Avian flu: taking flight?

On 16th November a duck breeding farm in Yorkshire tested positive for avian flu, so as you can imagine DEFRA was on the scene immediately, introducing a 10km restriction zone and culling all 6,000 birds on the farm to prevent further spread of the disease. But why? Is avian flu so contagious, that such an extreme response was warranted? Is it that dangerous? Let's find out shall we. Avian flu is caused by the influenza A strain of the influenza virus (although A does not stand for avian). All subtypes of influenza A are adapted to be able to use birds as a host, but are not always zoonotic (can pass from animals to humans). Influenza A is what is known as an  single stranded antisense RNA virus, meaning its single strand of RNA (3'-5') runs complementary to viral mRNA (5'-3')  that it encodes. This means it must carry RNA polymerase within the virion particle, as the viral RNA cannot be directly translated into protein, it  must be transcribed first, rather like DNA. Influenza A can be categorised into 2 subtypes based on the protein used to construct its membrane: Hemagglutinin (H) or Neuraminidase (N) . The H protein causes agglutination of red blood cells in the host, and mediates the binding of a virion particle to the host cell and entry of the viral genome into the host cell genome.  Whereas N is an enzyme that breaks the glycosidic bonds in the monosaccharide, neuraminic acid, commonly found in animal cells as glycoprotein and it also controls the release of new virion particles from host cells. Different strains of influenza virus encode for different types of N and H which all play a specific role in the viral lifecycle, for example H5N1 virus  contains type 5 Hemagglutinin and type 1 Neuraminidase (oh how I love arbitrary numbers used to name pathogens). Interestingly, these 2 proteins form the antigens that allow antibodies produced by B cells to bind to the microbe, so macrophages can phagocytose it. Ok,  I think I've indulged you enough in the virus' structure and nomenclature, so let's talk about the birds. The outbreak in Yorkshire recently was H5N8 strain, which is of very little threat to people. In fact no one has died from it. Ever. The avian flu strains that you really have to worry about are H7N9 continually reported in poultry in China, and H5N1, which has a 60% case mortality rate across 15 countries (since 2003). Migratory fowl can act as asymptomatic carriers for the virus, which is probably how it reached the UK in the first place, and strains like H5N1 are not limited to birds and people; in New England 400 harbour seals were killed by the pathogen in a 1978 epidemic. Ok, so we know that certain strains of influenza A are deadly to people, but if the H5N8 strain poses no threat to us, why were all the ducks killed? Well, because of the high mutation rate of the virus, and its ability to hybridise with strains from other species, it could quickly become a human epidemic. So before we start arguing over the 'poor ducks' that were 'murdered', we must consider the potential threat of any strain of influenza to humans.