Bird flu-Box 1

	Published by Australian Academy of Science
Bird flu – the pandemic clock is ticking Box 1 \| An ever-changing virus

Two factors combine to make the flu virus so dangerous. They are its genetic make-up and the fact that animals can act as 'mixing vessels' for the virus.

Genetic make-up of the flu virus

When a virus particle is enlarged, you can see that the outside of the capsule is studded with tiny spikes. Most of these spikes are a protein called haemagglutinin (or H). A minority of the spikes are another protein called neuraminidase (or N). These are the proteins that the immune system responds to, producing immunity to the virus after a dose of the flu.

The problem is that the H, and sometimes the N, change subtly causing new outbreaks of flu. The common flu epidemics that emerge every 2 to 3 years typically involve a change of less than 1 per cent of the building blocks that a protein is made from. Your body has a reasonable chance of 'recognising' this virus and fighting it off. New vaccines which are made every year are mostly effective, giving about 70 per cent protection against current strains and decreasing the severity of the flu.

Scientist use a number system to describe the different types of flu virus by looking at differences in the H and N proteins. The virus that caused outbreaks of bird flu since 2003 is known as H5N1 (short for haemagglutinin type 5 and neuraminidase type 1). Fifteen different types of H and nine types of N are recognised by scientists.

Animals are mixing vessels

By analysing the types of surface proteins of influenza virus that have caused previous pandemics, scientists believe that flu pandemics occur when the virus aquires a radically new H or N, or both. New H or N come from the rearrangement of existing H or N genetic material, which occurs when the influenza virus infects other species, such as pigs or birds.

Analysis of virus genetic material from lung tissue of flu victims had previously not provided any clues to the virulence of the 1918 Spanish flu. Further computer-based analysis of the information by Australian National University scientists Mark Gibbs, John Armstrong and Adrian Gibbs, showed how the virus evolved in pigs shortly before it moved to humans, thus causing a pandemic.

The Spanish flu pandemic was caused by a H1N1 virus. About half of the H genetic material had been swapped for pig flu virus genetic material. When such major changes occur, the resulting virus can cause severe symptoms, such as copious leakage of fluid into the lungs observed for the Spanish flu.

Although H5 is common in viruses that cause bird flu, it is unusual in viruses that cause human flu, so even adults who have previously had the flu have very low immunity to H5 influenza. In some parts of Asia, humans have close contact with potentially infected animals. For example in China, where H5N1 originated, ducks are kept in farmyards along with pigs. Importantly, researchers from China and Indonesia report that bird flu H5N1 has been found in pigs, so pigs may act as a mixing vessel for new types of virus.

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