Montserrat Torremorell, University of Minnesota, said that porcine reproductive and respiratory syndrome (PRRS) continues to be the most important disease affecting the US swine industry. PRRS virus (PRRSV) causes significant economic losses to US swine producers with most of the cost attributed to losses in the growing period, she said during the 2022 Leman Conference.

Despite the significant losses in the growing pigs, most of the investments in PRRSV control and prevention strategies happen in the breeding herds, she said. This is in part due to the recognition that control in growing pigs starts with weaning a negative pig and thus the need to focus on protecting the sow herd, she added.

However, the sole focus of PRRS control in the sow herd may be holding the industry back in the efforts to control and eradicate PRRSV from specific regions, Torremorell said.

Results from the Morrison Swine Health Monitoring Project indicate that PRRSV incidence has not decreased significantly since the project started more than 10 years ago. PRRSV incidence has ranged between 20% and 40%, and it has been only in the years when the industry has been concerned about the spread of emerging viruses that the incidence has been lower than 20%, she added.

Torremorell said that there is evidence that finishing pigs represent a risk of infecting sow farms with PRRS. An association between PRRSV prevalence in growing pigs and PRRSV detection in sow farms was documented in 2018.

The risk of infections in sow farms increases as infection rates in nearby growing pigs increase, she said.

Pseudorabies and PED

During the pseudorabies outbreaks and subsequent cleanup in the US it was not until intensive vaccination took place in finishing pigs that the rate of infection in sow farms decreased, she said.

This was also observed for porcine epidemic diarrhea (PED); the risk of PEDV infection into sow farms depended in part on the movement of growing pigs into an area around the sow farm, Torremorell explained.

Other local factors such as weather, vegetation and temperature also played a role, but by far pig movements into farms located near a sow farm served as a proxy for PED virus introduction into sow farms, she said.

PRRSV control in growing pigs

There is limited information on what happens to PRRSV in growing pigs, Torremorell explained. There is a lack information on estimates of wild type PRRSV introductions, risk factors that trigger PRRSV infections, biosecurity measures effective at preventing and containing PRRSV in growing pigs and control measures able to maximize productivity while limiting viral spread, she added.

Fortunately, there are research groups working with producers to investigate factors and measures that hopefully will be able to halt or limit virus spread between farms, Torremorell said.

Researchers reported a 44% cumulative incidence rate of PRRSV wild type virus introduction in wean to finish sites located in the Midwest; sites with wild type virus had higher mortality, she noted.

There is an opportunity to improve PRRSV control and biosecurity in grow/finish pigs that will no doubt help limit other diseases as well. Arguably, growing pigs are the Achilles’s tendon of the US swine industry, Torremorell emphasized.

Growing pigs represent the largest population of pigs in the US, close to 70 million, and unfortunately there is limited implementation of biosecurity measures, she said. The challenge and opportunity for the swine industry is to pay attention to PRRSV control and prevention in growing pigs while keeping production costs in check, she added.

The swine industry needs to continue to validate, create and assess measures and strategies that will help contain and prevent PRRSV infections in growing pigs in order to protect the sow farms and all pigs in general, Torremorell concluded.