A source population of 300 grow finish pigs was inoculated with PRRSV MN-184 and Mycoplasma hyopneumoniae strain 232, and air was sampled over 50 days. Four of 306 samples were PRRSV positive and six were M. hyopneumoniae positive. All four PRRSV positives were recovered 4.7 kilometres northwest of the source, all contained infectious virus, and all were at least 98.8 percent homologous to MN-184. The M. hyopneumoniae positives included two at approximately 2.3 kilometres and two at approximately 4.7 kilometres northwest, plus one southeast and one southwest at approximately 4.7 kilometres.
This is the source of the 4.7 kilometre figure, which also appears in the literature as 2.92 miles. Those are the same result expressed in different units and not two separate findings, a point worth holding because industry material sometimes lists both as though they were independent. Otake and colleagues extended the confirmed distance to 9.1 kilometres for PRRSV in 2010, and Arruda and colleagues reviewed the whole body of evidence in 2019.
The recovery of infectious virus, rather than genetic material alone, is what gives this study its standing. Detection of RNA at a distance establishes that viral material travelled; recovery of infectious virus establishes that what arrived could still cause infection. The genetic homology to the inoculated strain closes the remaining question of origin. Four positives out of 306 samples over 50 days is also part of the finding: airborne transmission at this range is infrequent and conditional, which is why it can be real and still be missed by shorter sampling campaigns.