SCIENCE/TECHNOLOGY: High risk of bird flu pandemic

Analysis

The World Health Organisation (WHO), having warned of the threat earlier in the year, now says a pandemic of bird flu is inevitable:

• A mutated virus that is capable of being easily transmitted between humans could not be stopped and would reach all the world's capitals within weeks. The only question is the exact genetic nature of such a strain, which would determine its lethality and the efficacy of current vaccines, and the timing of the outbreak.
• Worldwide, it is predicted that billions would become ill.

In the United Kingdom, with a well prepared and financed health service and large stocks of influenza vaccine and anti-viral drugs, the forecast is for one death per thousand of the population. In countries with poorer resources, the death toll is likely to be proportionately much higher.

Viral type and virulence. The influenza virus is a promiscuous parasite, able to infect many species of mammals and birds:

• It mutates quite rapidly, and different influenza virus strains co-infecting a single animal or person are able to swap units of genetic information, leading to new strains with different properties and altered host ranges (see SCIENCE/TECHNOLOGY: Concerns mount over hybrid virus - March 29, 2004; and see SCIENCE/TECHNOLOGY: Diseases remain a global threat - June 2, 2005).
• Such strains are partially characterised by the types of surface haemagglutinin protein (H) and internal nucleoprotein (N) that they contain.

The deadliest current strain -- H5N1 -- has high mortality in humans, who hitherto have only caught the disease rarely, through close contact with live infected birds. There have been 38 confirmed cases of human death from the H5N1 influenza virus strain in Vietnam, twelve in Thailand, four in Cambodia and three in Indonesia. A primary concern is the possibility of the avian virus mutating and evolving so that it can spread easily from human to human.

Efficacy of vaccines. The rapid mutation rate of influenza virus means that a vaccine prepared against last year's major flu strain is of only limited effectiveness against next year's:

• The major challenge in preparing effective flu vaccines is identifying the strain causing a new outbreak when it first appears in Asia, and developing an effective specific vaccine against it -- a process that takes about three months -- in time for the infection's arrival in Europe and the Americas.
• Because the genetic makeup is unknown of a future mutant form of avian influenza capable of causing a pandemic, and an effective vaccine can only be produced once the exact nature of the mutated virus became apparent, the vaccine against existing variants of the H5N1 strain is likely to offer only limited protection.

However, working on the basis that any protection is better than none, UK Health Secretary Patricia Hewitt has recently said that it would be prudent to purchase a limited quantity of H5N1 vaccine, which could be used to help protect those most at risk, such as health workers.

New control measures. Despite the culling of millions of chickens in South-east Asia since 2003, the current outbreak of avian influenza has not been contained or eliminated. Indeed, infection has spread to domestic pigs and ducks, which may provide a reservoir of infected yet symptom-free birds. Moreover, wild ducks, geese and seabirds have also become infected, creating an uncontrollable natural reservoir for the virus.

At a recent conference in Malaysia, new control measures were advocated. These include:

• a move to the mass vaccination of healthy animals;
• continued culling of infected ones;
• encouragement of better farming practices; and
• the need to ensure that farmers are financially compensated for any animals lost to culling, in order to encourage them to alert authorities to cases of infection.

The UN Food and Agriculture Organisation (FAO) and WHO have said that this plan will give health officials a chance of controlling outbreaks, but only if countries take concerted action. This includes the provision of 100 million dollars, most of which will be spent on vaccinating flocks and compensating farmers who report outbreaks.

Spread between continents. Large numbers of geese and other migratory wild birds have died from the disease at Qinghai Lake in western China, and Xinjiang province near Kazakhstan:

• It is feared that infected birds fit enough to fly will carry the disease during their annual autumnal migrations to wintering grounds in South-east Asia, India, Australia and New Zealand, leading to the likely spread of the disease to chicken farms in these countries.
• This would result in enormous potential loss of income because of disease-induced deaths and the culling that would be required, and additional concerns that the new environments might give the virus further opportunities to mutate into a pandemic strain.

Chinese caution. Official Chinese sources claimed that by July 1, 6,000 birds had died at Qinghai Lake, and the bird flu outbreak was "under control". Chinese scientists involved in investigating the outbreak claim the numbers to be much higher -- in the hundreds of thousands. Their genetic analyses have revealed that the strain of H5N1 avian influenza involved at Qinghai Lake is distinct from those in Vietnam and Thailand, but identical to virus isolates they had recently obtained from poultry markets in more eastern Chinese provinces.

However, the Chinese authorities -- which have restricted access by international experts from the WHO and banned all future research on the issue without official permission from the Agriculture Ministry -- have recently disputed the findings of the scientists' papers, declaring that there have been no outbreaks of avian influenza among Chinese chicken populations this year. This is reminiscent of the initial Chinese response to the SARS outbreak in 2003 (see INTERNATIONAL: SARS - shape of epidemics to come? - April 7, 2003), at a time when global control of this form of influenza will only be obtained through openness and international cooperation.