The PRRS virus continues to circulate within the U.S. swine population. Recent estimates put the cost to the swine industry at $664 million per year — a whopping $1.8 million per day. Broken down further, the cost across all U.S. sow herds amounts to $114.71 per sow or roughly $6.00 per pig in lost opportunity and cost.
Many techniques have been used to successfully eliminate this virus from herds. The majority of these elimination programs have been done in farrow-to-wean facilities, where it is much easier to go through an elimination program compared to farrow-to-feeder or farrow-to-finish sites.
This article focuses on farrow-to-wean sites, where the biggest challenge is to keep the virus from reentering the population once it has been eliminated. This continues to be a major hurdle that requires good biosecurity practices. For some farms in pig-dense areas, this means using air filtration technology to increase the level of biosecurity and reduce the odds of the sow farm breaking with a new porcine reproductive and respiratory syndrome (PRRS) virus.
There are many different methods that can be used to successfully eliminate PRRS virus from farrow-to-wean farms. Two basic strategies are commonly used — complete depopulation and repopulation (CDR) or herd closure and rollover of inventory (HCR). Each farm needs a herd- and site-specific program.
A complete depopulation and repopulation is traditionally used. The site is completely emptied of all inventory, washed and disinfected, then allowed to sit empty for a minimum of two weeks to ensure complete drying. The site can then be repopulated and breeding resumed. An option is to set up an off-site breeding project so that groups of bred females can be reintroduced into cleaned units to minimize production downtime. When farrow-to-wean facilities are repopulated,
farrowing begins relatively quickly.
In recent times, a more commonly used method of PRRS elimination on a sow farm is a herd closure using one of these four options:
1. Simply close the herd and allow no additional animals into the herd;
2. If available, utilize an on-site gilt developer unit (GDU) to hold extra inventory;
3. Utilize an off-site GDU to hold extra inventory for the closure; and
4. Create an off-site breeding
Before the herd is closed, the immunity of each animal within that herd must be homogenized (equalized). Planned exposure of virus to every animal increases the probability that each animal can mount an immune response. This can be done a number of different ways:
• Natural exposure from infected animals in the herd;
• Direct-virus delivery with the farm-specific virus that stays on site to avoid any potential contamination with other viruses, which should be done with veterinary supervision; and
With natural exposure, it is difficult to know if all animals are exposed, but it can be done with careful monitoring of animals and clinical signs. By definition, the herd is closed and will not be allowed to add animals for a minimum period of time.
Successful closure is indicated by three consecutive months of PRRS-negative, polymerase chain reaction (PCR) piglet testing and a specific amount of downtime. With the herd closure protocol, the length of time may vary from a minimum of 200 days up to about 240 days. Consult with your herd veterinarian to determine the right time for your herd and how to monitor it to make sure the plan is successful.
Closures of less than 200 days have not been successful. The success of the project depends on how the virus is acting within the herd, which is specific to each particular site and project.
By definition, the start date for the project begins when all sows have been exposed and immunity homogenized. After this date, no new animals are introduced into the population. Sow parity average gets older and the inventory erodes to try to maintain as much production in that time frame as possible.
In preparation for the herd closure, as many age groups of gilts as possible can be brought onto the site. Then, the herd is closed to any new introductions of animals and the homogenization of the herd proceeds in order to achieve immunity across the board.
A general herd cleanup list that can be used for any of the cleanup procedures described can be found below.
Pros & Cons of the Options
Following are the options and limitations of each type of herd closure program:
• Closure with no additional inventory to the site: The simplest method is just to close the herd and not make any additions to the inventory. The risk is not being able to achieve breeding targets, being forced to breed less productive animals and an aging parity structure, post-closure.
• On-site gilt development unit (GDU): An on-site GDU providing 200+ days of inventory allows for more normal breeding and culling decisions. An off-site location will be needed to maintain a PRRS-negative gilt pool flow once herd elimination is completed.
• Off-site GDU: Starting an off-site GDU allows for increased inventory during herd closure so that culling can continue as normal and inventory and parity structure can be maintained. These projects carry the risk that a new PRRS strain can come into the off-site location and not the sow farm. Testing is difficult since these off-site animals have had their immunity homogenized similar to the sow herd.
• Off-site GDU with a breeding project: To calculate when to start breeding off-site, you have to work backward from the date that negative gilts can come back into the herd and subtract the gestation length (e.g., 114 days) and how many days prior to farrowing you want to bring them back to the herd.
The goal is to maintain the breeding inventory each week by breeding negative gilts at the breeding project site to move back to the herd at the end of the closure.
This approach still has the risk of exposure to a new virus strain at the off-site breeding project. With negative animals on site, they can be easily monitored to make sure no new virus has been introduced.
Managing the breeding project can be difficult — providing adequate labor for heat checking, breeding, semen deliveries, etc. Once breeding begins, the clock is ticking. If something goes wrong at the farm closure site, then another plan to farrow the gilts elsewhere has to be made.
Producers often choose one of the four options or variations that combine parts of each.
The depopulation-repopulation approach is often chosen when there is more than one pathogen that needs to be eliminated from the herd. It gives producers an opportunity to remove several different pathogens at the same time.
Other reasons herds choose depopulation-repopulation is their parity structure is not where they want it to be or they want to change genetics. Depopulation-repopulation affords the ability to accomplish several goals at one time, provides the fastest return on the investment and minimizes cash outlay for maximum production gain.
A simple spreadsheet can be used to determine the cost of the PRRS outbreak, the impact it will have on herd health status, the cost of eliminating the virus and the potential outcomes of virus elimination programs. If exact input numbers are not available, estimates can be used to predict outcomes. Some assumptions will need to be made for first-time outbreaks.
Once inputs are in place, one can see the cost and opportunity for the herd through various cleanup scenarios. Table 1 details the factors that comprise the basic inputs needed for the model. A 1,000-sow herd is used as an example so it is easy to multiply the numbers to fit other herds.
In this example, we will use the production numbers from the publication, “An Economic Evaluation of PRRS Elimination in the United States Swine Herd,” compiled by Iowa State University’s Derald Holtkamp, DVM, since this is a large data pool and has just recently been compiled. An analysis of the study can be accessed at http://nationalhogfarmer.com/health-diseases/new-prrs-study-impact-0920  or in print in “PRRS Study: Impact Remains High,” September 15, 2011 issue of National Hog Farmer.
The outcome from these inputs computes the lost opportunities and additional costs for a herd that is PRRS negative and a herd that is continuously positive.
The total cost to a 1,000-sow herd, shown in Table 2, is $299,319 ($299.32/sow or $12.74/pig). The majority of the loss comes from lost pigs (farrowing rate, total born, stillborns, mummies and preweaning mortality) and additional cost of vaccinations and medications, resulting in 52% of the cost or lost opportunity.
Interventions include the use of complete depopulation and repopulation (CDR). Table 3 details the cost and rate of return for this procedure.
To return the cost of the CDR, the herd must stay negative for a minimum of 13.7 months if an off-site breeding project is done and 19.4 months if there is no off-site breeding project. The history of the farm staying negative must be considered before making this decision. Other benefits that may be achieved can be added into the model before it is rerun.
CDR will have more genetic costs because more animals are replaced than normal. If there are significant changes introduced, such as filtering, it can also alter the outcome. This model shows one year’s time frame. It can be extended to include multiple years for a look at the investment over a longer time horizon.
The other option for intervention is herd closure (HCR), detailed in Table 4. HCR is the easiest to understand, but it is also the most expensive cleanup procedure due to the loss of inventory, aging parity and the impact on productivity of having older parity animals in the herd until it can be rolled over. The cost of gilts will be included anyway, so it doesn’t impact the cost of the project.
Other factors to include for the off-site GDU programs are costs of transportation, health monitoring and rental management of the site tied up for the breeding project while the herd closure is in place. These are relatively minor factors. The cost for this intervention is reasonable, and the herd only has to stay negative for 1.7-3.4 months to return the investment. That is why this procedure has become so popular.
One Size Doesn’t Fit All
There are two primary ways we use to eliminate PRRS virus from a herd. Each farm will have to evaluate its unique conditions and decide what will work the best for them. There is no one-size-fits-all program. The industry knows the cost of PRRS is $1.8 million per day. It is important to understand one’s own costs.
Remember, the spreadsheets are just estimations of the cost and are only as accurate as the assumptions used. As Missouri swine consultant Dennis DiPietre points out, a spreadsheet can be precisely inaccurate. It is a good practice to compare estimates with the numbers from your accounting system or with someone you know and trust.
Using herd closure rollover has been very successful and the cost is relatively low, normally requiring just 2-4 months of negative production to return the cost.
The various numbers generated by the models will help determine if there is an opportunity to remove the virus from your herd and the time required to return the cost. Any additional time the herd stays negative is an additional benefit and a return on investment. It is also good to understand the opportunity cost of a PRRS infection and what can be done to reduce those costs.
Every project will carry some risk of failure, so each person must decide what risks they are comfortable with.