Evidence suggests that recent pandemic flu viruses originated in birds. During the last few years, the world has faced several threats with pandemic potential - principally from outbreaks of avian influenza. Many experts believe that the outbreak of highly pathogenic avian influenza A/H5N1 in Hong Kong in 1997 in which 18 people were infected, six of whom died, could have led to a pandemic, if it wasn' t for the prompt destruction of Hong Kong's entire poultry population - almost 1.5 million chickens. The current outbreak of highly pathogenic avian influenza A/H5N1 which has affected poultry in countries in Asia, Africa and Europe has resulted in human cases and deaths, has infected over 250 people (of whom over 150 have died) and has brought the threat of a flu pandemic close again. Information on animal cases is available from the World Organisation for Animal Health website and on human cases from the World Health Organization website.
Public health officials are alarmed by the unprecedented outbreaks in poultry for several reasons. First, most - but not all - of the major outbreaks recently reported in Asia have been caused by the HPAI 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.
This was the situation during the great influenza pandemic of 1918-919, when a completely new influenza virus subtype emerged and spread around the globe within 4 to 6 months. Several waves of infection occurred over 2 years, killing an estimated 40 to 50 million people world-wide.
Experts fear that HPAI H5N1 could trigger the next pandemic for several reasons. Firstly, it has already demonstrated an ability to infect people and cause severe disease - one of the key characteristics of a pandemic strain. Secondly, this particular virus has an ability to mutate and to acquire genes from viruses infecting other species. Experts fear that the virus could either adapt, (giving it greater affinity for humans) or exchange genes with a human flu virus (thereby producing a completely novel virus) and become capable of spreading easily between people and causing a pandemic.
The continued spread of HPAI H5N1 in birds increases the opportunity for direct infection of people. As more people become infected, the likelihood increases that they could be infected with human and avian influenza strains at the same time, and that they could serve as the ' mixing vessel ' for the emergence of a new virus capable of being easily transmitted from person-to person. Such an event could mark the start of an influenza pandemic. The likelihood of this occurring is not easy to predict.
Human infection with HPAI H5N1 has been rare up to now and the virus has not acquired the ability to pass easily from human-to-human. Should it acquire this characteristic, it would meet all the criteria of a pandemic flu strain. While there have been instances of possible human-to-human transmission, so far these have been one-off, isolated occurrences. Human-to-human transmission must be efficient and sustainable if the virus is to become capable of causing a pandemic.
While a new pandemic strain of the flu virus could first emerge anywhere, including the UK , it is most likely to emerge in China and the Far East , as most previous pandemics appear to have done. In this part of the world, dense human populations, domestic pigs and wild and domestic birds live in close proximity, facilitating the mingling of human and animal viruses through co-infecting and the consequent genetic exchange that could give rise to a pandemic strain. The circulating HPAI H5N1 virus that is of current concern arose in South Est Asia .
Last reviewed: 13 August 2008