Is bird flu a threat to humans?

The Asian bird flu outbreaks have been caused by a strain of influenza A called H5N1. At present, H5N1 is only slightly infectious to humans and cannot be transmitted from one human to another.

However, experts fear H5N1 may evolve into a virus that could be transferred among humans. This, they say, could lead to the first flu pandemic of the 21st century.According to the US Centres for Disease Control, the three great flu pandemics of the 20th century were the result of genetic material from bird flu viruses becoming incorporated into human flu viruses.

This led to a far more dangerous virus, which was able to rapidly spread worldwide.There are two ways H5N1 could become a greater threat to human health. One is that the genetic material of the virus could evolve, giving rise to new, more virulent strains. Alternatively, the virus could combine its genetic material with that from other influenza viruses that already infect humans. The more frequently humans come in contact with infected poultry, the more likely this is to happen.

Why are people worried about bird flu getting into pigs?

Both avian and human influenza viruses can infect certain animals, such as pigs. This creates a genetic 'melting pot' in which viruses can swap their genes and acquire each other's properties. For example, if a bird flu virus were to swap genes with a human flu virus, it could acquire the capacity to infect humans, and this could lead to human-to-human transmission. This could generate a new virus that would pose a greater threat to human health.

Can bird flu be treated?

The bird flu virus responsible for the recent outbreaks in Asia has been found to be resistant to the two oldest and cheapest flu drugs available, namely rimantidine and amantidine. However, researchers with Australia's Commonwealth Scientific and Industrial Research Organisation claim that flu drugs Relenza (zanamivir) and Tamiflu (oseltamivir) are effective treatments against the disease.

There is a worry, however, that antiviral drugs are expensive and in limited supply. Ira Longini at Emory University in Atlanta, Georgia, modelled Tamiflu's potential impact and came to this conclusion, according to a news report published in Nature in February 2004. In addition, a number of companies are trying to develop a vaccine against bird flu. Such vaccines present a particular challenge, as the flu viruses against which they are intended to provide protection (by stimulating the production of virus-fighting antibodies) mutate frequently. As a result, any vaccine against a flu virus needs to be modified as the virus itself evolves.

What has been done to minimise the risk of bird flu epidemics?

Culling (killing large numbers of infected and potentially infected animals) and putting infected farms in quarantine are two common measures for limiting the spread of bird flu. According to the World Health Organisation "most influenza experts … agree that the prompt culling of Hong Kong's entire poultry population in 1997 probably averted a pandemic."

A third option is vaccinating birds against the virus. This remains controversial as an effective means of controlling the disease remains contentious. In 1997, the Chinese government decided to vaccinate poultry in order to limit the spread of the disease. Concerns have been raised, however, that this decision may have in fact contributed to the spread of the disease. Inefficient vaccines are a concern as they can allow the virus to replicate without the animals showing any symptoms, a phenomenon known as a 'silent epidemic'.

In July 2004, however, Indonesia launched a poultry vaccination drive, and Thailand is considering the same course of action.The World Health Organisation recommends that people at high risk of being infected by the bird flu virus - mainly those involved in culling operations - should be vaccinated with the most recent human flu vaccine available. One reason for doing this would be to minimise the chances of anybody being infected by both the bird flu and human flu viruses simultaneously.

Should that happen, the two could combine their genetic material, creating a more harmful virus capable of spreading easily through the human population. Finally, there have been suggestions that wild migratory birds play a role in spreading bird flu across large distances. Despite such concerns, the UN Food and Agriculture Organisation, has recommended against culling wild bird populations, arguing that there is insufficient evidence to support it.

What is the history of bird flu outbreaks? What is their impact on human health?

Bird flu outbreaks are not uncommon. Usually, however, they do not affect humans. The oldest record of a bird flu outbreak having an impact on human health was the 1918 human flu pandemic. This, and the flu pandemics in 1957 and 1968, is believed to have been the result of a bird flu virus combining its genetic material with a human flu virus, thus becoming very infectious to humans. In 1997, the direct transmission of bird flu virus (H5N1) from bird to humans was reported for the first time. Six people died in Hong Kong after being infected in this way. In 1999, a different bird flu virus, identified as H9N2, infected two people in Hong Kong.

They recovered, and no additional patients were reported. In 2003, one person died from bird flu virus H5N1 in Asia, out of two reported infections. Both patients belonged to the same family from Hong Kong, and both were reported to have fallen ill after visiting mainland China.The same year an outbreak of bird flu virus H7N7 infected 80 people, in the Netherlands, killing one. During this outbreak, there were signs of human-to-human transmission of the disease - a rare event.

Also in 2003, a child in Hong Kong was reported infected with H9N2, but recovered. So far, in 2004, the World Health Organisation reports 23 human casualties of H5N1 outbreaks in Asia. Eight of these were in Thailand and 15 in Vietnam. Outbreaks have also been reported among poultry in Cambodia, China, Indonesia, Japan, Laos, and South Korea, but so far none of these is reported to have infected humans.According to the World Health Organisation, flu pandemics can be expected to occur three or four times every 100 years. The WHO quotes experts as agreeing, "another influenza pandemic is inevitable and possibly imminent".

Source: U.S. Department For Health & Human Services