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Avian & Pandemic Flu
What is Avian Flu (“Bird Flu”)?
Avian flu is an infection of birds, caused by a “type A” strain of the influenza virus. It is not new – it has been known for more than 100 years; it can occur all over the world and all bird species are believed to be susceptible to it.
It is normally only transmissible between birds, among which it causes a spectrum of illnesses ranging from minor to rapidly fatal disease, sometimes killing entire bird populations.
The current outbreak, which began in 1997, is the worst outbreak ever recorded. It has affected many millions of birds, and the strain responsible has proved to be an especially dangerous and virulent one.
Why are there so many strains of flu, and what’s the difference?
A hallmark of flu viruses (human, bird and other animal varieties) is their ability to evade immune defences over time by making changes to the proteins that are present on their surface coats.
The outer surface of the flu virus contains two main types of protein, that interact with the host’s immune system. These are called haemagglutinin and neuraminidase respectively – known as “H” and “N” proteins for short. (These are the “spikes” visible on the outer surface of the virus, as seen on electron micrograph images.)
Each of these proteins exists in the form of several possible variants, resulting in vast numbers of possible combinations and therefore vast numbers of possible strains. Immunity is normally specific to each individual strain.
Over a period of time, gradual, subtle changes in the character and combination of these protein variants (technically known as “antigenic drift”) enables flu viruses to always keep one step ahead of the their host’s immune system – which is why new flu vaccines have to be produced every year against each new season’s predominant strains.
H5N1 Influenza
The strain of avian flu now causing special concern is a subtype called H5N1. There are several reasons why this strain is especially important:
• H5N1 has a known ability to mutate easily into other types
• It has a documented ability to acquire genes from viruses that infect other animal species
• It has a known ability to infect humans as well as birds
• When humans have been infected, the illness has been very severe or even lethal (56% mortality)
• Birds that survive infection with this strain can go on to spread the virus orally and faecally for a period of up to ten days and can spread infection over a wide area, both by migration and through contact with large numbers of birds at live poultry markets.
What is the current state of the H5N1 outbreak?
The current outbreak was first evident in Hong Kong in 1997 and then resurfaced in Korea in mid-December 2003. Since then it has spread to several other countries. It is possible that the problem had been escalating in China and Indonesia for very much longer than this however and may even have been deliberately covered up.
The geographic spread of H5N1 has now been carefully documented (see our Links page for the latest information).
The spread of H5N1 is relentless and ongoing, despite the fact that only the most newsworthy milestones in its spread receive prominent media attention. The more birds that become infected, the greater the potential for spread between birds and humans.
How is Avian flu spread to humans?
With a tiny number of exceptions, human cases of H5N1 avian flu have so far resulted from direct contact between infected birds and humans.
The exceptions were a single, close family contact in Vietnam and a small cluster of cases (again involving close family contact) in Indonesia.
Birds excrete the flu virus in faeces. Transmission is believed to occur through inhalation and/or ingestion. Correctly cooked poultry is not capable of spreading the virus.
The major symptoms are fever, sore throat, cough and ultimately severe respiratory distress (shortness of breath, difficulty breathing) and pneumonia. Laboratory tests are available to confirm the condition.
Treatment
Antiviral drugs are available both for treatment of clinical cases, and for prevention of illness in people who have been exposed to cases. Drugs are most effective when given early (ideally within 12 hours) of the onset of symptoms.
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