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Building a better understanding of biosecurity and PRRS risks

At the center of any biosecurity program is the goal of preventing pathogens or diseases from entering and infecting a swine herd. Perhaps no disease has challenged producers and veterinarians as dramatically as porcine respiratory and reproductive syndrome (PRRS).

“The swine industry has committed significant resources to preventing PRRS. Yet, 20% to 30% of the breeding herds, according to the Morrison Swine Health Monitoring Project, report a new PRRS outbreak annually,” said Kimberlee Baker, second-year veterinary student at Iowa State University. “This tells me we still have room to improve regarding PRRS transmission between herds.”

So, what differentiates breeding herds with low-PRRS incidence from those with high-PRRS incidence? Baker conducted a study to learn more about the risk factors and which areas could offer greater results.1

Defining low- and high-PRRS herds

Baker focused on breeding herds and invited US production systems to enroll a “herd set” in her study. A herd set was defined as three breeding herds in the 25th percentile of PRRS outbreaks (low-PRRS incidence) and three breeding herds exceeding the 75th percentile (high-PRRS incidence) within a production system. A US production system could enroll breeding herds that met the following criteria:

  1. Weekly PRRS status recorded from January 2013 and May 2017.
  2. Willing to complete a detailed biosecurity survey.
  3. The site was limited to breed-to-wean production.

Baker’s study enrolled 14 herd sets/production systems for a total of 84 herds, representing 13 states. Average herd size for the low-incidence group was 3,453 sows, ranging from 543 to 7,200 head. The high-incidence group averaged 4,099 sows, with a range of 1,000 to 10,852 head.

The biosecurity survey involved 346 questions, including specifics on herd demographics, area swine density, PRRS-outbreak history, swine-transport biosecurity practices, people movement, mortality disposal, supply deliveries and various other risk events.

In all, 20 biosecurity-risk events were evaluated. From there, Baker ran a statistical model to determine how biosecurity practices differed between the low- and high-PRRS-incidence herds.

16 PRRS risk factors

“This revealed 16 risk factors, which fell into five general biosecurity categories,” Baker noted. These included: 1) frequency of events per month, 2) characteristics of herd and premises, 3) swine density, 4) employee turnover, 5) operational connections to other swine sites.

“If people take away nothing else from this study, it needs to be that a high frequency of events translates to a huge vulnerability for PRRS,” she added.

More specifically, five of the risk factors that separated the high-PRRS-incidence farms from the low-incidence farms in her study were the increased monthly frequencies of rendering, cull-sow removal, transferring supplies from another swine farm, visitor entry and bringing food onto the site.

Regarding the “food event,” Baker clarified, “I believe it’s a variable that’s related to the number of employees and herd size.”

Rendering = increased PRRS risk

Mortality disposal via rendering was the most significant difference between the high- and low-PRRS groups. Of the high-PRRS-incidence herds, 64.3% used rendering versus 31% of the low-incidence herds, Baker reported. Rendering’s risk factor is largely due to the fact that it increases the traffic flow on to the farm.

Also, the high-incidence herds used rendering 12.7 times per month on average, compared to 5.7 for low-incidence herds. More of the low-incidence herds used composting (52.4%) and incineration (16.6%) for mortality disposal than the high-PRRS herds where only 33.3% used composting and 2.4% used incineration.


Type of mortality disposal


Similarly, high-PRRS herds had more visitors (1 to 24) per month than the low-PRRS herds (0.5 to 8).

In contrast to that trend, Baker found that more frequent weaning was associated with reduced PRRS risk. “I thought that was a little strange,” she said. “As I looked into the low-incidence herds, while they did tend to wean pigs more often, those farms had very good weaned-pig biosecurity practices that made it a lower-risk event.”

Downtime and density

Downtime requirements played a role as low-PRRS-incidence herds had significantly longer downtimes (average 25.4 hours) for manure applicators than did high-PRRS herds (average 14.2 hours).

High-PRRS-incidence herds were located in areas with higher densities of show pigs and gilt-development units than low-PRRS-incidence herds.

“Interestingly, higher numbers of boars within a 3-mile radius were associated with low-PRRS-incidence herds,” Baker pointed out. “This may be due, in part, to the high level of biosecurity that boar studs practice.”

Operational connections to other swine sites, especially other PRRS-positive sites — such as sharing supplies, equipment or personnel — were related to high PRRS incidence. For example, high-incidence herds used trailers to deliver weaned pigs to and from more sites than the low-incidence herds.

Additionally, a greater percentage of high-PRRS-incidence herds (64%) also hauled PRRS-positive pigs on their weaned-pig trailers than the low-PRRS herds (31%). “So, this may offer another opportunity to reduce PRRS-exposure risk,” Baker said.

Three characteristics of high-PRRS herds

Three herd characteristics also were identified with high-PRRS herds: 1) larger herd size, 2) higher number of entrances onto the site and 3) on-site gilt isolation, especially when gilts remain in isolation for an extended number of days.

Higher on-farm employee turnover increased the odds of a herd being in the high-PRRS-incidence category. “So, if we can find ways to improve on-farm employee retention, we may be able to reduce our PRRS risk,” she added.


Annual employee turnover


Finally, the survey illustrated that a breeding herd’s vulnerability to PRRS is situation dependent.

“The biggest lesson I learned through this experience is that there is not, and never will be, a one-size-fits-all biosecurity program for PRRS,” Baker concluded. “Doing a thorough risk assessment to analyze vulnerabilities will be key to the future control of PRRS outbreaks.”


1 Baker K, et al. Comparison of biosecurity aspects between swine breeding herds with low and high incidence of porcine reproductive and respiratory syndrome virus outbreaks. Student Seminar, 49th American Association of Swine Veterinarians’ Annual Meeting. 2018;80.



Posted on August 1, 2018

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