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Why The Bird Flu Virus Is So Deadly?


It constantly swops genetic material and may do it with the human flu virus. Historically, humans were never at risk of being infected by the bird flu because the virus could not jump from birds into humans. Until 1997, that is. The H5N1 avian influenza virus broke through the barrier that year, with the first human infections emerging in Hong Kong. The threat has not ebbed since. Prior to the Hong Kong outbreak, which saw 18 human cases, six deaths and over 1.6 million chickens culled, previous outbreaks of the H5N1 virus in poultry were small and localised.

At the time, little was known about the virus, whose name is derived from the two types of protein that cover the particle surface hemaggluttin type 5 (H5) and neuraminidase type 1(N1). It is the hemaggluttin protein which recognises specific protein beacons on host cells. It sticks itself like a key onto the host protein - the lock - and gains entry that way. Neuraminidase helps the virus exit the cell after it has completed replication.

It was previously believed that humans lacked the correct proteins (locks) for the viral particles (keys) of the H5N1 to bind to. But now, some studies show that the H5N1 virus recognises some of the "locks" - specifically those in the air sacs of the human lung. Fortunately, because the H5N1 virus binds poorly to human proteins, infection has been limited to people with very close contact to infected poultry.

Once inside, the virus hijacks the cell's protein-making machinery to produce more copies of itself. The host cells effectively become virus factories. They eventually burst and die when the virus exits to infect other cells. It is also believed that, unlike other flu viruses, H5N1 can also infect cells outside the airways, such as the liver, kidneys and brain. Two infected children in Vietnam also suffered severe diarrhoea and seizures, which indicated that the virus had spread to the gut and nervous system.

But scientists do not know for sure yet how the virus affects other organs. Speaking at the Lancet Asia Medical Forum, virologist Robert Webster said that only three or four autopsies have been done of people who died of H5N1, which makes it difficult for scientists to study the virus' effect. The bird flu virus has infected 206 humans worldwide. It has also been reported in cats, pigs and even tigers.

With its affinity for mammals, scientists fear that the bird flu could mutate to spread easily between people. Right now, it is most likely to be spread through close contact with infected poultry. Dr Webster, a world-renowned virologist at the St Jude's Children's Research Hospital it t Memphis, predicted that it would take at least 10 mutations before the H5N1 virus could transmit from human to human.

He added that there was no way of predicting how soon - or if such an event would take place. The two-day international forum brought together many of the world's foremost experts on the avian flu. Altogether there were 25 speakers. Dr Malik Peiris from the University of Hong Kong has been researching viruses for over 20 years. It is "fortunate" that the H5N1 virus does not spread easily among humans, noted Dr Malik.

But what makes the virus especially dangerous, is its ability to, reassort. Peculiar to influenza, reassortment refers to how the viruses constantly swop genetic material among themselves. The new combination of genes may enable the virus to perform new functions. "H5N1 may become pandemic when it reassorts with the human flu virus," cautioned Dr Malik. Pigs pose a particular danger.

As virus "melting pots" pigs can act as a meeting place for the human and avian virus to exchange material, explained Dr Malik. Scientists believe it is just a matter of time before the H5N1 becomes a human virus. When that happens, many people will be infected and many may die. The oft-quoted example is the Spanish Flu pandemic of 1918, which is reported to have killed 20 to 50 million people. That too came from a bird flu virus - But there is no need to hit the panic button just yet.

While the prognosis may seem bleak, experts are doubtful that the virus will maintain its virulence it a pandemic occurs. Professor John Oxford of St Bartholo- mew's and the Royal London Hospital explained the most deadly diseases do not spread much because the host dies before the virus can be passed on. He gave the example of the Ebola virus, a horrific disease which has not led to a widespread outbreak because it burns itself out so quickly.

These viruses tend to become less deadly as they become more infectious since it does no good to them to kill their host, he said. Another promising outlook is that a vaccine for the H5N1 virus may not be far off. Initial human trials have been encouraging. Tests in the United States and France have showed that high doses of antiserum are effective and that patients have few or no side effects. It is, however, too early to tell how the battle against the flu will shape up.

Fortunately, the effects of a pandemic can be lessened with planning and preparation. Already a global effort is under way to be mount a pre-emptive strike against H5N1. One of the immediate priorities is to prevent the further spread of the virus within poultry populations. By implementing more hygienic farming practices and vaccinating persons at high risk of exposure, the chance of transmission can be reduced greatly.

Right now, not many countries are well-prepared to face the disease but global agencies like the World Health Organisation are trying to change that. The organisation has already drawn up a global response plan which includes the provision of antivirals and medical support teams to affected areas. Experts at the forum pointed out that this level of preparation against a pandemic has never happened before, and may be our best chance against it. The best defence is offence. As Dr Webster so aptly put about a pandemic: "It may never happen. We hope it doesn't happen. But we should not be complacent that it will not happen."

Frightening Mortality Rate

World-renowned bird flu expert Robert Webster said the H5N1 virus is the worst he has ever come across. The World Health Organisation estimates that the virus kills a frightening 55 per cent of the people it infects. By comparison, the 1918 Spanish flu pandemic which killed about 50 million people had a mortality rate of just 2.5 per cent. With over 30 years of flu research, the virologist at the St Jude's Children's Research Hospital in Tennessee describes the H5N2 virus as "like nothing I've ever seen".

Far from the innocuous cough and runny nose of the common flu virus, H5N1 manifests itself as a deep lung infection which steadily breaks down organ tissue. It has also been known to spread to the brain, liver and gut, Dr Frederick Hayden, a virologist at the University of Virginia Health Sciences Centre, has been researching flu viruses like H5N1 for decades.

Upon exposure, the viral particles travel down the lower respiratory tract, where they attach and gain entry into cells that line the lung walls, explained Dr Hayden. The average incubation period is about a week, during which time the virus multiples within the cells - with between 60 and 70 million particles.

 

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