Introduction/Background
Porcine reproductive and respiratory syndrome virus (PRRSV) has caused substantial economic losses in the swine industry since the 1980s. Holtkamp and Osemeke recently estimated the annual cost of PRRSV to be $1.2 billion in the US.1 Many sow farms aim to eliminate PRRSV when infected; however, the time required to reach herd stability has increased in recent years. According to the PRRSV Outbreak Management Program (POMP) database, the median time to stability has increased from 27 weeks in 2011 to 49 weeks for herds affected by the L1C.5 variant outbreaks. For herds in the 90th percentile, time to stability may reach 62 weeks.
Traditionally, farms are loaded with gilts, homogenized to the circulating PRRSV variant, and closed to any new live animal introductions until the herd tests negative. With herd closure periods now extending 49-62 weeks, a sizable gap in gilt entry is common, resulting in additional production loss. With traditional closures, farms often stretch their existing gilt inventory and breed females they would not normally use in order to maintain breed target. These practices lead to reduced sow performance and longevity, farm inefficiencies, and other unintended consequences.
Methods
To eliminate this gilt availability gap, farms have continued to acclimate feeder gilts to PRRS via herd-specific LVI in an off-site location prior to sow farm entry. In this approach, a gilt developer unit (GDU) is secured in an acceptable location off-site from the sow farm, shortly following a new PRRS introduction in the sow farm. The GDU is then populated with gilts ranging from feeder-pig age to nearly breeding eligible, depending on the needs of the recipient sow farm. The GDU site is acclimated to the recipient sow farm’s PRRS LVI and managed appropriately.
Gilts are tested thoroughly prior to monthly movement to the sow farm to ensure no additional PRRSV variants or other pathogens are present in the population. PRRSV sequencing is expected pre-shipment during the first 3-4 monthly deliveries to the sow farm. New feeder gilts are added to the GDU site monthly, with timing managed so that entry occurs shortly after the most recent group of acclimated, select gilts has moved to the sow farm. This allows for the largest window of time between new gilt entry and acclimated gilt departure. All incoming feeder gilts must be acclimated via LVI to the sow farm PRRSV within 48 hours of arrival, with the shortest possible timeframe preferred.
This cycle of entering and acclimating new feeder gilts in the GDU continues until the recipient sow farm has reached predetermined PRRS testing milestones. One common milestone is the completion of one full turn through the farrowing house of PRRSV PCR negative processing fluids (tested by room), followed by PRRSV PCR negative 60-piglet blood samples targeting the most clinically affected piglets across the farrowing house. Once this milestone is reached, no additional feeder gilts are added to the GDU for acclimation. Typically, gilts will continue to flow into the sow farm at their designated age until the acclimated GDU is empty. PRRS negative gilts are then staged at another location and enter the sow farm according to the farm’s typical monthly schedule after the last of the acclimated gilts have entered. Usually, when the first negative gilts enter the farm, the last positive PRRSV result from the sow farm occurred approximately five months earlier.
This approach has allowed the sow farm to make sound breeding decisions and maintain normal culling practices, thereby preserving parity structure throughout the PRRS elimination process. It also removes the need for a PRRS negative breeding project on the backside of a PRRS elimination.
Discussion
The option to continuously expose gilts for a period of time may not be possible for all sow farms. There are a few requirements for this program to be successful:
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An off-site GDU location in a good area with solid biosecurity that isn’t likely to be infected with another PRRSV variant or other pathogens.
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Strict PRRSV exposure and management protocols must be followed at the GDU.
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Timely and thorough testing and PRRSV sequencing of the GDU site to protect the sow farm.
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The recipient sow farm undergoing elimination should have an outbreak history that provides adequate time to receive all acclimated gilts into the farm prior to the next outbreak (14 months or more with no outbreaks would be desired).
References
Holtkamp D, and Osemeke O. “PRRS Now Costs Pork Producers $1.2 Billion per Year.” 2024. National Hog Farmer, July 30, 2024. https://www.nationalhogfarmer.com/livestock-management/prrs-now-costs-pork-producers-1-2-billion-per-year.
Linhares DCL. PRRS Outbreak Management Program (POMP) database. 2025. Accessed February 6, 2025.