20 000 viruses in one cough
2009-05-12 12:32
Hong Kong - The next time you cough or sneeze, there is good reason to cover your mouth.
As many as 20 000 viruses are expelled in an average cough,
which may be sufficient to infect many people - particularly
those who are not vaccinated.
Julian Tang, a consultant at Singapore's National
University Hospital's Division of Microbiology, said as many as
3 000 tiny droplets are produced in a standard cough.
Using previous research on influenza viral loads in nasal
secretions, and assuming that each coughed airborne droplet
measures between 1 to 5 micrometres in diameter, that adds up
to many, many viruses in a single cough.
A micrometre, or micron, is one millionth of a metre, or
one thousandth of a millimetre.
"Based on this research and assuming about 3 000 droplets
are produced per cough, this range of influenza viruses
produced per cough is about 195 to 19 500," Tang told Reuters.
He added that this figure of 3 000 droplets refers to those
that remain suspended in the air for considerable periods -
long enough to infect people.
"This (3 000) is also the number of droplets estimated to
remain suspended in air for long periods - so-called droplet
nuclei.
H1N1 influenza
Larger droplets carrying influenza viruses may also be
produced during a cough, but these will fall to the ground
relatively quickly and will no longer be considered to be
significant in the airborne transmission of influenza," he
said.
With a new H1N1 influenza virus spreading around the world,
governments in many countries have revived advertisements
entreating people to observe personal hygiene.
In Indonesia, a slick TV advertisement features a young
woman remonstrating a suitor at a village sing-a-long, telling
him to get to a health clinic because "that's not a normal
cough".
Aimed at the poorly-educated masses, the adverts are set to
a dangdut (traditional pop music) theme and feature an
easy-to-remember slogan and theme song.
Infective dose
However, experts are uncertain as to what constitutes an
infective dose when it comes to influenza viruses.
But as a guide, previous research has found that it takes
just one to 10 organisms to cause viral hemorrhagic fevers, and
10 to 100 organisms to cause viral encephalitis.
Tang said an infective dose of flu virus would depend on a
variety of factors, such as the constitution of people
breathing in these droplets and whether they had been
vaccinated.
"It is difficult to give an exact number for the infectious
dose - and this may even differ for the same individual
throughout the year.
"But probably for immune people, the
infectious dose will be higher than for non-immune people -
hence, the benefits of vaccination," Tang said.
Antibody response
"Those previously vaccinated or naturally infected to the
same or similar virus can develop a rapid antibody response and
clear the virus in the respiratory tract before the virus can
take hold and cause disease."
"(But) even immune hosts may develop symptoms if the viral
load exposure is sufficiently high," he added.
Doctors say most airborne flu transmissions occur within a
one metre range, although it can also be transmitted by direct
contact with contaminated objects, like toys.
"Wearing masks may well help to reduce the transmission of
these infections, as well as covering the mouth with your hand
or a tissue when coughing or sneezing - simply by using a
barrier to prevent dissemination of the virus," Tang said.