Objective 1 was to document, describe and study patterns of PRRS in the US sow herd. This project showed a highly repeatable pattern of infections was observed between 2009 and 2012. In each of the first four years of this study, the number of new PRRS infections was very low during the summer months, and then rapidly increased during the fall and winter. Using a statistical method to monitor these infections, we have documented the onset of the annual PRRS epidemic in the middle week of October. Additionally, we were able to show clustering of reported new PRRS infections in Northern/Northwestern Iowa. The results of this objective have been published in National Hog Farmer, presented at several scientific presentations, and are due to be published in peer review literature in January 2015.
Strikingly, the pattern of PRRS infections was very different in 2013. This year, the onset of the annual PRRS epidemic was delayed 2-3 weeks. Additionally, significantly fewer new PRRS infections were reported overall. Interestingly, the clustering of new infections remained in the same area as previously documented. The results of this year of the project are being reported separately in peer reviewed literature, discussed at scientific presentations and reported in National Hog Farmer. It is suspected that the introduction of porcine epidemic diarrhea virus into the US sow herd in late 2012, early 2013 might play a role in this pattern and studies are being conducted to analyze patterns of herds infected with one, both or neither of these diseases.
Taken together, this objective has documented a highly repeatable pattern in PRRS infections. Knowing that the PRRS virus epidemic is looming, has motivated swine producers to increase biosecurity practices and prepare thoroughly before the fall epidemic.
Objective 2 was to conduct a risk assessment based on density of swine farms around a sow herd and the odds of reporting a new infection. We utilized data from the Production Animal Disease Risk Assessment Program (PADRAP) which has a count of farms at various distances from the sow farm completing the survey. These data are then used to compute the odds of reporting a new infection each year. In this study we found that as number of farms increased, so did the odds of reporting a new PRRS infection. There are, however, some important limitations of this study. First, it is extremely difficult to obtain PRRS status of neighboring farms. Additionally, it is difficult to know for how long these farms would have active infection that could generate a plume of aerosol virus. Furthermore, there is some data to suggest that different strains of PRRS are more or less likely to transmit by air, and we do not have strain data in this study. Finally, some of the PADRAP data used in this study is more than 5 years old. We have concern that the number of neighboring swine farms may have changed since the data was collected.
Overall, this objective quantified the additional risk added to sow farms that have several other swine farms around them as identified by the PADRAP score. It may help larger producers with many sow farms direct limited time and money resources to herds at highest risk. Additionally, it may help spur farms in these regions to maintain a highly vigilant bio-security program. The results of this study are part of a larger, ongoing study that is identifying factors that explain the repeatable patterns of PRRS infections. As such, it will be published in peer review literature and presented at scientific conferences.