PRRS virus dissemination across pig production systems in the United States

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing the summary of a work done at North Carolina Sate University by Dr. Gustavo Machado and his lab on PRRS virus dissemination based on MSHMP data.


Despite improvements in virus characterization, mainly by viral sequencing, it is unclear whether most PRRSV infections are a product of viral circulation within a farm, between farms within a production system (local) or between farms across production systems (external). 


To investigate the dissemination dynamics of PRRSV among pig farms in three production systems. The understanding of PRRSV transmission routes has the potential to inform targeted strategies for its prevention and control, however, further studies are needed to identify the contribution of known local routes of transmission.

Materials and methods

ORF5 PRRSV sequences derived from the PRRSV MSHMP database were obtained from three commercially unrelated systems of sow-to-finish swine farms collected during 2014-2019 in a defined dense pig populated region of the United States. 

To determine the spatiotemporal patterns of PRRSV spread, the impact of different potential predictors on PRRSV spread, as well as to infer the migration rates between internal vs external introductions (between farm systems) we used a Bayesian phylodynamic approach This methodology consider the observed genetic diversities to estimate, for example the volume of PRRSV spread among farms of different production systems.

PRRSV migration events among pig systems are inferred through a structured coalescent model. Number of dissemination events originated in farms from the same pig system compared to events from farms integrated into another pig system. Grey background represents the months where the exponentially weighted moving average epidemic monitor surpassed the established PRRSv epidemic threshold based on the February 2020 MSHMP report (MSHMP, 2020), while dashed lines represent the beginning of each year


  1. PRRSv evidenced high levels of transmission between non-commercially related pig production systems, especially, from November until May when PRRSV tends to peak in the studied region. These results reinforce the importance of farm proximity on PRRSV spread.
  2. Sow farms were found to be the most dominant spreaders of PRRSV, however, significant number of samples collected in sow and nursery farms most likely originated from finisher farms.
  3. Between-farm dissemination becomes easier when farms are located in highly populated pig areas (from 500 to 1000 pig/km2), as well as farms located at a range within 0.5 km and 0.7 km from major roads showed more likelihood to be infect by PRRSV.
  4. On the other hand, PRRSV evidenced more difficulty reaching farms at higher elevation surrounded by dense vegetation, which acted as dissemination barriers.

Conclusions and implications

The understanding of PRRSV transmission routes has the potential to inform targeted strategies for its prevention and control, where reinforcement in biosecurity considering farm proximity as a relevant route for virus dissemination may be the best tool to prevent PRRSV spread. 

For more details please read the full paper here (

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