Tuesday, 29 November 2011

Much A-Flu About Nothing (Post 2 of 2)

I left the last post touching on the fact that we would look into 3 historic pandemics in more detail. We’ll start with the furthest back, and the most deadly, this being the Spanish Flu pandemic of 1918. In the 2 years this pandemic lasted, it managed to infect an estimated 500 million people (that was about third of the population at the time), furthermore it also managed to kill between 50 and 100 million of those it infected, meaning that between 3% and 6% of the global population died due to this virus. Additionally, instead of targeting those of advanced age, as seasonal flu does, it caused deaths in younger, healthier individuals, making the burden of disease far greater. The Spanish Flu pandemic was the result of an early avian virus managing to infect humans due to mutation, not re-assortment. This virus was an H1N1 virus.
A pretty famous image form the Spanish Flu pandemic. All are patients.

Fast-forward to 2009 and we hear from H1N1 again, something I’m sure many readers will remember. This was the Swine Flu pandemic, which lasted a little over a year. This H1N1 virus came from a quadruple re-assortment of strains from swine, avian and human flu viruses. So couple the terror of the possibility of a second Spanish Flu outbreak with a quadruple re-assortment and you can hopefully understand why there was plenty of fear surrounding this outbreak. Swine Flu had more than just letters and a number in common with Spanish Flu, in that it was more contagious than seasonal flu and was seen to target the young more than the old. Fortunately, however, the virus seemed to cause only a mild illness; of the 375,000 lab confirmed cases there were only 4,500 deaths.

Our final virus to consider is Bird Flu (aka Avian Flu), another outbreak which many people will probably remember hitting the headlines. This virus is yet to reach pandemic level, unlike Swine Flu, however there is a close eye being kept on it as there is a high potential that it could soon manage to become pandemic. The Bird Flu virus is an H5N1 virus that can pass from birds to humans and from 2007 to 2010 there have been 26 outbreaks resulting in human infection. The reason that H5N1 has failed to live up to the media hype of being a huge global killer virus is that its transmission from human to human is very limited, so any outbreaks rapidly burn out. The reason for this lack of transmission lies in our lungs. As a reminder, flu viruses have HA on their surface, which binds to cells and allows the virus to enter them for infection. All HA molecules target a receptor on cells known as the sialic acid receptor which is (in parts) made of sugar molecules. Avian HA targets sugar molecules which are linked in a certain way, known as an α2,3 linkage. However, human HA targets α2,6 linked sugar molecules. Both of these types of linkage are found in our lungs, but not with an even distribution. In our upper respiratory tract we have many α2,6 receptors, whereas in the lower respiratory tract we have α2,3 receptors. Flu is an upper respiratory tract infection and going to the lower respiratory tract means it will not be coughed or sneezed out as it is too far down, thus stopping human to human transmission. Here in lies the fear though, in order to switch from targeting α2,3 receptors to targeting α2,6 receptors the H5N1 HA only needs to pick up 2 mutations. Mutations are a rare event, but picking up 2 is not beyond the realms of possibility and is something that is constantly being looked for.

Hopefully now you’ll never consider flu in the same way. For so many people it is essentially just a severe cold, but spare a thought for those who suffer much more from the threat of flu. And the next time the media pick up on a global pandemic that will “kill us all” hopefully you’ll be able to fully understand the aspects of the virus in question and understand the real dangers it may or may not pose, as the case may be.

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