Bird Flu - WHO view on global threat

Futurist Topics - Health

Article Index
Bird Flu - WHO view on global threat
Page 2
Page 3
All Pages

Bird Flu information summarised from WHO

What is Avian Influenza? Avian influenza is an infectious disease of birds caused by type A strains of influenza virus. The disease was first found in Italy more than 100 years ago, but can occur worldwide.

All kinds of birds can catch avian influenza, though some species are more resistant than others. Infection causes many different symptoms in birds, ranging from mild illness to highly infectious and rapidly fatal disease resulting in severe epidemics. This more lethal can produce sudden illness and death in almost all cases.

How many kinds of Bird Flu are there? Fifteen variants of influenza virus are known to infect birds. Up until April 2005, all outbreaks of the highly pathogenic form were caused by influenza A viruses of subtypes H5 and H7.

Migratory waterfowl - mainly wild ducks - are the natural reservoir of avian influenza viruses, and these birds have some immunity to them. Domestic poultry, including chickens and turkeys, are particularly vulnerable to epidemics of rapidly fatal influenza.

Direct or indirect contact of domestic flocks with wild migratory waterfowl is a frequent cause of epidemics. Live bird markets have also played an important role in spread.

Mild strains can rapidly mutate into lethal ones as they spread throuhg poultry. During a 1983-1984 epidemic in the United States of America, the H5N2 virus initially caused few deaths, but within six months became highly dangerous, with death rates approaching 90%. Control required destruction of more than 19 million birds at a cost of nearly US$ 65 million.

During a 1999-2001 epidemic in Italy, the H7N1 virus was first mild in impact, but rapidly became highly dangerous. More than 13 million birds died or were destroyed.

Strict quarantine (isolation) of infected farms and destruction of infected or potentially exposed flocks are standard control measures aimed at preventing spread to other farms and across an entire country.

Apart from being highly infectious, avian influenza viruses are easily spead from farm to farm by contaminated equipment, vehicles, feed, cages, or clothing.

Highly dangerous Bird Flu viruses can survive for long periods in the environment, especially when temperatures are low. Strict hygiene and disinfection measures on farms can, however, help prevent this.

If you don't act fast at the beginning, and monitor very carefully for fresh outbreaks,, epidemics can last for years. For example, an epidemic of H5N2 avian influenza began in Mexico in 1992, as a mild virus, evolved to the highly fatal form, and was not controlled until 1995.

A constantly mutating virus: two results

All type A influenza viruses, including those that regularly cause seasonal epidemics of influenza in humans, are genetically unstable and well adapted to host defenses. Influenza viruses lack mechanisms for "proofreading" and repair of errors that occur during replication. As a result of these uncorrected errors, the genetic composition of the viruses changes as they replicate in humans and animals, and the existing strain is replaced with a new antigenic variant. These constant, permanent and usually small changes in the antigenic composition of influenza A viruses are known as antigenic "drift".

The tendency of influenza viruses to undergo frequent and permanent antigenic changes necessitates constant monitoring of the global influenza situation and annual adjustments in the composition of influenza vaccines. Both activities have been a cornerstone of the WHO Global Influenza Programme since it began work in 1947.

influenza A viruses, including subtypes from different species, can swap or "reassort" genetic materials and merge. This is, known as antigenic "shift", and the result is a novel subtype different from both parent viruses. As populations have no immunity to the new subtype, and as no existing vaccines can confer protection, antigenic shift has often resulted in highly lethal pandemics. For this to happen, the new subtype needs to have genes from human influenza viruses that make it highly infectious for a long period.

Conditions favourable for the emergence of antigenic shift have long been thought to involve humans living in close proximity to domestic poultry and pigs. Because pigs are susceptible to infection with both avian and mammalian viruses, including human strains, they can serve as a "mixing vessel" for scrambling of genetic material from human and avian viruses, resulting in new mutations.

Evidence is mounting that, for at least some of the 15 avian influenza virus subtypes circulating in bird populations, humans themselves can be such a"mixing vessel".

History of Human infection with avian influenza viruses

The first documented infection of humans with an avian influenza virus was in Hong Kong in 1997, when the H5N1 strain caused severe respiratory disease in 18 humans, of whom 6 died. This outbreak coincided with an epidemic of highly pathogenic avian influenza, caused by the same strain, in Hong Kong's poultry population.

Investigation showed that close contact with live infected poultry was the source of human infection - virus jumped directly from birds to humans. Limited transmission to health care workers occurred, but did not cause severe disease.

Rapid destruction - within three days - of Hong Kong's entire poultry population, estimated at around 1.5 million birds, reduced opportunities for further direct transmission to humans, and may have averted a pandemic.

That event alarmed public health authorities, as it was the first time that an avian influenza virus was transmitted directly to humans and caused severe illness with high death rates. Iin February 2003, an outbreak of H5N1 avian influenza in Hong Kong caused 2 cases and 1 death in members of a family who had recently travelled to southern China. Another child in the family died during that visit, but the cause of death is not known.

Two other avian influenza viruses havecaused illness in humans. An outbreak of highly pathogenic H7N7 avian influenza began in the Netherlands in February 2003, causing the death of a vet two months later, and mild illness in 83 other people. Mild cases of avian influenza H9N2 in children occurred in Hong Kong in 1999 (two cases) and in mid-December 2003 (one case). H9N2 is not highly pathogenic in birds.

The most recent cause for alarm occurred in January 2004, when laboratory tests confirmed the presence of H5N1 avian influenza virus in human cases of severe respiratory disease in the northern part of Viet Nam.

Why people are worried about H5N1

Of the 15 avian influenza virus subtypes, H5N1 mutates rapidly and is good at acquiring new genes from viruses infecting other animal species. It is known to cause severe disease in humans. Birds that survive infection excrete virus for at least 10 days, orally and in faeces, so further spread is a great risk at live poultry markets and by migratory birds.

The epidemic of highly pathogenic avian influenza caused by H5N1, which began in mid-December 2003 in the Republic of Korea and is now being seen in other Asian countries. H5N1 variants demonstrated a capacity to directly infect humans in 1997, and have done so again in Viet Nam in January 2004. The spread of infection in birds increases the opportunities for direct infection of humans, with growing risks of a new world-wide flu epidemic if viruses mix in humans.

Several measures can help minimize the global public health risks that could arise from large outbreaks of highly pathogenic H5N1 avian influenza in birds. First - urgent need to halt further spread of epidemics in poultry populations, and reduce opportunities for human exposure to the virus. Vaccination of persons at high risk of exposure to infected poultry, using existing vaccines effective against currently circulating human influenza strains, can reduce risk of co-infection of humans with avian and influenza strains, and risk that genes will be exchanged. Workers involved in the culling of poultry flocks must be protected, by proper clothing and equipment, against infection. These workers should also receive antiviral drugs as a prevention measure.

When cases of avian influenza are seen in humans, information must be made available quickly. Thorough investigation of each case is also essential.

Clinical course and treatment

In the 1997 Hong Kong outbreak, patients developed symptoms of fever, sore throat, cough and, in several fatal cases, severe respiratory distress secondary to viral pneumonia. Previously healthy adults and children, and some with chronic medical conditions, were affected.

Tests for diagnosing all influenza strains of animals and humans are rapid and reliable. Many laboratories havehigh-security facilities and reagents for performing these tests as well as experience. Rapid bedside tests for the diagnosis of human influenza are also available, but do not tell us if the virus is spreading between humans.

Antiviral drugs, some of which can be used for both treatment and prevention, have some effect against influenza A virus strains. Some of these drugs are expensive and supplies are limited.

Experience in the production of influenza vaccines is considerable. However, at least four months would be needed to produce a new vaccine, in significant quantities, capable of conferring protection against a new virus subtype.

For more see http://www.who.org

< Prev		Next >