Avian Flu Fact Sheet

What is Avian Flu (“Bird Flu”)?

Avian flu is an infection caused by a type A strain of the influenza virus.  It is not new – it has been known for more than 100 years; it can occur all over the world, and all bird species are believed to be susceptible to it.  It is normally only transmissible between birds, and causes a spectrum of illnesses ranging from minor to rapidly fatal disease, sometimes resulting in the death of entire bird populations.  However, this is the worst ever outbreak, affecting many millions of birds, and the strain responsible is an especially dangerous one.

There are so many different strains of flu: why are there so many, and what’s the difference?

The hallmark of flu viruses (both bird and human varieties) is their ability to evade immune defences over time by making gradual changes to the proteins that are present on their surface coats.  Flu viruses have two important types of surface proteins that interact with the immune system.  These are called haemagglutinin and neuraminidase respectively – known as “H” and “N” proteins for short.  Each of these proteins exists in the form of several possible variants, so there are vast numbers of possible combinations, and therefore vast numbers of possible strains. Immunity is specific to each individual strain.  Over a period of time, gradual changes in the combination of these protein variants (technically known as “antigenic drift”) enables flu viruses to always keep one step ahead of their host’s immune system – which is why new flu vaccines have to be produced every year against each new season’s predominant strains.

The strain of avian flu now causing special concern is a subtype called H5N1.  There are several reasons why this strain happens to be especially important:

• H5N1 is a strain that has a known ability to mutate rapidly into other types

• It has a documented ability to acquire genes from viruses that infect other animal species

• It has a known ability to infect humans as well as birds, having been responsible for human cases during an outbreak in 1997

• In the small number of instances (up to 120 cases) where humans have been infected, the illness has been very severe or even lethal (~50% mortality).

• Birds that survive infection with this strain can go on to spread the virus orally and faecally for a period of up to ten days, and can spread infection over a wide area, both by migration and through contact with large numbers of birds at live poultry markets.