Hugh Pennington hopes our luck will hold

Avian influenza is bad news for birds.  A virulent strain can kill a chicken within a day.

First, it shows signs of paralysis, because the virus attacks the brain, and it gets diarrhoea, because it grows in the gut.  It has been feared by poultry farmers for three quarters of a century.

A bird flu outbreak started in Hong Kong in March 1997 in rural chicken farms.  A three-year old boy died from flu in May.  It took till August to work out the connection, because it had never been made before.  He had been killed by the bird flu.  The virus was subtype H5N1.  There were more human cases in November; by Christmas 18 had been infected and six had died.  This was very frightening.  The virus was as lethal – and probably more so – than the most vicious smallpox.  On 28 December Hong Kong started to kill all its chickens.  All 1.6 million had perished by the end of the year.

Evolution

When I was starting my scientific career I worked on bird flu.  We cared not at all about protecting ourselves because the strains we worked on only attacked birds.  Hong Kong and evolution changed all that.

What makes influenza such a difficult adversary is that it mutates often (and has no proof-reading mechanism to correct errors in genome replication) and has the ability to swap its genes with ease.  A strain specializing in killing birds can pick up genes that enable it to infect humans while keeping the virus proteins recognized by the immune system.  So when it moves into the human population, it behaves like a brand new virus; our immune systems have never seen it before.  This is why influenza causes pandemics, world-wide outbreaks.  The big worry about H5N1 is that it might mutate and do the same.

The slaughter in Hong Kong in 1997 did not eradicate it.  It continued to circulate in China.  Since then it has spread to poultry in most countries in the continental Far East.  So far it has only killed humans who have had very close contact with chickens – in them, mortality rates have been about 60 per cent.  But the most worrying developments have been mutations which have helped it to infect migratory geese in China, and its spread westwards into Siberian chickens.  Now it is in Europe.

Will H5N1 come to Britain?  Nobody knows.  There is no convincing evidence that the mutations necessary for it to turn into a virus that can spread easily from person to person have happened – yet.  So if it does, it is much more likely to come as an efficient killer of birds than of humans.

Protecting the population

What is being done to protect us?  As with any impending disaster, good intelligence is crucially important.

The World Health Organisation has had long experience of influenza surveillance and it is doing its best in the countries where H5N1 is known to occur.  Sick wild birds in Britain are being tested through our own veterinary surveillance system.  Traditionally, avian influenza outbreaks in poultry have been controlled by slaughtering the infected flocks.  This policy would be put into effect if H5N1 came.

For humans, the main option in the past has been immunisation.  Influenza vaccines protect – but are not as good in practice as the live virus vaccines such as MMR.  And because influenza virus evolves rapidly, new vaccines have to be made at frequent intervals.

Particular problems

For H5N1 there are additional problems.  The virus is difficult to grow because it kills eggs quickly (embryonated eggs are the system of choice for vaccine production). It also needs very careful handling because of the threat it poses to people working with it. And it appears to be poor at stimulating an immune response.  But at the end of the day, even a feeble immunity will be better than none at all.

Antiviral drugs are also available.  Ones like tamiflu work against a wide range of influenza viruses and don’t have the disadvantage of specificity like a vaccine.  The UK government has ordered millions of doses.  Whether antivirals could nip an epidemic in the bud is not absolutely certain, because they have never been used on any scale.  Mathematical modelling suggests that they might – but they would have to be used early, soon after the arrival of the virus in the population.

So readiness to meet H5N1 is not perfect.  We have been fortunate so far in that H5N1 is still primarily a bird virus.  Let us hope our luck holds.

Hugh Pennington is President of the Society for General Microbiology.