Viruses don’t have wings but many can be airborne and transmit between farms…so how do they travel?

When viruses become aerosolized they associate to particles of different natures and sizes. Common types of particles include dust, feces, feed, respiratory secretions and other organic debris. The size of particles in air influences the distances they (and associated viruses) are transported, the locations in the respiratory tract where they are deposited after inhalation, and the survivability and infectivity of the viruses that are contained in aerosols. The size of particles is also important when it comes to determining whether biosecurity systems like air filtration will work, since filter efficiency is particle-size dependent.

However, overall we have very limited information on the size of particles that are associated with swine pathogens. While seeking more information, we recently designed a study to measure the size of particles that are associated with influenza, porcine reproductive and respiratory syndrome (PRRS) and porcine epidemic diarrhea virus (PEDV). In this study we infected pigs with these three viruses. Air samples were collected with an air sampler that is able to discriminate between particle sizes. The amount of virus in the air was measured using quantitative reverse transcription polymerase chain reaction tests (RT-PCR).

We measured viruses in particles of the respiratory range (0.3 to 10 microns) and found that these viruses can be associated with particles of most sizes measured. Indeed, influenza and PED viruses were detected in all particle size ranges from 0.3 to 10 microns, while PRRS virus was detected in those same size ranges, with the exception of 0.7 to 2.1 microns.

It is important to note, based in this study we are not concluding that the PRRS virus cannot be found in those particle ranges since more work considering other PRRS virus strains and more field conditions is needed.

More importantly, we found that there was more virus quantity in those particles of larger size, but the total contribution was different for each of the viruses. Quantity of virus ranged from 10^5 RNA copies/m3 of air to almost 10^7 RNA copies/m3 of air. Overall this information provides insight into particle size ranges that viruses are associated with when they are suspended in the air. An important finding of this study is that all these viruses are found in the particle size ranges that air filters are designed to remove. Although in this study we did not measure the particle size of viruses while they travel between farms, this study furthers our understanding of factors that contribute to disease transmission for airborne pathogens.