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Close to the finish: How grower pigs spread PRRS to sow farms

It’s time to consider the role finishing pigs play in the spread of porcine reproductive and respiratory syndrome virus (PRRSV) to sow farms, according to Montserrat Torremorell, DVM, PhD, University of Minnesota.

Most PRRSV-control efforts have focused on sow herds. As examples, Torremorell cites gilt acclimatization, PRRSV stabilization and herd closure — all aimed at protecting the sow herd and producing a PRRSV-negative pig.

Likewise, she says, biosecurity investments have focused on protecting sows. Examples include air filtration; disinfection of supplies; showering in; bench-entry systems; cleaning, disinfection and drying of weaned trailers; as well as downtime, dedicated personnel and dedicated transport.

Despite these efforts, various studies indicate the annual incidence of PRRSV hasn’t dropped significantly and is still found in 20% to 30% of US sow herds (Figure 1).

The focus on protecting sow herds is a starting point, but to further improve PRRSV control, there needs to be a focus on PRRSV in finisher pigs, Torremorell says.

Currently, there’s little effort to prevent infections in finishing sites, yet there is evidence that PRRSV is a growing problem in finishers and that these animals pose a risk for infection to sow farms, she says.

PRRSV in growers

In one study, 38% of finishing-pig groups (243/639) negative for PRRSV at weaning were positive at marketing; 39% (247/639) of groups were positive at both weaning and marketing.1 In another study, 26% (31/120) of groups negative at weaning were positive at marketing.2

Annual losses due to PRRSV based on records from 2005 to 2010 were $664 million, and 55% of that amount was attributed to losses in growing herds.

Data from 2016 indicates that losses due to PRRSV were $581 million (adjusted for changes in prices and the national herd size), and the percentage attributed to losses in growers had risen to 62%,3 Torremorell says.

The prevalence of PRRSV in finishers varies widely by region and may be as high as 62% to 84% based on information Torremorell obtained from colleagues at the Ohio State University.4 “So, there’s a lot of PRRSV in the finishing population,” she says.

Spread to sow farms

In addition, emerging data from investigators at the University of Minnesota5 indicate that when pigs are moved to finishing sites near sow farms, they may introduce pathogens to sows through area spread. In other words, when pigs positive for PRRSV move into a farm “neighborhood,” there’s an increased risk that neighboring sow farms will become infected with the virus, Torremorell says, noting that the median number of pigs moving weekly into an area can range from under 2,000 to tens of thousands.

Unfortunately, not much is known about risk factors for PRRSV infection in finishing pigs, she continued, but her personal experience indicates biosecurity is sorely lacking on many finishing farms.

“I don’t go to any sow farms in the US where I don’t have to shower. I can’t say the same for most of the grow-finish sites I visit,” she continues. “We don’t even try to do some of the basic things on finishing farms…and don’t really monitor them.”

The failure to aggressively prevent PRRSV on finisher sites could be due in part to mindset. Producers may figure that if they’ll be emptying the site in the weeks ahead, it doesn’t matter if the animals get infected. “But if you start thinking in terms of implications for the neighborhood, then that assumption isn’t very good,” Torremorell observes.

Effect of marketing pigs

The veterinarian also addressed marketing strategies that may contribute to PRRSV infection. Anecdotal evidence indicates the virus in previously negative pigs is associated with marketing the first load of pigs. Infection with PRRSV could be linked to contamination of trucks, trailers and drivers, to loading procedures or perhaps to collection of cull animals at multiple sites.

Other marketing practices that appear to be linked to PRRSV infection in finishers include failure to wash the facilities between groups of pigs and poor employee retraining about biosecurity procedures, Torremorell says.

Because information about PRRSV in finishers is limited, the University of Minnesota has planned a study. It will focus on determining the PRRSV incidence among pigs in medium-density areas.

The study will also evaluate risk factors associated with PRRSV infection in growers and the production and economic impact of the disease, she says.

“My assumption is that if we can control the risk of infection in finishers, we could potentially lower the risk of infection on sow farms,” an approach that could bring PRRSV control to the next level, Torremorell concludes.


1. Holtkamp DJ, Kliebenstein JB, Neumann EJ, et al. Assessment of the economic impact of porcine reproductive and respiratory syndrome virus on United States pork producers. J Swine Health Prod, 2013;21(2):72-84
2. Robb C, Holtkamp D, Yeske P, Lower A, Lowe J, Polson D, Lasley P. Evaluation of biosecurity measures and management variables as risk factors for infection of growing pigs that are negative at placement with porcine reproductive and respiratory syndrome virus. Proc Am Assoc Swine Vet. p:75.
3 Dr. Torremorell’s personal communication with Dr. Derald Holtkamp.
4 Dr. Torremorell’s personal communication with Dr. Andreia Arruda.
5 Swine Health Monitoring Project, 2017 Feb 24.

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