PRRS 2019-2021 Exponentially Weighted Moving Average by State

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, the MSHMP team shares the 2019-2021 PRRS incidence by state and how it influenced exponentially weighted moving average (EWMA).

Reminder: What is Exponentially Weighted Moving Average?


  • We continue to observe different EWMA patterns across states.
  • Iowa, Minnesota and Nebraska are undergoing an epidemic of larger magnitude and duration when compared to the previous year.
  • Even though fall and winter continue to be the seasons with overall higher incidence, sporadic cases in warmer months were observed throughout the states.

We continue to analyze the MSHMP 2019-2021 PRRS incidence seasons through the summary of outbreaks and EWMA pattern comparison for individual states. We specifically looked at the six most important states from an MSHMP sow herd size standpoint. These include: Minnesota, Iowa, North Carolina, Oklahoma, Nebraska, and Illinois. Both the scale of the EWMA and the number of outbreaks is the same across all charts. The red line is the last calculated threshold of the epidemic curve for the national EWMA during 2019-2021 (0.35%) and is included here as a reference.

Iowa: The 2020-2021 PRRS season started in September 2020. The magnitude of this epidemic was slightly higher compared to the same period for the 2019-2020 PRRS season. However, although cases seemed to be decreasing in January 2021, a second peak was detected in February.

North Carolina: The 2020-2021 PRRS season for North Carolina started in December 2020 and declined rapidly in February but with activity again March 2021. The previous PRRS season seemed to have occurred in two epidemics, comprising a smaller epidemic peak between October 2019 and December 2019, followed by a second epidemic that started in February 2020 lasting up to June 2020. This pattern was not observed in the current season.

Minnesota: This season EWMA crossed the epidemic threshold by the end of September 2020. Overall, EWMA was higher in the 2020-2021 season than the 2019-2020 season and represented one of the highest EWMA of the six states showed here. It is still unclear if the new L1C variant was responsible for this trend.

Figure 1: EWMA (blue line) in six states since July 2018. Green dots represent weekly number of PRRS cases and red line the overall epidemic threshold.

Oklahoma: PRRS incidence during the 2020-2021 season has been much lower than in the previous year, similar to the pattern observed in the 2018-2019 season. This suggest that the higher PRRS occurrence in the 2019-2020 season could have been atypical for the region.

Nebraska: Compared to the previous season, the 2020-2021 PRRS season was characterized by an outbreak of a higher magnitude and duration, starting in November 2020. Together with Minnesota, Nebraska also presented one of the highest EWMAs showed here.

Illinois: Also presented fewer cases, although slight increases in EWMA occurred during spring (April-June) 2020. The 2020-2021 season experienced fewer cases, with a slight increase in November 2020.

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