Greatest fear: Bird-flu virus may mutate

Four types of the virus in birds are known to have caused infections in humans. These are known as H5N1, H7N3, H7N7 and H9N2.

Generally, these lead to mild symptoms and not many cases of severe illness.

But there is one notable exception: the H5N1 strain, which can lead to serious illness and death.

It is this virus that has infected more than 100 people, mainly in Asia, and caused more than 60 deaths.

All these cases have resulted from close contact with wild birds and poultry and so far no infections have been reported to have been spread from person to person.

Experts are keen to stress that, at least at present, only those who have close contact with birds are at risk of bird flu.

Symptoms include a sore throat, aching muscles, lethargy, eye infections, breathing problems and chest pain.

Deterioration in patients with H5N1 can happen very quickly and within a few days they can be dead.

Antiviral drugs may help to reduce the length of symptoms, but they have to be given near the start of the illness.

Patients can quickly develop severe symptoms and die as a result, with many treatments proving ineffective in the end.

By far the greatest concern has been that the bird-flu virus will change into a form that is easily spread between humans, leading to a flu pandemic.

There are two main ways in which this could happen.

The first is by “reassortment”, whereby genetic material is exchanged between human and avian viruses during co-infection of a human or pig.

In this mechanism the human could act as a mixing pot, with the human flu virus mixing with the avian flu virus and creating a fully transmissible pandemic virus.

The second mechanism would involve a more gradual process of mutation, with the virus adapting so it is more successful in infecting humans.

This would usually emerge in a small cluster of cases among humans, which would give the rest of the world some time to take action if it was detected early enough.

But experts accept that it is only a matter of time before a new flu pandemic emerges, with bird flu its most likely initial source.

Pandemic flu spreads rapidly to affect most countries and, unlike ordinary flu, which occurs every winter in Ireland and the UK, it can emerge at any time of year.

In the last 100 years there have been three flu pandemics, leading to millions of deaths.

In the UK it is estimated that a new pandemic could kill more than 50,000 people.

It is thought that few people, if any, would have any immunity to a new pandemic virus, allowing it to spread widely and to cause more serious illness.

The British government has already developed a flu pandemic action plan and taken other measures to try to prepare in case it happens.

The UK is stockpiling some 14.6 million courses of antiviral drugs, which could help contain any outbreak and reduce the length of illness.

It has also initiated plans to be able to vaccinate the whole UK population.

But the problem is that a vaccine cannot be created until the exact pandemic strain is known, and even then it could be months before enough is produced to vaccinate everyone.

Chief Medical Officer Sir Liam Donaldson has said the UK would be “very lucky” not to have pandemic flu arrive from elsewhere in the world before the vaccine was produced.

The British government has invited vaccine manufacturers to tender for so-called “sleeping contracts”, meaning they would have to build up capacity ready to produce the vaccine once the strain was identified.

Seasonal flu jabs will not protect against pandemic flu.

The British government’s contingency plan to tackle a flu pandemic includes measures such as stopping large public gatherings like football matches and concerts.

But it is unlikely that screening people entering the country or restricting travel will prevent the virus arriving.

The UK, and every other country in the world, is playing the waiting game, unable to predict whether or not a pandemic will emerge.