January 25, 2016

4 Min Read
New strategies for an old foe

For the past month in central Illinois, weather has been perfect for porcine reproductive and respiratory syndrome transmission, with temperatures in the 30s, fog, rain and sloppy conditions. Our expected seasonal epidemic of PRRS has started. The onset of a PRRS outbreak in a sow farm is extremely demoralizing to the staff. Virulent strains of PRRS are reported to be lingering longer in herds, with some reported weaning positive pigs for 40 to 50 weeks. Our focus must be to eradicate the wild-type virus from the farm reliably and on a predictable timeline.

Time to negative pig

This term was coined by Daniel Linhares, D.V.M., at the University of Minnesota (now at Iowa State University). It is a measurement of how long it takes after the outbreak to wean negative pigs on a consistent basis using the American Association of Swine Veterinarians PRRS Classification – Positive Stable (II) herd. This is defined as:

  • test serum from weaning-age pigs by polymerase chain reaction

  • no positive results over a 90-day period (four consecutive negative herd tests sampling every 30 days or more frequently)

  • no clinical signs consistent with PRRS observed in breeding herd

When a herd meets this testing definition, the interval from the break date to the date of their first negative test is the interval for TTNP. A study (D. Linhares, 2014) following 61 sow herds after a PRRS introduction identified a median time of 26.6 weeks post-exposure to consistently wean negative pigs. When you add on 12 weeks to determine that the herd is negative using the AASV definition, 40 weeks is the average closure time needed before entering gilts back to a sow herd. This means you should expect to run out of gilts and will struggle to maintain breed target. Additional work is being done to identify risk factors in order to shorten that time to negative pig production. With this understanding that most herds will run out of gilts during a closure time period, there are three strategies employed to not miss your breeding target by running out of gilts.

Off-site breeding project

The first has been to simply start with the expectation that you will do an off-site breeding project at the end of the closure. This will buy you up to 16 weeks, if needed, of additional closure time; 46 weeks total. When you initiate a closure, you must plan that you will stock an offsite gilt developer with 16 weeks of inventory in order to have the ability to close the herd up to 46 weeks. This strategy also couples very easily with mycoplasma eradication projects and is commonly done concurrently.

Delayed herd closure

The second strategy has been to delay herd closure by two to three months. After the farm breaks, continue to enter gilts into the system and expose them to the virus early and/or vaccinate those gilts with a modified live vaccine. The program would look similar to running the farm “positive stable.” This allows all of your sow population an additional eight to 12 weeks to individually eradicate the virus. The value with this strategy would be for farms that do not have good off-site gilt development space. Additionally, this takes the stress and hassle away from completing breed projects. There are drawbacks. There is increased risk for longer-term viral transmission within the farm. Additionally, the last group of gilts entered still only has 210 days to eradicate the virus. This last gilt group is a much smaller population than your whole sow farm and generally will burn out the virus within that time frame.

Wean down

The third strategy is based off of what we have learned about porcine epidemic diarrhea virus’ viral load and pig age in the farrowing house. In PEDV farms, our ability to wean down farrowing houses in age was highly successful in eradicating the virus out of the farrowing house. Wean pigs start and perform well down to about 12 days of age as well as sows breeding back on the first cycle. Twelve days of age has generally become our line for a wean-down event, plus or minus a few days.

After a PRRS outbreak, farms need to begin tail swab or serum sampling pigs on day zero of age at 12 weeks post-break. Repeated negative sampling indicates pigs are being born negative to the virus, but are positive yet at weaning. This is your opportunity to perform a wean-down event to try to eradicate that infected and susceptible late suckling population. Only a few herds have tried this to date, but it will be practiced moving forward as a way to potentially reach TTNP on a faster timeline.


Data aggregation at a national level continues to reveal different ways to manage PRRS virus in sow herds to reduce the clinical and economic impact. Continue to be creative with ideas, understanding the biology of the pig and pathogenesis of the virus, in order to minimize the effects of this pathogen. 

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