|A cough - Tang et al 2008|
Respiratory pathogens are pro's when it comes to person-person transmission; easy in and easy out. Viral replication - and also I guess bacterial - followed by the subsequent immune response, induces a range of behaviors in the host which results in the expulsion of doses of infectious particles. These include your runny nose, coughs and sneezes - you've all experienced them. Although even talking and breathing may also be spreading virus.
But, how exactly does the virus cause this? Which mechanism is potentially the most potent transmitter of virus? How many particles can be found in your average cough? And, how do these behaviors physically transfer infection? Perhaps most important - what can we do to stop it?
A recent review states, talking about influenza:
Three modes of transmission have been postulated, which are not mutually exclusive: aerosol transmission, transmission by large droplets and self-inoculation of the nasal mucosa by contaminated hands.
So, turns out we're not all that sure which is more important as we seem not to be able to discern whether one mechanism is better than another despite it's obvious importance in healthcare and epidemic situations. Remember the whole influenza/face mask situation a few years back? From epidemiological studies we can clearly see that for the most part, these viruses are spread through the air but we lack experimental evidence confirming it, especially the specifics. This probably stems from the difficulty in setting up well-controlled experimental situations on human participants in the lab.
What many of these mechanisms have in common is the induction of liquid droplets potentially containing infectious virus which can be inhaled into (or added directly to) the respiratory tract and restart another infection. With every cough we generate and expel all kinds of these droplets: big ones and little ones (from 1mm to 100um in diameter); each has it's own physical characteristics. Big ones - considered droplets - may not hang around in the environment for long (they'll drop to the ground and even if they're inhaled, they might not make to the lower respiratory tract) and the small ones - considered aerosols - will persist for longer. These two general categories appear to be important for both transmission and viral disease in different ways. But perhaps most importantly when thinking about the capacity to spread disease, it's the small ones we have got our eye on - these are the ones that can be inhaled into your lung alveoli. One theory that has been passed around is the ability of these viruses to infect through the conjunctiva surrounding the eye, but again, it has been difficult to test such an hypothesis.
But check out this recent PLoS ONE paper:
But check out this recent PLoS ONE paper:
|'Flu in aerosols|
They show clearly (perhaps for the first time) that influenza positive human subjects in the clinic, expel a range of influenza-laden droplets when asked to cough. These aerosols come in a range of sizes, with each size class containing different amounts of virus. See the graph to the right. It appears that the bigger particles contain lots of virus, but there isn't many of these droplets compared to the smaller ones. The populous smaller ones on the other hand contain less virus. But there are more of them.
Of particular note is the fact that they were only able to find infectious particles in 2 out of 21 subjects tested, despite finding influenza RNA in all of them. Whether or not this reflects the real amount of virus or it's some sort of technical problem of the assay being not sensitive enough I don't know. Maybe once you've got a virus inside a small aerosol, you only need very little to infect? Also they say that their patients in this study would have been passed the peak transmission phase of the infection at the time of entrance to the clinic.
These caveats aside, this work characterizes clearly the presence of infectious influenza virus in expelled respiratory secretions of humans. It did not address the biological causes and effects of this however; no transmission was actually observed or tested. Are one class of droplets better suited at transferring virus between hosts? We don't know. But work following on from this, in both human and animal models, may shed some light on the comparative roles of respiratory transmission versus direct contact of viral pathogens in epidemics and pathogenesis.