Avian flu - facts and fiction

In the next few weeks the annual flu vaccination campaign will begin against the backdrop of worldwide concern about bird flu.

It is feared that avian flu could jump the species barrier and cause a lethal pandemic of the illness, which could kill millions of people.
Health Editor Madeleine Brindley asked the Health Protection Agency to separate fact from fiction about avian flu and the risks to human health

Q What is avian flu?
A Avian influenza, or bird flu, is a contagious disease of animals caused by viruses that normally infect only birds and, less commonly, pigs. While all bird species are thought to be susceptible to infection, domestic poultry flocks are especially vulnerable to infections that can rapidly cause epidemics in poultry.

Q What is the present situation?
A Since mid-December 2003, a growing number of south east Asian countries have reported outbreaks of highly pathogenic avian influenza in chickens and ducks. Infections in several species of wild birds and in pigs have also been reported. The rapid spread of highly pathogenic avian influenza, with outbreaks occurring at the same time in several countries, is historically unprecedented and of great concern for human health as well as for agriculture.

Q Why is there a risk to human health?
A Of particular concern, in terms of risks for human health, is the detection of a highly pathogenic strain - known as H5N1 - as the cause of most of these outbreaks of bird flu. H5N1 has jumped the species barrier, causing severe disease in humans, on two occasions in the recent past - in 1997 and 2003 - and is now doing so again. In the last year, in gradually growing numbers, there have been cases in Vietnam, Thailand and, more recently, Cambodia.

Q Apart from H5N1, have other avian influenza viruses ever infected humans?
A Yes. Two other avian strains have caused illness in humans, but the outbreaks were not as severe as those caused by the H5N1 strain. The H9N2 strain, which is not highly pathogenic in birds, caused mild cases of illness in two children in Hong Kong in 1999 and in one child in mid-December 2003, also in Hong Kong. An outbreak of highly pathogenic H7N7 avian influenza in birds, which began in the Netherlands in February 2003, caused the death of one vet two months later, and mild illness in 88 poultry workers and members of their families. An outbreak of highly pathogenic H7N3 avian influenza also occurred in British Colombia, Canada in 2004 causing two cases of mild illness in humans.

Q Are all of the currently reported outbreaks in birds equally dangerous for humans?
A No. Outbreaks caused by the H5N1 strain are presently of the greatest concern for human health.

Q Can humans be infected by bird flu?
A Yes, but this occurs rarely. Human cases were described in Hong Kong in 1997 during a large outbreak of bird flu which affected the large live poultry markets in the country. Similar cases occurred again in Hong Kong in 1999, in the Netherlands in 2003 and in British Columbia in 2004. All human cases were probably infected through close contact with live infected poultry. There have been 55 cases of H5N1 flu in humans in Vietnam, Thailand and Cambodia since January 2004, causing 42 deaths.

Q How are humans infected?
A Humans are usually infected through close contact with live infected birds. Birds shed influenza virus in their faeces.

Q Why is there so much concern about the current outbreaks?
A Public health officials are alarmed by the unprecedented outbreaks in poultry for several reasons. Firstly, most - but not all - of the major outbreaks recently reported in Asia have been caused by the highly pathogenic H5N1 strain. There is mounting evidence that this strain has a unique capacity to jump the species barrier and cause severe disease, with high mortality, in humans. A second and even greater concern is the possibility that the present situation could give rise to another influenza global epidemic - a pandemic - in humans. Scientists know that avian and human influenza viruses can exchange genes when a person is simultaneously infected with viruses from both species.
This process of gene swapping inside the human body can give rise to a completely new subtype of the influenza virus to which few, if any, humans would have natural immunity. Moreover, existing vaccines, which are developed each year to match presently circulating strains and protect humans during seasonal epidemics, would not be effective against a completely new influenza virus.

If the new virus contains sufficient human genes, transmission directly from one person to another - instead of from birds to humans only - can occur. When this happens, the conditions for the start of a new influenza pandemic will have been met. Most alarming would be a situation in which person-to-person transmission resulted in successive generations of severe disease with high mortality.