What causes bird flu?
Bird flu is caused by a strain of the flu virus called influenza A H5N1.
Influenza A viruses occur naturally in wild birds. Although these birds aren't affected by the virus, domestic poultry such as chickens and turkeys are - and so are people.
H5N1 is common among birds in Asia, who shed the virus in their saliva, nasal secretions and faeces. More than 90 per cent of birds who get H5N1 die, and mortality among humans is also high.
The H5N1 virus was first shown to have passed from birds to humans in 1997, during an outbreak of avian flu among poultry in Hong Kong. The virus caused severe respiratory illness in 18 people, and six died. Outbreaks have occurred intermittently since then, particularly in Asia where the disease is now considered to be endemic.
In 2011 a new mutant strain has been reported in Vietnam and China which may be resistant to the current vaccine for bird flu.
Bird flu symptoms
The symptoms of bird flu include:
- Fever
- Sore throat
- Muscle aches
- Headache
- Lethargy
- Conjunctivitis (eye infections)
- Breathing problems
- Chest pains
How flu viruses mutate
Flu viruses continually alter through small changes in their make-up called antigenic drift and occasional abrupt major changes called antigenic shift. This means that although you may have fought and won a miserable battle against flu a year or two ago, the next time the virus appears your body won't recognise it because the antibodies your body made against flu last time won't work. This is why people need to be immunised against flu each year, using the most up-to-date strains of the virus.
There are three types of influenza viruses: A, B and C. Type C only causes mild problems in humans. Type B can cause more serious illness and seasonal epidemics, but as it only changes through the slower process of antigenic drift there is little risk of a pandemic. That threat is more likely to come from type A, which can undergo rapid shift.
New flu strains tend to emerge in Asia, Africa and the East, where people live in closer quarters with their animals, and different flu viruses may mix to cause new strains. Research has shown that H5N1 has changed so it's even more deadly in chickens and mice, and can now infect cats too. H5N1 is also resistant to some of the drugs used to treat flu (such as amantadine), and the new strain which has emerged in 2011 may be resistant to the current H5N1 vaccine..
Millions of chickens and ducks have been slaughtered across South East Asia in an effort to prevent the virus spreading from birds to humans.
If H5N1 becomes able to pass from human to human then the situation will be even more serious, as most people have little immunity to the strain.
The first possible case of human-to-human transmission was reported in Thailand in September 2004.
Bird flu vaccination
A vaccine against H5N1 has already been developed, and others are in development.
The World Health Organization (WHO) provides up-to-date statistics on bird flu cases and deaths worldwide. The UK flu pandemic contingency plan is available from the Department of Health.
