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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Best of Leman 2019 #3: Three systems views on PRRS decreasing incidence

A few days ago, we shared a report that PRRS incidence decreased in naive breed-to-wean farms. Interesting questions were brought forward by our readers so this month, we are sharing three practitioners’ experiences with PRRS in their systems: Dr. Tara Donova with the Hanor company, Dr. Jay Miller with The Maschhoffs, and Dr. Pete Thomas from Iowa Select Farms.

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Comparison of PRRS incidence rate in status IV breeding herds during 2009-2019

This post is from the MSHMP team looking at status IV farms and their PRRS incidence between the years 2009 and 2019. Quick reminder, in 2011, Holtkamp et al. defined status IV farms as PRRS-negative per their shedding and exposure status.

Key points

  • PRRS incidence rate in status IV farms varied significantly from 2009 to 2019.
  • The incidence rate in status IV farms during 2019 was one of the lowest on record and comparable to the one observed in 2013 and 2014; the years of the PED epidemic in the US.
  • PRRS incidence in status IV farms during 2019 was significantly lower than the one of 2015 and 2016 and marginally non-significant lower (p=0.07) than the one of 2018.
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