRS on a polymerase chain reaction test) before they are permitted entry into the gestation barn, says Wagner.

Realize, too, that the use of serum therapy will extend the isolation/acclimation period from 30-60 days to 60-120 days, says Loula.

Serum therapy is also used two other ways, in the face of an active outbreak and for chronic PRRS infections (see case study on p. 22), says Huinker.

“We have had farms that just have been really frustrated because they can't get a handle on their nursery death loss, and this straightens them out in about 10 weeks,” says Huinker.

Use Caution

Fairmont veterinarians Huinker and Wagner stress that producers frustrated with the failure of PRRS control programs should not attempt to try serum therapy on their own. It is critical that they work with their veterinarian to set the best course of action.

And remember that serum therapy is just another tool in the veterinarian's toolbox, says Loula. Its use must be evaluated on a farm-by-farm basis.

“Some good results can still be achieved with PRRS vaccine, so that is probably an approach that should be used first. But if the strain in your vaccine doesn't match up with the strain identified on your farm, or if the vaccine doesn't protect against that strain, then serum therapy is an option,” Loula observes.

Ongoing Research

Eric Neumann, DVM, director of Swine Health for the National Pork Board, says they are looking into funding projects to review the safety and efficacy of serum therapy.

The American Association of Swine Veterinarians has formed a committee to look at establishing a set of guidelines for serum therapy, but no conclusions have been reached, says Tom Burkgren, DVM, executive director.

Case Study on Serum Therapy

A 1,000-sow, farrow-to-finish operation in southern Minnesota broke with porcine reproductive and respiratory syndrome (PRRS) in November 2001, according to Mark Wagner, DVM, Fairmont (MN) Veterinary Clinic.

Ten percent of sows aborted, pigs weaned/sow dropped by one pig to 8.5 pigs and nursery mortality climbed from 1.8% to 8.5%. The outbreak lasted for 16 weeks.

The herd was closed (internal multiplication), sows were exposed to “seeder” pigs (infected suckling piglets) and lung and intestinal material from infected pigs were fed back to gestating sows. Herd health seemed to stabilize, says Wagner.

Then in November 2002, the herd had a relapse with the same strain of PRRS virus.

This time, serum-based live virus from suckling pigs was injected into all sows. Four percent of sows aborted and problems lasted for seven weeks.

But results have been positive for the last 12 months. Nursery mortality runs 3%, prewean mortality is 8% and pigs weaned/sow has returned to 9.5. Gilt replacements are serum-injected with the farm's PRRS virus as a maintenance program.

“Before we used serum therapy in the sow herd, we weren't able to fully stabilize it,” points out Wagner.

by Joe Vansickle, Senior Editor

Closed-Herd Concept Cleans Up PRRS

Kansas swine veterinarian Steve Henry is following a management program that disavows the use of vaccines and medications to clean up porcine reproductive and respiratory syndrome (PRRS).

To date, 32 of 34 known, PRRS-infected herds following this procedure have gone negative through the nursery phase, based on serologic screening of nursery pigs and the testing of sero-negative sentinels, says Henry, Abilene (KS) Animal Hospital.

Twenty-seven of these herds have become PRRS-negative through herd rollover with immune animals, and some have replaced the sow herd with negative animals. Five herds are negative due to depopulation-repopulation.

Henry defines immune animals as those that have been infected with, then recovered from, PRRS infection, verified by viral sequencing.

He prescribes two tests to provide confidence that animals have remained immune and non-infected: the enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR). Mixing known, naïve sentinels into the population, then testing the sentinels by ELISA and PCR, provides added assurance the herd remains non-infected.

Two other herds have failed the process and have experienced repeated PRRS outbreaks, says Henry.

Natural Immunity

The process of moving from positive to negative PRRS status hinges on development of natural immunity following infection with the PRRS virus.

Development of what Henry calls “sterilizing immunity” requires many weeks to develop and is dependent on uniform exposure, infection, recovery and immunity.

It also requires lengthy post-infection isolation of the affected population, and strict isolation to prevent infection of naïve animal additions to the herd, he points out.

Sterilizing immunity is an immune response where the animal not only recovers from PRRS infection, but the virus is also cleared from the body, Henry explains.

Another requirement for sterilizing immunity is that only known, PRRS-naïve animals can be added to the herd.

Henry's PRRS elimination program is an outgrowth of four other control steps:

  • Use of PRRS vaccine is strongly discouraged. No client herds have been exposed to live or killed PRRS virus vaccines since October 1996.

  • Replacement breeding animals, whether raised internally or purchased, male or female, are entered into the herd at as young an age as possible, preferably as weaned pigs.

  • Where possible, nursery through finishing operations should be located on separate sites from the breeding herd and segregation procedures should be carefully practiced.

  • Reduce the number of management procedures that might transmit infectious bodily fluids among pigs. Examples include injections, identification and surgery.

After the sow herd has achieved negative immune status, testing a random sample of weanling and nursery pigs by ELISA on a monthly basis, and exposing those pigs to naïve sentinels, confirm production of negative pigs.

Henry observes: “Results are most promising in many herds under a variety of production methods. Most gratifying of all has been the lack of new cases in sow herds and the great reduction in nursery infection. While progress has been slow, it has been steady.”