Everyone knows of flu, or to give
it its full name, influenza. For most people it’s nothing more than a passing
illness characterised by symptoms such as fever, fatigue, coughing, sneezing,
aches and pains. However, for some people, it can be a much more serious
disease, with estimates from the World Health Organisation believing it to
cause 250,000 to 500,000 deaths a year. Those people at the highest
risk have probably already been contacted with regard to receiving the flu
vaccine, which is an annual occurrence around this time of year (in the
northern hemisphere). What I’d like to talk to you about in this blog is what
flu is and why every couple of years there seems to be a huge scare about a
pandemic outbreak, the most recent of these being Bird Flu and Swine Flu. I
don’t want this to be a scare-mongering blog about how there could be a
pandemic flu outbreak that could kill us all (as the papers like to report it)
but just to help people understand what is going on next time there are reports
of a potential outbreak.
First things first; flu is caused
by a virus. One of the defining features of viruses is that, unlike bacteria,
they are completely unable to replicate without a host, and the aim of any
living thing is to replicate. So without something to infect, viruses would
just sit there as a ball of DNA, proteins and fats doing nothing. The influenza
virus has 3 main hosts, these being humans, pigs and birds. The virus must
therefore infect one of these animals, replicate inside their cells and then
spread to new hosts. In humans, the virus infects cells in the upper
respiratory tract, which is why we cough and sneeze a lot when we have flu.
Now for a bit of virology. A
virus is unable to replicate without a host because of the fact that, in
general, all a virus consists of is genetic material (either DNA, like us, or
RNA) and a particle which transports this material, made up of proteins and
fats. This particle is known as an envelope. In this envelope there are
proteins, which stick out (see the picture) known as haemagglutinin (HA) and
neuraminidase (NA). Both of these have very important roles for the virus. HA
allows entry into the cells and without entry the virus cannot live. NA on the
other hand allows the virus to leave the cells and spread to more cells. And
yes, if you were wondering, these are where names of flu viruses such as H1N1
come from. These notations refer to which distinct type of HA and NA is on the
surface of that flu virus. We currently know of 16 HA variants (numbers 1-3
infect humans) and 9 NA variants (only numbers 1 and 2 infect humans).
The last bit of background
regards the influenza genome. The genetic material of influenza is RNA and
inside each influenza particle there are 8 different segments of RNA. This is
like having a jigsaw of 8 different pieces, all of which have different detail to them
and all of which are necessary to have the final image. Each of these 8 RNA
molecules will produce different proteins that enable the virus to replicate
inside host cells and then spread.
Now that we’ve got the background
down, things start to get interesting. As long as the flu virus has all 8 of
the RNA segments it needs in the genome, it doesn’t care where they all come
from. So let us think of a typical human flu virus with its 8 RNA segments. If
segments 4 and 5 (for example) are exchanged with a flu virus that infects
birds, we have a whole new virus that may be capable of infecting humans. We
can take this idea further and include pig viruses as well and get what is
known as triple re-assortments, where we form a completely new virus with RNA
segments from human, pig and bird viruses. This is what scares virologists and
epidemiologists the most.
Most people never get anything
worse than a severe cold from flu infection due to the ability of our immune
system to fight it off. This system of cells and molecules can detect the flu
virus by means of molecules known as antigens. Our immune system is essentially
blind and has to fumble around feeling for things it recognises as being an
invader before it can destroy them. The main antigens that we use to detect flu
are the HA and NA proteins I described earlier. Now let us consider
recombination: if a flu virus recombines the segments of its genome which code
HA and NA, then there is the potential to produce a flu virus with completely different
HA and/or NA which in turn will be completely un-recognisable to our immune
system, the system won’t know what that feels like so will ignore it.
Being that this new flu virus has
a whole new set of antigens, no-one in a population will have immunity to it,
which allows for the very rapid spread of a highly infectious virus. This will
start as an epidemic and spread from there to eventually become a pandemic. A
small caveat to this is that it is not always necessary for there to be
recombination for a pandemic to occur; sometimes flu viruses which infect
animals can simply mutate to aquire the ability to infect humans instead,
bringing with them a whole new set of antigens.
So the question is: what,
potentially, can the consequences of all this be? And why are we so scared? In
the next blog we will look in detail at one of the worst global pandemics in
history and also at two more recent examples of a pandemic and near pandemic
that never really lived up to the hype, and why they didn’t. So come back soon
to learn more…
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