This review tabulates the PRRSV airborne transmission studies published through 2019 and assesses what they collectively support. The evidence is consistent for transmission up to 9 kilometres. The review also names the gaps that remain, principally the role of meteorological factors and the extent of differences between viral strains.
It is the careful review level citation for the long distance claim, which matters because that claim is frequently repeated with the wrong figure attached. The underlying field results are specific. Dee and colleagues in 2009 recovered infectious PRRSV at 4.7 kilometres, equal to 2.92 miles, in four of 306 samples over 50 days, all at the same northwest sampling point. Otake and colleagues in 2010 recovered infectious PRRSV in five of 114 long distance samples at 2.3, 4.6, 6.6 and 9.1 kilometres, with only one of the three variants present in the source population appearing at distance. Alonso and colleagues in 2013 then showed the epidemiological consequence, an approximately 80 percent reduction in risk of novel introduction with filtration in place.
The knowledge gaps this review names bear directly on how a figure like 9 kilometres should be used. Both long distance recoveries were a small number of positive samples out of many, which is what airborne transmission at that range looks like: infrequent, conditional, and dependent on weather that was not controlled. The distance is therefore a statement about what is possible under some conditions, not about what happens routinely. For siting decisions, it defines the scale over which neighbouring sites are relevant rather than a radius within which infection is expected.