2005-12-05-Bird flu: it's on its wayUnderstanding Avian Influenza
2005-12-05-Bird flu: it's on its way
In a first extract from her new book our correspondent explains why the avian flu virus is such a threat
In the past two years we have seen country after country struggling in vain to eradicate a virus that has leapt across national boundaries to infect countless millions of chickens and geese. At the moment it is still a disease of birds, but the great fear is that it will evolve into an infection that is contagious between human beings, which could cause a worldwide flu pandemic. We have already seen TV bulletins showing the first victims, mostly healthy young people, struggling to catch their breath as their lungs become clouded with the infection.
There is a chance that the virus causing so much concern, a flu sub-type knows as H5N1, may never be able to pass from human to human, but that possibility is receding fast as every week that passes reveals that bird flu is embedded in more ducks and chickens across a larger region of the globe than anyone thought possible. An estimated 120 million birds have already been culled in the Far East but the disease keeps returning.
Since 2003, 130 human cases of bird flu ? and 67 deaths ? have been reported in Cambodia, China, Indonesia, Thailand and Vietnam. In the words of Britain's chief medical officer, Sir Liam Donaldson, a pandemic is a "biological inevitability" and one that the World Health Organisation has classified as the globe's most serious health challenge. The chances of containing the disease within one country when it emerges in a "humanised" form are slim, Sir Liam feels.
"I would be very surprised if we found out about human-to-human spread within 48 hours, quick enough to give the drugs," he says. "What is more likely to happen is that we will see multifocal outbreaks [in several places at once]. You still have to try to contain it, but I am pessimistic about the chances of doing so."
In Britain there are two dangers. First, there is a risk that H5N1 will arrive here in birds soon, because all kinds of waterfowl migrate from the east into the UK over the winter months. If it does arrive, there are serious implications for free-range poultry farmers and for those who run bird reserves.
The greater worry is that a human influenza pandemic is overdue, and the dice are loaded in favour of that pandemic being caused by H5N1. A reservoir of this disease probably lies in more than half a billion birds in the Far East, and it would take only a few genetic changes for it to move into people. Mathematical modelling suggests that the disease could be in the UK within two to four weeks of being detected in South-East Asia.
The virus's greatest asset is that it is airborne. Once the virus has invaded the cells lining your lungs, throat and nose, it sheds tiny particles which can be expelled at great speed in mucus when you sneeze, or in droplets of moisture from a cough or splutter. One study demonstrated that a viral particle can travel from one end of a train carriage to another at 80 miles (128km) an hour, such is the force of the common sneeze. That is why covering the mouth and nose when you sneeze is more than an issue of social etiquette; it is about preventing germs from escaping at high speed and infecting others near you.
At the moment it is extremely difficult for a human to catch the disease from a bird, and most of those who have caught it have come into close contact with ducks or hens. The bird flu virus can live in the droppings, saliva or nasal secretions of a bird. The wild birds that carry the virus, mostly water-fowl that fly between rivers and lakes, may fall ill or have no symptoms, but any droppings they leave behind at their stopping- places can infect other birds.
Will H5N1 bird flu mutate to become a form that is easily transmissible from human to human? John Oxford, professor of virology at Queen Mary's School of Medicine and Dentistry at the University of London, explains one possible scenario: "If a child comes home from market carrying a chicken, and that chicken is slaughtered in the back yard, it is possible that one of the family will breathe in particles of virus from the bird. If that family member has a more common strain of flu which is around at the time, the H5N1 could mix its genes with the other form and we could get a completely new form.
"This strain could evolve as it spreads, acquiring new characteristics between people. It may become slightly less virulent because what matters to the virus is that the people it occupies don't die too quickly but survive for long enough to pass it on to others. But it would still be a very powerful strain against which we have no defences."
The fact that H5N1 has also spread between certain animals ? tigers, peacocks, cats, pigs and several different bird species ? suggests that it would be possible for transmission to become efficient from birds to humans.
The effect of the infection in humans can be devastating, as Dr Dominic Dwyer, a virologist and member of Australia's National Pandemic Planning Committee, explains: "The virus gets into the lungs. The body makes a really strong immune response to it. The lungs fill up with fluid, with proteins, with cells that are all trying to get rid of the infection, but what happens is that the lungs fill up and the person can't breathe. They die of respiratory failure."
Apart from the financial damage caused by a pandemic ? it could wipe out 2 per cent of the global economy ? mortality rates could be high. Experts have calculated that if the disease has a "clinical attack rate" of 25 per cent of the UK population, and 1 per cent of those people die, there would be 141,000 deaths. If the virus proves more lethal and its "overall case fatality rate" rises to 1.5 per cent, the number of deaths would rise to 212,000. If it has the same fatality rate as was seen in the 1918 pandemic (2.5 per cent), the number of dead would increase to 355,000.
Sir Liam Donaldson hopes that when the virus looks as though it might be moving closer to a human pandemic strain, GPs will be given detailed guidance on the exact symptoms that they should look for, because the disease will have its own "signature" or pattern of illness.
"If it emerges in the Far East you could characterise the early symptoms and have a bit more specificity about what to look out for." But with medications that need to be given within 48 hours, "it is very dramatic, it is different from any other comparable situation or life-threatening emergency. We will have to invent a different way of getting the medicine to the patient as quickly as possible."