Bird Flu Briefing16th February 2007
As the spectre of 'bird flu' or avian influenza reaches our shores we take a look at its history and consider the likelihood of a pandemic.
1 What is bird flu?
Avian influenza or 'bird flu' is a highly contagious bird disease, caused by influenza A viruses. In birds, the viruses can present with a range of symptoms from mild illness and low mortality, to a highly contagious disease with a near 100% fatality rate. 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 of illness, 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.
2 Where has the disease come from?
Avian influenza known as ‘fowl plague’, first appeared in Italy in the late 1870s, and the US in 1924-25.
Prior to the present situation, outbreaks of highly pathogenic avian influenza in poultry were considered rare. Since 1959, only 24 outbreaks of highly pathogenic avian influenza had been recorded worldwide. The majority have had limited geographical spread with a few confined to a single farm or flock; only one spread internationally.
However in 2003, a serious outbreak occurred in the Netherlands and spread to Belgium and Germany. More than 250 farms were affected and 28 million poultry slaughtered. In 2004 the H5N1 strain of avian influenza surfaced in South East Asia and spread west through Europe and Africa. Cases were also reported in the US and Canada, but these were attributed to different strains of the virus – H2N2 and H7.
3 How does it spread?
The bird flu virus can be spread by:
• Direct contact with secretions from infected birds, especially faeces.
• Contaminated vehicles, equipment, personnel, clothing, water or feed.
In addition, the virus can remain viable in contaminated droppings for long periods and this helps it to spread either through ingestion or inhalation.
Some scientists also believe that migratory waterfowl / wild birds can transmit H5N1 to domestic poultry. It is known that wild flocks suffer from the virus, and it is thought that they can pass the virus on to domestic poultry as they migrate. This has yet to be proved scientifically.
4 What are the different strains of the virus?
There are many different subtypes of influenza A virus. The most virulent are called highly pathogenic avian influenza (HPAI) and can reach epidemic levels among birds. Of these, subtype H5 is of greatest concern to human health. There are 9 different forms of H5 ranging from highly pathogenic to relatively harmless. However it is the strain H5N1, and 4 associated sub types, which causes greatest concern. All are deadly to birds, and can cause disease - and death - in humans.
The bird flu virus currently affecting poultry in Britain and some parts of Asia and other areas is the highly pathogenic H5N1 strain of the virus.
5 Why are we so worried about H5N1?
There is most concern about the H5N1 virus because:
• It is able to cross the species barrier from birds to humans.
• It has caused the greatest number of cases of infection and deaths.
Consequently it has the potential to develop into a flu pandemic, although at present H5N1 is not able to spread easily to / between humans. However the characteristics of this virus could change over time, and give it the ability to spread easily within the human population.
This could occur if:
• The virus mutates and improves its’ ability to adhere to human cells.
• Genetic material is exchanged between human and avian viruses during co-infection of a human or pig. A process known as ‘reassortment’.
6 How do humans catch bird flu?
Bird flu was thought only to infect birds until the first human cases were seen in Hong Kong in 1997.
H5N1 is able to infect people because it is able to cross the species-barrier. 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.
Pigs are thought to be important to the potential transmission of the virus because they can host both bird and human ‘flu. Scientists believe that they could act as ‘mixing vessels’ for the reassortment of the virus.
Human infection with avian influenza viruses usually causes mild conditions such as conjunctivitis (eye infection) and mild flu-like symptoms, with one notable exception, the highly pathogenic H5N1 virus. More severe infection can lead to pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.
7 How many people have caught bird flu?
Since the recent outbreak of bird flu in 2003, the World Health Organisation has been notified of 271 cases of human infection, resulting in 165 deaths worldwide, as at the beginning of February 2007. This comprises laboratory confirmed cases only.
Reports have been made by the following countries:
All human cases have coincided with outbreaks of highly pathogenic H5N1 avian influenza in poultry. To date, Vietnam has been the most severely affected country, with 93 cases and 42 deaths.
8 Could the virus mutate?
Between 1918–1920, it is estimated that 50 – 100m people died in the Spanish Flu pandemic. The disease is thought to have affected 20% of the world’s population, killing approx. 2.5 – 5% of sufferers. This pandemic was caused by H1N1, a subtype of the species Influenza A virus (avian flu virus).
Research into the H1N1 genome has indicated a relatively small number of changes can have disastrous consequences. When the 1918 virus was compared with today's human flu viruses, scientists noted that only 25 - 30 of the virus's 4,400 amino acids had changed. However these changes allowed the virus to spread easily through the human population.
In 2003, world-renowned virologist Robert Webster published an article titled "The world is teetering on the edge of a pandemic that could kill a large fraction of the human population" in American Scientist. He called for adequate resources to fight what he sees as a major world threat to possibly billions of lives. On September 29, 2005, David Nabarro, the newly-appointed Senior United Nations System Coordinator for Avian and Human Influenza, warned the world that an outbreak of avian influenza could kill anywhere between 5 million and 150 million people.
Many governments worldwide are preparing for a pandemic, and the UN ands WHO are monitoring the situation on an ongoing basis.
9 Is a vaccine available?
At present there is no specific vaccine available, although several products are under development. Only antiviral drugs, such as Tamiflu, are available – these may help limit symptoms and reduce the spread of the disease if a pandemic occurs. The Government has stockpiled sufficient drugs to treat 25% of the population if a pandemic occurs. However reports from Vietnam suggest that patients have become partially resistant to Tamiflu.
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