PRRS: Devil in the Details

PRRS has mutated and evolved to create hundreds of viruses

May 15, 2013

4 Min Read
PRRS: Devil in the Details


Porcine reproductive and respiratory syndrome (PRRS) virus entered the United States in the late 1980s. Through the years, the virus has mutated, evolved and recombined to create hundreds of virus strains today.

The virus has shown great propensity for spread within the industry and for rapid replication in the pig. It has thwarted most attempts at suppressing transmission and infection by vaccination and management procedures.

Technically, we know how to prevent the introduction of this virus into herds. But prevention requires close attention in implementation, and is not always achievable. Dirty trucks, people traffic, recycled lagoon water, improperly acclimated gilts, area spread, crossfostering and many more issues contribute to the recirculation of PRRS virus. 

Case Study No. 1

A 1,200-sow farm brought in gilts to its on-site isolation facility every eight weeks. The gilts were given live virus inoculation (LVI) at arrival and tested to verify seroconversion (PRRS-positive). The animals were retested in seven weeks to ensure they had turned PRRS-negative by polymerase chain reaction (PCR). Rope testing was used to confirm status. Eight ropes were sent to the Iowa State Diagnostic Lab, and one turned up suspect.

Gilts were needed to achieve production targets. The manager wanted to move the gilts into the gestation area since there was only one suspect and no clinical signs. However, it was decided that the suspect case wasn’t normal, and the animals would be retested a week later.

Of the eight rope tests, three were suspects and one was positive for PRRS. The positive was analyzed, and the sequence revealed the farm was infected with a new virus. The gilts were shipped and the isolation building was cleaned, disinfected, dried and disinfected again.

New gilts were left unexposed and monitored. They stayed PRRS-negative for six weeks and then were acclimated to the farm.

Monitoring procedures averted a major catastrophe; the sow farm is continuing to produce PRRS PCR-negative pigs.

Case Study No. 2

A 400-sow, farrow-to-finish, single-site farm had a history of producing PRRS-negative pigs. The farm batch-farrowed and produced its own replacement gilts.

Production exceeded space on the farm, so a neighbor’s vacant site was used as a gilt production unit (GDU). Gilts were segregated and placed on gilt developer diets, then added to the sow farm for breeding.

However, after the first group of gilts was introduced to the sow herd for breeding, the farm experienced a PRRS outbreak. Upon investigation, it was found the gilt site had gone PRRS-positive.

The sow farm owner learned a very expensive lesson: Never add animals to the herd unless their status has been verified. The farm was stabilized with LVI (the process of giving animals material to stimulate immunity) and vaccine, and is now producing PRRS-negative pigs again.

Case Study No. 3

A 200-sow, farrow-to-finish, single-site farm was PRRS-negative. The producer raised his own gilts internally, utilized home-grown corn and hauled his pigs to market. A large, heated machine shed was used to wash his trailer. This was his method of operation for a number of years, and periodic testing showed his herd remained PRRS-negative.

Over time, several wean-to-finish sites were built surrounding his farm. Sources for these pigs were different each “turn.” The client was concerned that these sites, located close to his farm, could infect his sows with PRRS. When the situation was explained, veterinarians for these sites were contacted and they shared information.

Using this information, we developed a sow vaccination plan. Sows turned PRRS-positive, but no clinical signs were seen. It appears that both the batch farrowingsystem and herd vaccination helped prevent a devastating clinical outbreak.


PRRS can be a challenging disease for many farms. There are literally hundreds of PRRS viruses that exist today. Most pig-dense areas have similar viruses that circulate locally. Most common risks can be reduced with a sensible prevention plan.

That plan should be developed with your health adviser with information from your herd and the surrounding area. The plan should include herd monitoring, internal and external biosecurity, isolation and testing of incoming animals, vaccination, filtration and other management procedures.

Owners and farm staff must be trained and committed to performing all of the steps discussed in the PRRS prevention plan for the farm. The veterinarian or health adviser should be instrumental in developing a strategic plan for each farm.     

Subscribe to Our Newsletters
National Hog Farmer is the source for hog production, management and market news

You May Also Like