What is Bird Flu?

Avian influenza ("bird flu") is an infectious disease caused by particular strains of the influenza virus. Experts believe that all birds are susceptible to infection, though some species are more resistant than others.

Symptoms can be mild to very serious, even leading to death. Fifteen subtypes of influenza virus are known to infect birds. So far, all outbreaks of the highly pathogenic form have been caused by influenza subtypes H5 and H7.

Migratory waterfowl – most notably wild ducks – are the natural reservoir of avian influenza viruses, and these birds are also the most resistant to infection. Domestic poultry, including chickens and turkeys, are particularly susceptible.

Danger for humans

Bird flu viruses do not normally infect species other than birds and -- in some instances -- pigs. The first documented infection of humans with an avian influenza virus occurred in Hong Kong in 1997, when the H5N1 strain caused severe illness in 18 people, six of whom died. All those affected had close contact with infected poultry. More than 60 people have died of bird flu since 2003, all in Asia.

The H5N1 strain is particularly dangerous because it mutates very rapidly and can mix with genes from other viruses thus allowing it to spread to other species. In addition it is highly pathogenic and can cause severe disease outbreaks.

With the infection spreading in birds there's an increased risk for direct infection of humans, too. And if more people become infected, it also becomes more likely that humans, if infected with human and bird flu strains at the same time, could serve as the "mixing vessel" for the emergence of a novel strain that could easily be transmitted from person to person. Such an event would mark the start of an influenza pandemic.

Symptoms in humans

The symptoms appearing a few days after infection are those of a severe flu -- a fever, sore throat and cough. In many cases the victims also suffer from diarrhea and stomach pains. About every second person who contracted the disease also died.

Antiviral drugs, some of which can be used for both treatment and prevention, are effective against the virus, but they can be expensive and supplies are limited.

There is also considerable experience in the production of flu vaccines, particularly as vaccine composition changes each year to match changes in circulating virus due to mutation. However, at least four months would be needed to produce a new vaccine, in significant quantities, that is capable of protecting against a new virus subtype.