A source population was established carrying a mixed infection of three PRRSV variants together with Mycoplasma hyopneumoniae strain 232. Five of 114 long distance air samples were PRRSV positive, collected at 2.3, 4.6, 6.6 and 9.1 kilometres from the herd, and all five contained infectious virus, more than 99.2 percent homologous to variant 1-8-4. Six of 114 samples were M. hyopneumoniae positive, all 99.9 percent homologous to strain 232, and three of those, collected at 3.5, 6.8 and 9.2 kilometres, were infectious. In source exhaust air, PRRSV was found in all 21 samples and M. hyopneumoniae in 8 of 21.
Two distances appear in this study and they belong to different organisms. The confirmed outer distance for infectious PRRSV is 9.1 kilometres. The 9.2 kilometre figure is Mycoplasma hyopneumoniae. Secondary material sometimes attaches 9.2 kilometres to PRRSV, and that attribution is wrong. The earlier work by Dee and colleagues in 2009 recovered infectious PRRSV at 4.7 kilometres, and Arruda and colleagues reviewed the whole body of PRRSV aerosol work in 2019, finding the evidence consistent for transmission up to 9 kilometres.
The strain specific result is the finding that receives least attention and deserves more. Only variant 1-8-4 was recovered at distance, with no evidence of 1-18-2 or 1-26-2 in any long distance sample despite all three being present in the source population. Long distance airborne transport is therefore not a uniform property of the virus but something that varies between strains, which means a regional risk assessment depends in part on which strains are circulating rather than on distance alone.