When it comes to the porcine reproductive and respiratory syndrome, prevention and control are full-time challenges.
While every herd is different, there are some common measures you can take. Sit down with your veterinarian to establish a rough idea of the steps you'd take if PRRS infiltrates your herd.
Here's a look at the steps that swine practitioners at Carthage Veterinary Services in Carthage, Ill., usually take in such cases:
- Increase sow and gilt matings by 20 percent to 30 percent for at least 12 weeks.
- Reduce culling.
- Line up as many weeks of future gilt replacements as possible. Acclimatize these gilts via natural exposure. Use internal replacements.
- Vaccinate with modified-live or killed-virus PRRS vaccine, depending on your herd strategy.
- Do not introduce any additional replacement animals until absolutely necessary.
- Maintain strict all-in/all-out pig flow in all farrowing rooms.
- Restrict piglet movement between litters to the first 12 hours after farrowing.
- Euthanize any pigs with chronic PRRS symptoms.
- Wean the most severely affected pig groups out of the system if possible.
- Maintain all-in/all-out pig flows through the growing phases. Include broad-spectrum, feed-grade antibiotics in the late-nursery diets.
An alternative strategy would be to:
- Stop all replacement animals from entering the sow herd for four months.
- Line up PRRS-negative replacements.
- Complete an off-site breeding project to maintain breeding targets.
- Add PRRS-negative sentinel pigs to the main herd on a trial basis.
Until the sow population becomes PRRS-stable, continue to acclimatize gilts naturally, and provide a long recovery period before adding them to the sow herd. You can accomplish natural acclimatization by providing contact with "seeder" animals (those shedding the PRRS virus) along with a feedback program.
This aggressive contact should be completed in the first few days of the acclimatization phase. Seeder animals should be low parity such as non-breeding gilts, gilts that have aborted or been rebred, or nursery pigs. Nursery pigs have the advantage of longer viral activity and are often the population with highest PRRS prevalence.
As PRRS stabilizes in the system, virus transmission within the seeder population diminishes. Serological profiling or polymerase chain reaction testing can identify the exact animals to use for contact exposure.
It's important that seeder animals are dispersed among all the animals being acclimatized. You can do this by housing them within all the groups or by moving them daily between the various groups. Rotate them once or twice daily during the early acclimatization period to increase exposure.
The exposure recovery period remains an inaccurate science and usually involves statistical profiling using ELISA, PCR, immunoflorescent assay, serum neutralization tests or combinations of those. Profiling is used to determine the PRRS status of gilts prior to acclimatization, their status following acclimatization, and their recovery length. Various profile patterns have been used to indicate the recovery, which coincides (hopefully) to reduced risk of animals shedding virus. PCRs can be used to identify high-risk animals.
However, if the animals are serum-PCR-negative, allow an additional 30 days of recovery to reduce the shedding risk. The best test to assess shedding may be sentinel monitoring, where known PRRS-negative animals are intermingled with the acclimatized gilts 90 to 100 days after natural exposure.
The sentinel pigs are profiled in 30 days. If the sentinels are PRRS-negative, it tells you the gilts are not shedding PRRS virus and are of minimal risk to the sow population.
These procedures of natural acclimatization, profiling and recovery continue until the sow population is determined to be stable with little or no viral activity. The length of time to achieve stability in many herds is 12 to 24 months. n
4 Steps Toward Eliminating PRRS
Controlling or eliminating the porcine reproductive and respiratory syndrome virus may not be as easy as one, two, three, but there are steps you can take to work toward that goal.
Swine practitioner Joe Connor and his partners at Carthage Veterinary Service, Carthage, Ill., take a four-step approach to controlling and/or eliminating PRRS from herds. These swine practitioners have adapted results from University of Minnesota veterinary researcher, Scott Dee's studies to develop the procedures.
Naturally, with any animal health protocol, you should consult with your own veterinarian to determine how to best approach your own herd.
STEP 1 The herd veterinarian runs tests to establish the PRRS classification of the sow herd first, then follows with the rest of the pigs. This is a necessary step in determining the herd's virus stability and selecting PRRS control and elimination strategies.
STEP 2 The Carthage veterinarians discontinue vaccine use if the herd has been stable for an extended period. Modified-live vaccine virus transmits may provide continual virus circulation, says Connor. Vaccines also make the interpretation of diagnostics more challenging.
STEP 3 PRRS-negative replacement gilts enter the sow herd. However, timing is critical. Adding PRRS-negative gilts to an unstable sow herd may contribute to a major epidemic within the group. PRRS-negative gilts can be added to the sow herd when:
- Clinical PRRS episodes subside.
- There are stable and declining ELISA or other serological parameters.
- Weaned pigs test negative for PRRS.
- PRRS-negative sentinel hogs that are co-mingled with the sow herd remain negative.
PRRS-negative sentinels can be derived from an outside source or from internal PRRS-negative pig flow. Add one sentinel animal per 200 sows. Serologically profile these animals routinely.
If the sentinel pigs seroconvert to PRRS, remove them and do not introduce the PRRS-negative gilt replacements. If the sentinel pigs remain negative, you can add PRRS-negative replacements to the sow herd. Flow the PRRS-negative replacements into a single barn on the site and keep them separate as long as possible.
STEP 4A Complete a breeding project of replacement gilts off-site. This step may not be necessary in many herds but may be critical in others, depending on the repetition of the PRRS-elimination program. By not adding animals for a four-month period you let the population stabilize and avoid introducing gilts that may be shedding the PRRS virus.
STEP 4B In some herds, an alternative to Step 4-A is to set up a test-and-removal plan without a breeding project. In this process, your veterinarian tests the whole herd, using a combination of ELISA, PCR, IFA and SN. Hopefully, the testing procedures identify virus-positive animals, which have some correlation to shedding or the risk of shedding. The success of removing shedders still needs to be proven.