What is bird flu?
What is bird flu?
Last Updated: 12:01am GMT 21/11/2006
What is bird flu?
Avian influenza or 'bird flu' is a contagious disease of birds, caused by influenza A viruses that can cause a range of symptoms, from mild illness and low mortality to a highly contagious disease with a near 100% fatality rate. The bird flu virus currently affecting poultry and some people in Asia and other areas is the highly pathogenic H5N1 strain of the virus.
How is it spread?
As the virus can remain viable in contaminated droppings for long periods, it can be spread among birds, and from birds to other animals, through ingestion or inhalation. All bird species are thought to be susceptible to avian influenza. Migratory birds such as wild ducks and geese can carry the viruses, often without any symptoms, and show the greatest resistance to infection. Domestic poultry flocks, however, are particularly vulnerable to epidemics of a rapid, severe and fatal form of the disease.
What kind of virus is it?
There are many different subtypes of influenza A virus. The most virulent are called highly pathogenic avian influenza and can reach epidemic levels among birds. Of these, subtype H5, and more particularly subtype H5N1, pose the greatest concern for human health. According to the World Health Organization (WHO), there is mounting evidence that the H5N1 strain has a unique capacity to jump the species barrier and cause severe disease, with high mortality, in people.
How did the current outbreak start?
The outbreak of avian influenza of most concern - H5N1 - began in poultry in South Korea in mid-December 2003, and has affected birds in many countries in Asia, Europe, Middle East and Africa. It involves a variant of the same virus subtype as that associated with the 1997 Hong Kong outbreak.
How can bird flu infect people?
H5N1 is able to infect people although it does not do this easily. In human populations, where domestic pigs and wild and domestic birds live in close proximity with people, the mingling and exchange of human and animal viruses can more easily occur. Those who have become infected have had close direct contact with infected birds.
What symptoms does it cause?
Human infection with avian influenza viruses usually causes conjunctivitis (eye infection) and mild flu-like symptoms, with one notable exception, the highly pathogenic H5N1 virus which can be deadly. The first documented cases in people appeared in Hong Kong in 1997, when 18 people infected with an H5N1 virus strain were admitted to hospital, six of whom died. As of 15 November 2006, 258 reported cases of H5N1 infection in people have occurred in ten countries, Thailand, Vietnam, Cambodia, Indonesia, China, Turkey, Iraq, Azerbaijan, Egypt, and Djibouti. One hundred and fifty-three of these have been fatal.
Risk of a human influenza pandemic
"We do not know what the virus is that will cause pandemic 'flu. What we do know is that Mother Nature has the recipe book and its just a matter of time before she starts cooking,." said Sir Liam Donaldson, Chief Medical Officer. "Wherever in the world a flu pandemic starts, perhaps with its epicentre in the Far East, we must assume we will be unable to prevent it reaching the UK. When it does, its impact will be severe in the number of illnesses and the disruption to everyday life."
What is a pandemic, and what causes it?
We are used to epidemics of 'ordinary' flu, which occur seasonally, every year, around the world. An epidemic is a widespread outbreak of disease occurring in a single community, population or region. A pandemic, on the other hand, occurs on a much greater scale, spreading around the world and affecting many hundreds of thousands of people across many countries.
Three influenza pandemics occurred in the last century - 1918 to 1919 (Spanish flu), 1957 to 1958 (Asian flu) and 1968 to 1969 (Hong Kong flu). All affected large numbers of the population, causing many deaths and huge economic and social disruption. In the case of the 1918 outbreak, around 50 million people are thought to have died.
What are the most important signals that a pandemic is about to start?
The most important warning signal comes when clusters of patients with clinical symptoms of a new influenza, closely related in time and place, are detected, as this suggests human-to-human transmission is taking place. For similar reasons, the detection of cases in health workers caring for H5N1 patients would suggest human-to-human transmission.
Why are they worried about H5N1?
The H5N1 virus is one of 16 different known subtypes of influenza virus. Experts fear that the H5N1 subtype could trigger the next pandemic for several reasons. Firstly, it has already demonstrated an ability to infect people and kill - one of the key characteristics of a pandemic strain. Secondly, the virus has the ability to mutate and acquire genes from viruses infecting other species. Experts are concerned that the virus could either: adapt, giving it greater affinity for humans, or; exchange genes with a human flu virus, thereby producing a completely new virus strain capable of spreading easily between people, and causing a pandemic. Alternatively the pandemic could arise from a strain of influenza A unrelated to H5N1.
Why is this influenza virus called H5N1?
Subtypes of influenza virus are named according to two specific proteins, hemagglutinin and neuraminidase, on the surface of the virus. Hemagglutinin allows the virus to "stick" to a cell and initiate infection, while neuraminidase enables newly formed viruses to exit the host cell. Currently, there are 16 known variants of hemagglutinin protein and 9 known variants of neuraminidase proteins. This particular subtype of influenza virus has hemagglutinin type 5 and neuraminidase type 1, so it is known as H5N1.
If there was a flu pandemic, what could I do?
You can reduce, but not eliminate, the risk of catching or spreading influenza during a pandemic by: covering your nose and mouth when coughing or sneezing, using a tissue when possible; disposing of dirty tissues promptly and carefully – bag and bin them; avoiding non-essential travel and large crowds whenever possible; maintaining good basic hygiene, for example washing your hands frequently with soap and water to reduce the spread of the virus from your hands to your face, or to other people; cleaning hard surfaces (e.g. kitchen worktops, door handles) frequently, using a normal cleaning product; making sure your children follow this advice.
Are drugs effective in treating avian influenza in humans?
The recently circulating H5N1 strains are susceptible to two antiviral drugs —oseltamivir (sold as Tamiflu) and zanamivir (sold as Relenza). However, these medicines need to be started early enough—usually within the first two days of infection—to be effective. Many of the recently circulating H5N1 influenza viruses have been shown to be resistant to two older, inexpensive antiviral drugs, rimantadine and amantadine. Scientists are studying how the H5N1 viruses became resistant to these older drugs and watching for any signs of resistance to the newer drugs.
What is the status of vaccine development and production?
Vaccines effective against a pandemic virus are not yet available. Vaccines are produced each year for seasonal influenza but will not protect against pandemic influenza. Although a vaccine against the H5N1 virus is under development in several countries, no vaccine is ready for commercial production and no vaccines are expected to be widely available until several months after the start of a pandemic.
Some clinical trials are now under way to test whether experimental vaccines will be fully protective and to determine whether different formulations can economize on the amount of antigen required, thus boosting production capacity. Because the vaccine needs to closely match the pandemic virus, large-scale commercial production will not start until the new virus has emerged and a pandemic has been declared. Current global production capacity falls far short of the demand expected during a pandemic.
Is the world adequately prepared?
No. Despite the advance warning the world is ill-prepared to defend itself during a pandemic. The World Health Organisation has urged all countries to develop preparedness plans, but only around 40 have done so. WHO has further urged countries with adequate resources to stockpile antiviral drugs nationally for use at the start of a pandemic. Around 30 countries are purchasing large quantities of these drugs, but the manufacturer has no capacity to fill these orders immediately. On present trends, most developing countries will have no access to vaccines and antiviral drugs throughout the duration of a pandemic.
Sources: Department of Health, National Institutes of Health, World Health Organisation
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