Porcine reproductive and respiratory syndrome continues to be the most important disease affecting the U.S swine industry. The PRRS virus causes significant economic losses to U.S. swine producers with most of the cost attributed to losses in the growing period. The latest estimate by Holtkamp et al (2017) attributed 62% of the $581 million USD losses to growing pigs.
Despite the significant losses in growing pigs, most of the investments in PRRSV control and prevention strategies happen in the sow herd. This is in part due to the recognition that control in growing pigs starts with weaning a PRRS-negative pig and thus the need to focus on protecting the sow herd. However, the sole focus of PRRS control in the sow herd may be holding us back in our efforts to control and eradicate PRRSV from regions.
Results from the Morrison Swine Health Monitoring Project indicate that PRRSV incidence has not really decreased significantly since the project started 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%. Both 2013-14 and 2018-19 seasons had lower incidence and coincidentally those were the seasons when porcine epidemic diarrhea virus emerged in the United States and African swine fever in China, which no doubt heightened the industry awareness to disease spread and risk of introduction into U.S. herds.
Evidence is also growing of the risk that finishing pigs represent to the introduction of PRRSV infections into sow farms. An association between PRRSV prevalence in growing pigs and PRRSV detection in sow farms has been documented by Angulo and Yeske (2018). Temporality of PRRSV infection in growing pigs and sow farms has been observed and there is a general understanding that the risk of infections in sow farms increases as infection rates in nearby growing pigs increase.
The same scenario was observed during the pseudorabies clean up in the United States, and it was not until intensive vaccination of PRV took place in finishing pigs that the rate of infection in sow farms decreased. This was observed more recently for PEDV by Machado and collaborators where they described that the risk of PEDV infection into sow farms depended in part on the movement of growing pigs into a neighborhood area around the sow farm. Other local factors such as vegetation, weather and temperature also played a role, but by far pig movements into farms located near a sow farm served as a proxy for virus introduction into sow farms. Thus realistically, in my opinion, we need to advance control of PRRS in growing pigs if we want to significantly advance control in sow farms.
However, we have limited information on what happens to PRRSV in growing pigs. We 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.
Fortunately there are groups at the University of Minnesota and Iowa State University that have started to investigate these factors and measures that hopefully will be able to halt or limit virus spread between farms. Angulo and collaborators reported a 44% cumulative incidence rate of PRRSV wild-type virus introductions in wean-to-finish sites located in the Midwest and sites with wild-type virus had higher mortality also (2019). Thus there is an opportunity to focus our attention to improve PRRSV control and biosecurity in grow-finish pigs that with no doubt will help limit other diseases also.
Arguably growing pigs are the Achilles's tendon of the U.S. swine industry. Growing pigs represent the largest population of pigs in the United States and unfortunately there is limited implementation of biosecurity measures. Thus the challenge and opportunity for the swine industry is to put attention on PRRSV control and prevention in growing pigs while keeping production costs at check. We need to continue to validate, create and assess measures and strategies that will help us contain and prevent PRRSV infections in the growing pigs so that we continue to protect the sow farms. By focusing on improving disease prevention in growing pigs we will not only improve PRRSV control but we will have a more resilient industry capable to prevent the spread of endemic and emerging diseases in the United States.