NHF: PRRS is also a summer disease

Our latest collaboration with the National Hog Farmer was written by the Morrison Swine Health Monitoring Program team regarding the incidence of PRRS in the summer.

Although our understanding of disease and control methods has improved in recent years, we continue to learn new features of PRRSV epidemiology in part thanks to the Morrison Swine Health Monitoring Project. One of the most recent questions that we have addressed based on enquires from MSHMP participants is whether PRRS incidence during the summer was higher in recent years (i.e. 2016-17) compared to previous years (i.e. 2009-15). We know that PRRSV outbreaks tend to have a seasonal pattern and that they are more frequent during the fall and winter, but we know little about the breaks that happen in the summer and spring.

In order to dig into this question, we analyzed MSHMP data from 2009 to 2017 which included 1,329 outbreaks. Of these, 66% of the breaks occurred during fall and winter and 14% and 20% of the breaks occurred during summer and spring, respectively. Although there were fewer breaks in the spring and summer, the number of breaks in warmer seasons was still significant which represents an on-going frustration to producers because the “PRRSV season” is supposed to be over.

As part of the analysis we learned that between 3% and 6% of the herds break yearly during the summer and spring seasons, respectively. This represents approximately 83 herds out of the 917 reporting in the MSHMP database. If we estimate that the average sow farm has 3,000 sows, then almost a quarter of a million sows break yearly during these two seasons.

Remember, although the risk of PRRSV introduction is lower during the spring and summer, PRRSV breaks still happen, so biosecurity efforts should not be decreased. PRRSV is a sneaky virus so keep your biosecurity up, even in the summer.

All of our collaborations with the National Hog Farmer can be found here.

Science Page: PRRS incidence in status 4 sow farms

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 a report by the MSHMP team on PRRS incidence in status 4 sow farms.

Key points

  • In the last 9 years, on average 10.2% (Range 3.7% – 22%) of status 4 farms have had a PRRS outbreak during the MSHMP season and in the 2017-2018 season, the cumulative incidence (July to April) is 9.6%.
  • The lowest PRRS incidence was observed during the 2013/2014 PRRS season; the year that PED entered the US.
  • PRRS incidence in status 4 farms during the current MSHMP season is not higher than the ones observed in the previous MSHMP seasons.

Reminder: Status 4 sow farms are the farms that considered negative both in shedding and exposure status in the classification document published by the AASV.

Objective

Has there been an increase in PRRS outbreaks incidence in status 4 sow farms?

Method

PRRS incidence in status 4 farms from 2009 to April 2018 was compiled and compared with the current MSHMP year using Fisher’s Exact test.

Results

During the current MSHMP year (July 2017- April 2018), 27 status 4 farms have had a PRRS outbreak (6.9% incidence). The average incidence of status 4 farms from 2009 to April 2018 was 9.6%. However, PRRS incidence have varied greatly among years (figure 1). PRRS incidence had its minimum value during the 2013/2014 MSHMP season with a 3.4%. This coincides with the year that porcine epidemic diarrhea virus (PEDv) entered the US.

When comparing the incidence during the 2017/2018 MSHMP year with the incidence observed during the 2015/2016 MSHMP year, a borderline significant difference (p=0.06) was observed.

PRRS incidence in status 4 sow farms
Figure 1. Percentage of PRRS incidence in Status 4 farms by year (July-April)

Summary

PRRS incidence in status 4 farms (July 2017 –April 2018) was overall similar to previous years, although slightly higher than July 2016-April 2017, and significantly lower than July 2015-April 2016. Other factors, such as region, may be contributing to the
perception of increased PRRS incidence in status 4 farms.Exploring these factors may help explain the perception of increased
incidence.

Science Page: PRRS cumulative incidence by status

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.

How does PRRS incidence vary based on farm status? This is the question answered in this week’s edition of the Science Page. Three different formulas were used to calculate the incidence in each of the group over type. First, the initial number of farms of each status at the beginning of the year was used as the denominator. Then, the denominator was changed to the total number of farms that entered each status since the beginning of the year. Lastly, weekly incidences calculated for each of the group since the beginning of the year were added. Calculations went back for the last 3 years.

Key points from this week edition:

  • Cumulative incidence is higher in those farms that are under status 2, 2vx and 2fvi.
  • The incidence is lower in farms that had recently an outbreak or those that are completely negative.
  • Different ways of calculating incidence by herd status lead to the same overall conclusion.

Take a look at PRRS incidences in farms of group 2 status, vaccinated or inoculated with live virus over the past years.