Translating Field PRRS Data into Practical Management

In swine production, safeguarding animal health is crucial, and combating diseases like porcine reproductive and respiratory syndrome (PRRS) virus is a central part of this effort. Vaccination remains the primary defense strategy, but determining the optimal dosage for the day-to-day realities of swine production is complex.

To help determine this, we collected closeout data and health-specific information from nursery and grow-finish sites in the Midwest that had experienced lateral PRRS virus breaks over a three-year period. Next, we used a visual mapping technique known as Directed Acyclic Graph (DAG) to help us see how various factors like the vaccine dose, vaccination type, timing of vaccination, other diseases and farm characteristics link to the health outcomes we cared about, such as mortality rates, cull rates, grade A and medicine costs. By laying out these connections, the DAG helped us spot the relationships between the variables, potential confounders and treatment outcome pathways and identify biases that affect our results.

With the insights from the DAG, we moved to perform Propensity Score Matching (PSM) for both the nursery and grow-finish datasets. This is a statistical causal inference technique that helped us balance groups on confounding factors to make them comparable except for the vaccine dose they received.

After we matched pigs based on their propensity scores, we applied linear mixed-effects models to assess differences between vaccination strategies (full vs. half-dose), incorporating the weights from the PSM and considering factors such as season, year, vaccine type, vaccination timing, farm status, other diseases and PRRS virus strain, with a random intercept being the flow. To see how the vaccine dose was related to pig health outcomes such as mortality, culling, grade-A pigs and medicine costs.

Advantage Goes to Full-Dose Vaccinated Pigs
Our analysis suggests that full-dose PRRS virus vaccination is more effective than half-dose strategies in reducing mortality rates in grow-finish herds with up to 3.69% improvement in mortality. This observation that full-dose vaccinated pigs often showed more favorable health outcomes than their half-dosed counterparts was a critical qualitative insight that mirrored our quantitative findings. The consistent efficacy of full-dose vaccines across different regions and in the presence of other diseases further reinforces this recommendation.

While our study provides robust insights, it is not without limitations. The observational nature of the study means that there might be unmeasured variables that could affect the outcomes. Although we used propensity score matching and linear mixed effects models to minimize biases, there is still a possibility of unaccounted factors influencing the results.

Further research could delve into questions such as:

• Is the improvement gained from full dose versus half dose of specific break rates offset by the overall cost of the vaccine program?

• Can we refine antibiotic strategies in the face of a PRRSV break that is more effective and more responsible (less usage) based on perceived expectations of specific PRRS virus strains?

Investigating the economic implications of these strategies, including cost-benefit analyses, to further evaluate the return on investment around full dosing every lot in a region versus the break rate.

By integrating field data with advanced statistical methods, we have provided a framework to guide more effective and efficient disease management practices in swine production. This research contributes to the ongoing efforts to enhance animal health and production outcomes, paving the way for more informed and strategic approaches to PRRSV management in the swine industry.