Sixteen years of Morrison Swine Health Monitoring Project data, running from July 2009 through December 2024, covering 25 production systems across 12 states and roughly 1.6 million sows, produced the largest field assessment of air filtration against PRRS to date. It compared 245 non-filtered against 178 filtered sow farms. Filtered herds ran roughly half the PRRS incidence rate of non-filtered herds, and that held regardless of how the filtration was scheduled or which ventilation pressure the system used. Year-round filtration returned an incidence rate ratio of 0.495 and partial-year 0.502; negative pressure returned 0.492 and positive pressure 0.416, all against non-filtered farms and all statistically significant. Crude incidence was 35.30 outbreaks per 100 farm years for non-filtered herds against 22.75 for year-round filtered.
A halving is a smaller effect than the earlier field literature reported, and the authors address why. Dee and colleagues found odds 7.97 times higher before filtration than after across 38 herds; Alonso and colleagues reported approximately 80 percent risk reduction across 37 herds over seven years; Havas and colleagues found 20 times greater odds without filtration across one management group. This study covers a far broader population over a far longer period, and over sixteen years most farms eventually break, whereas a two-year window captures the immediate difference. More virulent PRRSV-2 variants emerged during the period, and baseline biosecurity on non-filtered farms improved, which narrows filtration's relative advantage without reducing what it does.
Two comparisons invite misreading and should be handled carefully. The positive and negative pressure figures are each measured against non-filtered farms, not against each other; compared directly the difference was inconclusive, and in the most recent period negative pressure carried the lower ratio. Year-round against partial-year was likewise inconclusive, on only 22 partially filtered farms. What the study does establish is that the protective effect strengthened across the sixteen years, which the authors read as filtration technology improving over time. The study was funded by American Air Filter Company, which states it had no role in design or analysis.