Bird Flu: Current Evaluation Of Risks To Humans From Avian Influenza Virus Following Recent Reports

WHO has been concerned about this virus, influenza A(H5N1), because of its threat to humans both in farm settings in Asia and its greater, potentially global risk. Several countries in Asia have witnessed this virus crossing the species barrier, moving from infected chickens or ducks directly into humans in three documented outbreaks since 1997. These direct human infections have produced severe and sometimes fatal outcomes. Moreover, the virus has the potential to acquire the ability to spread easily from human to human, and thus, trigger a global influenza pandemic.

Now, two research reports have added to our understanding of this virus. First, members of China's Ministry of Agriculture and colleagues reported in a paper published last week in the Proceedings of the National Academy of Sciences that the virus appears to be widespread in domestic ducks in southern China. Further, the scientists found that the virus is causing increasingly severe disease. However, these trials were done in mice and may not have a direct implication for humans.

This week, the journal Nature published a report which indicates domestic and wild birds in the region may have contributed to the increasing spread of the virus and suggests that the virus is gaining a stronger foothold in the region. These observations suggest that control of the virus may be even more difficult than thought in the spring.

Effective risk management tools exist to control outbreaks of influenza A(H5N1) when they are detected in poultry operations. China, for example, was quick to employ these tools last week when an outbreak was discovered in Anhui province. These risk management measures include the culling of infected and exposed birds, stringent biosecurity measures and vaccination. While this approach can still take months or even years to contain the virus completely, these methods have been effective in the past.

However, tools to assess the risk to human health are less well developed. While recent reports indicate the virus has been present consistently in the environment for the last several years, it has still not acquired the ability to infect humans easily. Why? Is there something about this virus which resists this development? Given the recent reports, WHO urges and offers assistance that such risk assessment activities, including surveillance in animals and humans, and strain analysis, be undertaken as soon as possible.

More knowledge of the virus could be acquired if WHO had full access to all virus isolates and clinical specimens from recent outbreaks. All H5N1 viruses are not the same, and how they differ could provide important insights. For example, the work reported in Nature suggests that the Indonesian avian influenza virus, while belonging to the genotype of viruses seen in Viet Nam and Thailand, is also distinct. What, if any, impact does this difference have? With this information, public health planners would know that they are confronting the same virus in all of the recent outbreaks in Asia. This is another set of the many questions that need to be answered imperatively.

Pandemic preparedness activities started by WHO in the wake of the outbreaks reported earlier this year continue. Less than two weeks ago, WHO hosted a meeting in Kuala Lumpur with experts from 13 countries and areas of the Asia-Pacific region. Among other activities, the meeting participants were provided with a WHO preparedness self-assessment tool. WHO is collaborating with scientists and the pharmaceutical community on a global surveillance system to monitor changes in the virus's susceptibility to known antivirals. Finally, pandemic vaccine development continues. Two vaccine manufacturers, both based in the United States, have produced a supply of trial vaccine which will be tested for safety and efficacy in humans.

In summary, recent developments suggest that:

• the virus is more widespread than previously thought and found in wild birds, and therefore it may be more difficult to eliminate.

• virus isolates and specimens from all recent outbreaks need to be shared with the WHO laboratory network to monitor the circulating viruses and to assess the adequacy of the current pandemic vaccine strain.

• as control measures are strengthened, national governments are encouraged to provide human influenza vaccinations to culling workers.

• all people, especially culling workers, exposed to infected birds need to be provided with antivirals.

• human trials of experimental influenza pandemic vaccines should be accelerated.

• while early identification of avian influenza cases in humans is difficult, stepped up surveillance for the early detection of the disease in humans is essential.

The risk of emergence of a new human pandemic virus will remain as long as the avian influenza virus exists in the environment. WHO's concern and activities continue at a high level following recent reports. Because the H5N1 threat is unlikely to be resolved in the shortterm, WHO is working with other international agencies, including the Food and Agricultural Organization (FAO), and World Organization for Animal Health (OIE), to monitor events.