H7N9: The Silent Killer

H7N9 is a particular variant of flu that first reared its ugly head in China in 2013 and has been worrying people ever since. A recent World Health Organisation (WHO) assessment reports that the virus has infected a total of 571 people, killing 212. To understand the threat posed by H7N9, it is first important to understand a bit more about flu itself.

Flu loves birds. It feels so at home in our feathered friends that birds contain every variant of H and N (tiny spikes on the surface of the virus) that have ever been documented.

chicken
This bird is happy to contain every variant of H and N.

As you’ve undoubtedly seen, these Hs and Ns are tossed around whenever people are talking about flu: “H5N1 this…”, “H1N1 that…”.  The letters stand for haemagglutinin and neuraminidase (really can’t see why we abbreviated them…), which are parts of the virus that are incredibly important during infection. There are 18 different Hs and 11 different Ns, giving a staggering number of combinations.

But why is the H7N9 combination so special and why are people so worried about it? The key is that, like a youngster yet to experience the hardships of the world, humans are naive. H7N9 may seem like just another combination of the H and N proteins; however it is a particular combination to which humans have never been exposed before. This means that when we are confronted with the virus, our immune systems can’t handle it and lots of us die.

This is how pandemic viruses like H7N9 differ from the viruses that cause seasonal outbreaks of the flu. The combinations that cause seasonal flu are always H3N2 and H1N1. These types mutate a bit before they do their rounds each winter – meaning a new vaccine needs to be produced every year – however the actual combination of Hs and Ns is one that humans have been exposed to before. Pandemic viruses like H7N9 are formed when several different strains all muck together to make a completely new combination, as demonstrated by the following infographic:

h7n9-reassortment-diagram
This handy infographic from the CDC shows how H7N9 is a combination of viruses from 3 different birds

However, there’s more. This H7N9 combination is particularly sinister because it has the potential to cause a pandemic with less warning than other viruses. Unlike previous bird flu threats such as H5N1, when birds are infected with H7N9 they do not display any symptoms. This means that it is incredibly difficult to tell how many birds are infected with the virus as it silently spreads throughout the avian population.

This sounds like an ideal combination for large-scale pandemonium, so how come H7N9 hasn’t decimated large swathes of the human race? Luckily, the virus finds it hard to transmit from poultry to humans and much harder to transmit between humans. The Centres for Disease Control and Prevention (CDC) – who are basically the American WHO – summary clearly states that there has been no sustained human-human transmission.  The vast majority of infections have been as a result of close contact with poultry in markets and on farms, not by being sneezed at on a crowded commuter train.

Chicken_market_in_Xining,_Qinghai_province,_China
H7N9 lurks in poultry markets like this one

So the important question (as shown by its presence in both the WHO and CDC‘s H7N9 FAQ sections) now is how likely the virus is to mutate into one capable of transmitting between humans.

The blunt answer to this is that nobody really knows. However a recent paper in the scientific journal Nature (the paper is a pretty difficult read, but Nature kindly provided an easily-digestible summary of its findings) gives some interesting information on how H7N9 is mutating. The study found that as the virus has spread south, it has diversified into 3 distinct branches. This diversification is a normal process, but such evolution is always a potential threat because of the risk that the virus will pick up mutations that will allow it to spread more easily among humans.

‘The threat from H7N9 is unlikely to go away any time soon’

Although the virus is yet to acquire pandemic-enabling mutations, the Nature researchers were clear that H7N9 is here for the long-haul: “The virus is now endemic and entrenched in poultry populations across swathes of China, making it likely that people will continue to be infected sporadically.”

So, it is clear that H7N9 has the potential to become a threat, however the lack of human-human transmission means that it is unlikely to cause a pandemic any time soon.

Pandemic Potential = 6/10

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