Crafting a system-specific PRRS outbreak management program

Facilities, logistics, business structures and pig flow within each swine production system must be considered.

March 5, 2024

5 Min Read
The Maschhoffs

By Ana Paula Poeta Silva, Christopher Rademacher, Gustavo Silva, Giovani Trevisan and Daniel Linhares, Iowa State University; and Amy Maschhoff, The Maschhoffs

Outbreaks due to porcine reproductive and respiratory syndrome virus limit the profitability of the production site while disrupting pig well-being. Overall, field veterinarians and practitioners will use different strategies and health interventions to stabilize the infection in the sow herd as soon as the virus is identified.

A PRRSV outbreak management program succeeds when the affected herd quickly recovers its baseline production indices. Still, the facilities, logistics, business structures and pig flow within each unique swine production system must be considered for an efficient outbreak control program. In this scenario, collecting and standardizing outbreak-related data has become essential to understanding the performance of PRRSV outbreak management programs across different systems.

The Maschhoffs system has implemented an ongoing data collection and standardization process focusing on three critical components of a PRRSV outbreak management program:

Diagnostic data

  • PCR testing: PRRSV statuses in the herd are defined using polymerase chain reaction testing of weekly collected processing fluids. PRRSV statuses during the outbreak follow the Association of Swine Veterinarians guidelines of PRRSV breeding herd classification.1

  • Sequencing: Whole genome sequencing to identify PRRSV RFLPs and Lineages from the strain associated with the outbreak (PRRSV wild-type exposure).

Production data

Sow inventory and number of weaned pigs per week are collected for 21 weeks before and 52 weeks after the PRRSV outbreak. This data is used to calculate metrics that are implemented in the PRRSV outbreak management program offered by Iowa State University Field Epidemiology Team. The metrics are standardized and comparable across different breeding herds and swine production systems for benchmarking purposes:

  • Time-to-Stability refers to the number of weeks required to consistently wean negative pigs.

  • Time-to-Baseline-Production refers to the number of weeks needed to achieve the baseline of total weekly pig wean.

  • Total Losses per 1,000 sows represents the number of pigs that the farm did not wean during the course of the outbreak.

Practices and interventions

Detailed information is gathered and uploaded to master Excel spreadsheet regarding the diagnostic data, production data, alongside with practices and interventions implemented during the outbreak period, such as herd PRRSV status at the PRRSV outbreak event, response plans (elimination vs. control), herd closure, dates between herd closures and no gilt entry, immunologic solutions, concurrent porcine epidemic diarrhea outbreaks, depopulations, etc.

Up to date, the dataset included standardized information on 177 PRRSV outbreaks occurring between October 2010 and December 2023. Different epidemiologic metrics (incidence rate, e.g., number of new outbreaks within a year divided by the number of breeding herds at risk) and statistical analyses can be done with the dataset. For instance, as preliminary data analyses, univariate models including mixed-effect Cox regressions using site as random effect and controlling for the year of the outbreak are used to associate practices and interventions with faster TTS and TTBP. Univariate mixed-effect zero-inflated Poisson regressions are used to associate practices and interventions with lower total losses.

Across the 13 years of PRRSV outbreaks, the highest incidence rate was in 2013 and 2017, with 25 and 22 new PRRSV outbreaks. Of those, approximately 50% occurred in herds that were negative at the moment of the PRRSV wild-type exposure. A descending incidence trend was observed in 2019 and 2020 followed by an increase in the incidence in 2021 and 2022.

Figure 1 shows key events related to PRRSV outbreaks, and the PRRSV AASV statuses of 10 sites located in the same region where the system manages farms. Four sites classified as negative changed to positive stable with vaccination after two consecutive outbreaks. Further, it was observed that PRRSV RFLPs clustered by region and time in the production system, and that consecutive outbreaks occurred most frequent due to different PRRSV RFLPs.


Preliminary analyses using the last six years of outbreak data showed that:

  • Sites that were negative at the moment of PRRSV wild-type exposure were associated with 3.5 times higher total losses compared to positive unstable or stable with vaccination sites (p-value < 0.0001). 

  • Sites that did whole-herd closure were associated with 2x faster TTS or an average of 10 weeks shorter TTS (p-value<0.0001).

  • Sites that performed immediate closure after the PRRSV wild-type exposure identification were associated with the lowest total losses (p-value<0.0001). That is, herds that closed the herd (no gilt entry) between five days to four weeks were associated with 182% higher total losses compared to herd were closed in less than five days post PRRSV wild-type exposure identification.

Although the outbreak-associated PRRSV strain will play a role regarding the PRRSV impact on production indices, the ongoing collected and analyzed data has provided the system with opportunities to make more data-driven decisions when managing future PRRSV outbreaks. Herd closure and immediate closure were practices that showed statistically significant association with faster recovery and lower pig losses. Combining production, diagnostic and health intervention data has been a key step to understanding what The Maschhoffs system does and will do in the future to successfully control PRRSV outbreaks.


1. Holtkamp D et al. Proposed modifications to porcine reproductive and respiratory syndrome virus herd classification. J Swine Health Prod. 2021;29(5):261-270.

This project was funded by the Iowa Pork Producers Association.

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