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, the MSHMP team updates a valuable statistical parameter: the PRRS incidence in status 4 status 4 sow farms. As a reminder, status 4 farms are defined as negative for PRRS in both exposure and shedding status.

Key Points

  • In the last 13 years, on average 9.3% (Range 3.2% – 20.3%) of status 4 farms have had a PRRS outbreak, with an apparent cyclical peak at around 10% every 2-3 years.
  • The cumulative incidence of the current MSHMP year (July 2020 to May 2021) for status 4 farms is 9.9%.
  • PRRS incidence in status 4 farms during the current MSHMP season is not higher than the ones observed in the previous MSHMP seasons.
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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?

Keypoints

  • 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.
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Incidence Risk and Incidence Rate: an Overview

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, Dr. Juan Sanhueza helps us understand better two epidemiologic concepts: the incidence risk and incidence rate.

Key Points

  • Incidence risk is a measure of disease occurrence over a defined period of time. It is a proportion, therefore takes values from 0 to 1 (0% to 100%)
  • Incidence rate takes into account the time an individual is at risk of disease. It is not a proportion since it defines the number of cases per animal or farm time at risk.
  • Incidence risk and incidence rate are often confused. Incidence risk and rate are numerically the same when the period at risk does not vary across individuals being studied.
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Review of Chart 3: PRRS incidence rate by status at break

Key Points

  • Incidence rate takes into account the time a farm is at risk for disease.
  • Chart 3 incidence rate has replaced incidence risk by status which did not account for a farm’s contributed time at risk.
  • While incidence rate is less visually intuitive, it more accurately reflects PRRS outbreak rate by status.
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Yearly incidence rate of PRRS per MSHMP Participating Production System

This week, the MSHMP team is sharing the yearly PRRS incidence rates in the 39 participating systems.

Keypoints

  • The average incidence rate for all systems during the entire period described was 0.27 PRRS outbreaks per farm per year.
  • The incidence rate of PRRS outbreaks within a system varies across years with some participants continuing to have lower incidences.
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