PRRS infection in boar studs

Veterinarians, diagnosticians and other researchers still are searching for answers to last winter's PRRS outbreaks in many boar studs.

PRRS continues to be a big industry problem as indicated by the disease sessions at the recent University of Minnesota Allen D. Leman Swine Conference.

Because of the outbreak of PRRS in many boar studs last winter, much of this year's Leman discussion was focused on PRRS-virus infection in boars

Joe Connor, swine veterinarian with Carthage Veterinary Services, Ltd., Carthage, Ill., for example, discussed a major survey on risk factors for boar studs becoming infected with PRRS.

Results of the American Association of Swine Veterinarians (AASV) sponsored survey won't be available until next year's AASV annual meeting in Orlando, Fla., says Connor. The University of Minnesota is completing the analysis of the survey, which was completed by 37 veterinarians, according to Connor.

Alarming outbreak
Connor says that last winter's outbreak in "many unrelated studs and wide geographical area is alarming."  That's why it is important to try to identify risk factors and then implement protocols to reduce the risk factors, he says.

"The impact of PRRS infection in studs can be far-reaching," remarks Connor. Depending on the number of herds receiving virus-contaminated semen and other factors, it could take one to three years for an infected stud to eliminate PRRS, he says.


Joe Connor

Prior to the outbreak last year, the majority of boar stud populations had become PRRS- negative through sourcing, depopulation or rollover, explains Connor. "All of the major breeding stock companies have production of PRRS-negative sources, allowing depopulation and/or stocking of new studs with PRRS-negative boars."

Connor says that last winter's sudden PRRS infection of boar stud populations continues to be perplexing. "In the majority of studs, the biosecurity is higher and risk of infection lower than sow populations. Studs are usually well located with consideration of risk factors, such as aerosol transmission, traffic and other factors. Isolation of incoming stock is generally longer.

"Isolation populations are frequently 100 percent sampled rather than statistically sampled. Decisive actions of sacrifice and further testing of serum and tissues of any suspected ‘false positives" are routinely completed."

Further, Connor adds, monthly or more frequent monitoring is maintained within the stud population. "Stud population per facility is low in comparison to a typical sow unit. Routine biosecurity procedures are easier to implement, maintain and monitor because education and dedication of staff with a low number of employees and turnover is the norm."

Studs adapting
 Clark Huinker, a swine practitioner with Fairmont Veterinary 


 Clark Huinker

Clinic, LLP, Fairmont, Minn., discussed at the Leman meeting how boar studs are adapting to the recent PRRS breaks.

Huinker says that PRRS is "the disease of most concern in boar studs" because of the health and economic devastation it can cause, as well as its potential ability to transmit via boar semen.

In his Leman presentation, Huinker described the outbreak of PRRS in one boar stud last January.

Lessons learned
"The biggest lesson learned from the recent PRRS outbreaks revolves around early detection and prevention of transmission," Huinker says.

He says that at the time of the outbreak, PRRS virus PCR diagnostic testing turnaround time was five to six days. Since then, and partly as a result of the PRRS outbreak in boar studs, the laboratory has reduced that time to 24 hours, he says.

"Twenty-four-hour turnaround time has greatly increased the probability of identifying PRRS infections earlier. This is especially true when clinical signs are subtle or not apparent."

Huinker says that 100 percent PCR testing and retention of all doses until PCR results are found negative further decreases the risk of infectious semen doses reaching the farm level and transmitting PRRS virus infection. "Each stud and farm will need to decide for themselves if the risk is clear enough to warrant the extra expense of semen PCR testing."

Although semen PCR testing was not readily available during the first years the boar stud was up and running, the PRRS-vaccinated stud went for several years with no PRRS activity or PCR-positives, according to Huinker.

Huinker speculates that perhaps part of the reason that so many different boar studs across the country experienced PRRS outbreaks last winter was that many of them had just become PRRS-naive during the last several years.

"One could make a good argument either way as to whether or not it is important to have a PRRS-naive stud if every dose of semen is PCR tested and any positive studs are well vaccinated. The answer may be different depending on whether the recipient sow herd is PRRS-positive or naive."

Huinker concludes that testing doesn't prevent infection of a boar stud, "but it may help decrease the risk of transmitting PRRS to recipient sow herds.

"Many studs are re-thinking the aggressiveness of their PRRS testing protocols. Due to the quicker turnaround times available today, more studs are considering at least some semen PCR testing."

Huinker believes that until PRRS is eliminated or more easily controlled, many will choose a more aggressive PRRS testing approach, particularly in boar studs. 

Some PRRS-related terms defined
Kelly M. Lager, DVM, PhD, a veterinary researcher with USDA ARS National Animal Disease Center in Ames, Iowa, provides the following definitions for terms frequently used in discussions related to how PRRS virus might be spread.

Horizontal transmission

Transmission of virus from one pig to another pig requiring close contact between them. In addition, the use of PRRS-virus-contaminated semen collected from virus-positive boars can be included.

Vertical transmission

Transmission of virus from one generation to the next. In the case of PRRS virus transmission from dam to pig.


An object that can harbor a pathogen and serve as an agent of transmission.


A carrier of infectious agent from one host to another host, can be mechanical i.e., just transmitting the agent or biological the vector replicates the agent before transmission to a susceptible host.

Area spread

Recognition of virus infection among swine herds in a selected geographical region at about the same time.

Direct area spread

Transmission of virus among herds in an area related to horizontal or vertical transmission.

Indirect area spread

Transmission of virus among herds in an area not related to direct area spread, assumed transmission owing to fomites, aerosol, or vectors.