Bird Flu Special

What is bird flu?
Bird flu - known technically as avian influenza - is a highly contagious viral disease affecting mostly chickens, ducks, turkeys, quails and other birds, and was first identified more than 100 years ago.
It can be caused by any one of about 20 different strains of the influenza virus. The recent outbreaks in Asia, however, have been largely caused by a highly contagious and virulent strain, known as H5N1. (The name H5N1 refers to the specific nature of two proteins, haemagglutinin and neuraminidase, found on the surface of the virus.)

Is bird flu a threat to humans?
The Asian bird flu outbreaks have been caused by a strain of influenza A called H5N1. At present, H5N1 is only slightly infectious to humans and cannot be transmitted from one human to another. However, experts fear H5N1 may evolve into a virus that could be transferred among humans. This, they say, could lead to the first flu pandemic of the 21st century.
According to the US Centres for Disease Control, the three great flu pandemics of the 20th century were the result of genetic material from bird flu viruses becoming incorporated into human flu viruses (see 'Influenza Pandemics', CDC). This led to a far more dangerous virus, which was able to rapidly spread worldwide.
There are two ways H5N1 could become a greater threat to human health. One is that the genetic material of the virus could evolve, giving rise to new, more virulent strains.
Alternatively, the virus could combine its genetic material with that from other influenza viruses that already infect humans. The more frequently humans come in contact with infected poultry, the more likely this is to happen.

Why are people worried about bird flu getting into pigs?
Both avian and human influenza viruses can infect certain animals, such as pigs. This creates a genetic 'melting pot' in which viruses can swap their genes and acquire each other's properties. For example, if a bird flu virus were to swap genes with a human flu virus, it could acquire the capacity to infect humans, and this could lead to human-to-human transmission. This could generate a new virus that would pose a greater threat to human health.

Can bird flu be treated?
The bird flu virus responsible for the recent outbreaks in Asia has been found to be resistant to the two oldest and cheapest flu drugs available, namely rimantidine and amantidine. However, researchers with Australia's Commonwealth Scientific and Industrial Research Organisation claim that flu drugs Relenza (zanamivir) and Tamiflu (oseltamivir) are effective treatments against the disease.
There is a worry, however, that antiviral drugs are expensive and in limited supply. Ira Longini at Emory University in Atlanta, Georgia, modelled Tamiflu's potential impact and came to this conclusion, according to a news report published in Nature in February 2004.
In addition, a number of companies are trying to develop a vaccine against bird flu. Such vaccines present a particular challenge, as the flu viruses against which they are intended to provide protection (by stimulating the production of virus-fighting antibodies) mutate frequently. As a result, any vaccine against a flu virus needs to be modified as the virus itself evolves.

What has been done to minimize the risk of bird flu epidemics?
Culling (killing large numbers of infected and potentially infected animals) and putting infected farms in quarantine are two common measures for limiting the spread of bird flu. According to the World Health Organisation "most influenza experts … agree that the prompt culling of Hong Kong's entire poultry population in 1997 probably averted a pandemic." (see 'Influenza pandemics: can they be averted?' in 'Avian influenza')
A third option is vaccinating birds against the virus. This remains controversial as an effective means of controlling the disease remains contentious. In 1997, the Chinese government decided to vaccinate poultry in order to limit the spread of the disease. Concerns have been raised, however, that this decision may have in fact contributed to the spread of the disease. Inefficient vaccines are a concern as they can allow the virus to replicate without the animals showing any symptoms, a phenomenon known as a 'silent epidemic'.
In July 2004, however, Indonesia launched a poultry vaccination drive, and Thailand is considering the same course of action.
The World Health Organization recommends that people at high risk of being infected by the bird flu virus - mainly those involved in culling operations - should be vaccinated with the most recent human flu vaccine available. One reason for doing this would be to minimize the chances of anybody being infected by both the bird flu and human flu viruses simultaneously. Should that happen, the two could combine their genetic material, creating a more harmful virus capable of spreading easily through the human population.
Finally, there have been suggestions that wild migratory birds play a role in spreading bird flu across large distances (see Bird flu 'could pose global threat to humans'). Despite such concerns, the UN Food and Agriculture Organization, has recommended against culling wild bird populations, arguing that there is insufficient evidence to support it.

What is the history of bird flu outbreaks? What is their impact on human health?
Bird flu outbreaks are not uncommon. Usually, however, they do not affect humans.
The oldest record of a bird flu outbreak having an impact on human health was the 1918 human flu pandemic. This, and the flu pandemics in 1957 and 1968, is believed to have been the result of a bird flu virus combining its genetic material with a human flu virus, thus becoming very infectious to humans.
In 1997, the direct transmission of bird flu virus (H5N1) from bird to humans was reported for the first time. Six people died in Hong Kong after being infected in this way.
In 1999, a different bird flu virus, identified as H9N2, infected two people in Hong Kong. They recovered, and no additional patients were reported.
In 2003, one person died from bird flu virus H5N1 in Asia, out of two reported infections. Both patients belonged to the same family from Hong Kong, and both were reported to have fallen ill after visiting mainland China.
The same year an outbreak of bird flu virus H7N7 infected 80 people, in the Netherlands, killing one. During this outbreak, there were signs of human-to-human transmission of the disease - a rare event.
Also in 2003, a child in Hong Kong was reported infected with H9N2, but recovered.
In 2004, the World Health Organisation reports 23 human casualties of H5N1 outbreaks in Asia. Eight of these were in Thailand and 15 in Vietnam. Outbreaks have also been reported among poultry in Cambodia, China, Indonesia, Japan, Laos, and South Korea, but so far none of these is reported to have infected humans.
Latest outbreak have been reported in China, Indonesia and the one that really concerns the world is Bird Flu deaths in Turkey, an alarm for Europe.
According to the World Health Organisation, flu pandemics can be expected to occur three or four times every 100 years. The WHO quotes experts as agreeing, "another influenza pandemic is inevitable and possibly imminent".

More on Bird Flu

What is the difference between bird flu and SARS?
Both bird flu and Severe Acute Respiratory Syndrome (SARS) are respiratory diseases, and both have caused widespread epidemics in Asia in the last 12 months. Both diseases are also zoonoses, meaning they originate in animals but are transferred to humans.
But while the media coverage of the bird flu and SARS epidemics has made it easy to confuse the two, SARS and bird flu are very different. Bird flu is caused by influenza viruses, similar to those that cause human flu. In contrast, SARS is caused by a previously unknown type of 'coronavirus', the same family of viruses as that which causes the common cold. Also, the SARS virus is highly dangerous to humans, whereas at present, the bird flu viruses are not.

What drugs are available for treatment?
Two drugs (in the neuraminidase inhibitors class), oseltamivir (commercially known as Tamiflu) and zanamivir (commercially known as Relenza) can reduce the severity and duration of illness caused by seasonal influenza. The efficacy of the neuraminidase inhibitors depends, among others, on their early administration ( within 48 hours after symptom onset). For cases of human infection with H5N1, the drugs may improve prospects of survival, if administered early, but clinical data are limited. The H5N1 virus is expected to be susceptible to the neuraminidase inhibitors. Antiviral resistance to neuraminidase inhibitors has been clinically negligible so far but is likely to be detected during widespread use during a pandemic.
An older class of antiviral drugs, the M2 inhibitors amantadine and rimantadine, could potentially be used against pandemic influenza, but resistance to these drugs can develop rapidly and this could significantly limit their effectiveness against pandemic influenza. Some currently circulating H5N1 strains are fully resistant to these the M2 inhibitors. However, should a new virus emerge through reassortment, the M2 inhibitors might be effective.
For the neuraminidase inhibitors, the main constraints – which are substantial – involve limited production capacity and a price that is prohibitively high for many countries. At present manufacturing capacity, which has recently quadrupled, it will take a decade to produce enough oseltamivir to treat 20% of the world’s population. The manufacturing process for oseltamivir is complex and time-consuming, and is not easily transferred to other facilities.
So far, most fatal pneumonia seen in cases of H5N1 infection has resulted from the effects of the virus, and cannot be treated with antibiotics. Nonetheless, since influenza is often complicated by secondary bacterial infection of the lungs, antibiotics could be life-saving in the case of late-onset pneumonia. WHO regards it as prudent for countries to ensure adequate supplies of antibiotics in advance.

Can a pandemic be prevented?
No one knows with certainty. The best way to prevent a pandemic would be to eliminate the virus from birds, but it has become increasingly doubtful if this can be achieved within the near future.
Following a donation by industry, WHO will have a stockpile of antiviral medications, sufficient for 3 million treatment courses, by early 2006. Recent studies, based on mathematical modelling, suggest that these drugs could be used prophylactically near the start of a pandemic to reduce the risk that a fully transmissible virus will emerge or at least to delay its international spread, thus gaining time to augment vaccine supplies.
The success of this strategy, which has never been tested, depends on several assumptions about the early behaviour of a pandemic virus, which cannot be known in advance. Success also depends on excellent surveillance and logistics capacity in the initially affected areas, combined with an ability to enforce movement restrictions in and out of the affected area. To increase the likelihood that early intervention using the WHO rapid-intervention stockpile of antiviral drugs will be successful, surveillance in affected countries needs to improve, particularly concerning the capacity to detect clusters of cases closely related in time and place.


IMPORTANT NOTES:

~ Influenza viruses are grouped into three types, designated A, B, and C. Influenza A and B viruses are of concern for human health. Only influenza A viruses can cause pandemics.

~ The H subtypes are epidemiologically most important, as they govern the ability of the virus to bind to and enter cells, where multiplication of the virus then occurs. The N subtypes govern the release of newly formed virus from the cell

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