Avian influenza, or “bird flu” is a contagious viral disease that infects birds. Avian influenza viruses are species specific but do occasionally cross the species barrier to infect humans and other mammals. In poultry, avian influenza causes two main forms of the disease. These are known as the low and high pathogenic forms.

• The low pathogenic form generally causes only mild symptoms, for example reduced egg production, and may go un-noticed by the farmer or keeper.
  
• By contrast the high pathogenic form causes severe symptoms with the rapid death of individual birds. It spreads quickly within flocks and leads to very high overall mortality rates.

Highly pathogenic avian influenza has probably infected birds for millions of years, however it has only been noted as a specific disease within the last century or so. Prior to the outbreak at a turkey farm in Suffolk in February 2007, the last case recorded in a commercial flock in the UK had been in 1991 (also turkeys but in Norfolk).

Type A viruses are of particular interest because of their ability to cause high pathogenic influenza in poultry and to seriously affect human health.

Type A influenza viruses are divided into “H“ and “N” subtypes. There are a total of 16 H subtypes and 9 N subtypes but only two of the H subtypes are capable of causing highly pathogenic avian influenza; these being known as H5 and H7. However, H5 and H7 do not automatically cause high pathogenic influenza as both can circulate in poultry populations in the low pathogenic form before periodic mutation into the highly pathogenic form. The presence of the H5 and H7 subtypes is therefore of concern even if there are no obvious signs of influenza.

Virus spread
The mechanisms of spread of avian influenza are not fully understood but it is believed that wild waterfowl which naturally carry type A viruses can or may introduce the subtypes into poultry flocks where they can mutate into the highly pathogenic form. Transmission of the virus from bird to bird is usually a result of direct contact with the secretions from infected birds, especially their faeces.

International Situation
In 2003 a new highly pathogenic avian influenza emerged in South East Asia. Caused by the H5N1 virus, outbreaks were recorded throughout the first half of 2004 in China, South Korea, Thailand, Vietnam and Indonesia. Further spread of the virus occurred throughout the region and beyond so that parts of Russia and Eastern Europe were affected by late 2005. By early 2006 further outbreaks had occurred and the extent of spread had widened to cover parts of Western Europe and Africa.

The current worldwide outbreak is characterised by its rapid spread and persistence despite the destruction of millions of birds in an attempt to control the virus. This may be in part due to recent changes in H5N1 which now appears to be able to circulate longer in the environment. It also appears that both domestic ducks and wild waterfowl are now carrying the highly pathogenic form of the virus without symptoms and therefore acting as both reservoir and agents for direct transfer.

Control of avian influenza
The principle mechanism for the control of avian influenza is through the slaughter and incineration of infected birds, and other birds that may have been in close contact with them. In some countries vaccination is also adopted however the circumstances for vaccine use will vary according to both local and national conditions at the time.

Avian influenza and human health
H5N1 carries risks for human health in two ways.

In the first instance the virus may pass direct from poultry to humans. This is a process known as crossing the species barrier that to date has proved extremely rare. It is therefore considered to be of low risk to human health.

As at 3rd February 2007 a total of 271 cases (recorded worldwide since the start of the current outbreak) had resulted in 165 deaths - the majority being in Indonesia and Vietnam. It is believed that in all nearly all of these cases, those involved had been infected through direct close contact with infected birds or their contaminated faeces.

In the second instance there is a hypothetical, but potentially much greater risk to human health. In this instance the virus may change into a form that can spread more easily from person to person. Although deemed hypothetical, the chances of it occurring are growing.

With the H5N1 virus now endemic in parts of South East Asia, there is a risk that the avian influenza virus may exchange genetic material (during simultaneous infection) with a human influenza virus, resulting in a “reassortment” event. Such a reassorted virus could be fully transmissible, cause serious illness and in the absence of human resistance, lead quickly to a pandemic.

Although reassortment is the most likely cause of change in H5N1, adaptive mutation of the virus is also possible. Adaptive mutation is a more gradual process where the ability of the virus to infect humans grows with subsequent infections. Eventually it can become sufficiently infectious that a pandemic results.

The severity of a pandemic
The severity of any pandemic cannot be established prior to the emergence of a virus and determination of its influenza characteristics. However, we know that in the past pandemics have infected as much as a third of the population and resulted in mortality rates as high as 2.5%. The most well known pandemic (Spanish Flu – from its place of origin) spread throughout the world in 1918 killing at least 40 million people, more than had died throughout the whole of the First World War. Unusually, Spanish Flu particularly affected young and middle aged adults, rather than the usually susceptible groups of children and the elderly.

It is because of the potential severity of a pandemic that the issue of avian influenza has come to the forefront of human health planning at both the national and international level. This has resulted in co-operation in the search for potential vaccines (difficult until a pandemic strain of the virus emerges) and the waiving of patent rights to allow the manufacture and stockpiling of certain antiviral agents.

Avian influenza in the UK
Avian influenza is a notifiable disease and suspected cases in the UK must be notified to DEFRA. In the event of an outbreak the Government’s Animal Disease Contingency Plan would come into force where disease control would include killing birds on infected premises and those premises likely to be contaminated by a direct link with the infected one. Movement controls around the infected premises would also apply.

To facilitate information flow to poultry keepers and to ensure up to date information as to the location of all commercial flocks, DEFRA established a Poultry Register in February 2006 for poultry, gamebirds and certain other bird species. Registration for all commercial flocks with more than 50 birds is required by law.

Further information:

DEFRA - avian influenza
World Organisation for Animal Health
World Health Organisation

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