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.
Today the MSHMP team shares the most recent EWMA chart of weekly PRRSV incidence and helps readers understand what it all means.
The incidence exponentially weighted moving average (EWMA) chart shows a smoothing of our weekly PRRSV incidence with higher weights attributed to data that are more recent. Smoothing incidence allows us to visualize overall patterns leaving irregular noises out. Additionally, we also calculate the epidemic threshold, which corresponds to the upper control limit of the EWMA which is recalculated to ensure that it is relevant to the current period. Whenever the EWMA crosses the epidemic threshold, it means there are an excess number of new cases, which is then considered as an epidemic period (Figure 1). Here, we describe the duration of these epidemic periods as well as the time between epidemics.
We found that the duration of the epidemic periods that occurred between July 2009 and June 2014 ranged from 16 to 31 weeks, with a median of 20 weeks and an interquartile range of 18 to 21 weeks (Figure 2A). On the other hand, the epidemic periods that occurred from July 2014 to June 2019 ranged from 26 to 36 weeks, with a median of 34 weeks and an interquartile range of 31-36. Lastly, epidemic periods between July 2019 and June 2024 ranged from 14 to 38 weeks, with a median of 34.5 weeks and an interquartile range of 18-35. The duration of the epidemic periods between July 2009 and June 2014 was statistically different from the duration of epidemic periods from July 2014 to June 2019 (Kruskal-Wallis p=0.02). The duration of PRRS epidemics in the last five years were not statistically different from that of the previous decade. Similarly, Figure 2B shows that the time between the end and beginning of the new epidemic ranged from 18 to 37 weeks between July 2009 and June 2014 (median 30, interquartile range 19-33), from 15 to 29 weeks during July 2014 to June 2019 (median 17, interquartile range 16-23), and from 9 to 19 weeks from July 2019 to June 2024 (median 16, interquartile range 10-17). The time between epidemics from July 2009 to June 2014 was statistically different from the time between epidemics during July 2019 and June 2024 (Kruskal-Wallis p=0.02).
We continue to observe a wide range of duration in epidemic periods in the past five years. However, the interval between epidemics is now shorter than in 2009-2014. This could be partially due to spring epidemics (some associated with the L1C144 variant), as well as the higher number of epidemics in the past five years compared to other periods. An important caveat of this analysis is that the population of sow farms has been changing overtime as new systems continue to join MSHMP. However, this highlights that even though PRRS incidence remains amongst the lowest recorded, the industry’s PRRS burden continues to be high, as illustrated not only by this analysis but also by the continuously high prevalence throughout recent years.