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Bird (Avian) Flu Prevention, Treatment & Tamiflu

The avian or bird flu has been making news recently as increasing numbers of domestic and wild birds in Asia and later in Europe tested positive for the feared H5N1 influenza virus. In addition, several cases of human transmission of this virus were documented, with a very high mortality rate, raising fears that we are on the threshold of a major influenza pandemic that has the potential to kill millions of people around the world.

A History of Avian / Bird Flu

We know today the bird (avian) flu has been around for a while. There have been three influenza pandemics.

The first one, also called the Spanish flu, killed more people in 1918 than the bubonic plague. The 1918 virus was an influenza A (H1N1) virus whose origin is likely to have been a mutated H1N1 swine or avian virus.

The second and third bird flu pandemics happened in 1957 and 1968, caused by viruses that contained components of previous human and avian influenza viruses.

In 1997, 18 people developed respiratory disease in Hong Kong during an outbreak of H5N1 influenza in live-bird markets. Almost two thirds of these patients developed pneumonia, half of them required intensive care treatment and one third died as a result of the disease. This was clearly a very aggressive virus.

Avian / Bird Flu Virus Primer

There are three major categories of influenza viruses: A, B and C. Type A viruses are further classified in subtypes based on two glycoproteins - hemagglutinin (HA) and neuraminidase (NA) -embeded in the virus surface membrane. Type A flu viruses are responsible for the annual outbreaks of influenza and also for the flu pandemics, when they occur.

You may wonder why most of the time we only have epidemics, while pandemics are - fortunately - a rather rare occurrence?

The answer lies in a very interesting characteristic of the influenza viruses: they are dynamic, changing continuously. To understand this changes, think of the virus replicating mechanism as an inaccurate copying machine that places wrong letters every once in a while. As a result of these so-called "point mutations", the virus changes in time. This is a gradual process, called antigenic drift. As you may have guessed, it is the antigenic drift that is responsible for the annual need for another influenza vaccine. By the time another cold season arrives, the flu viruses circulating have changed enough that last year's antibodies you developed after being vaccinated are no longer able to protect you... And it is exactly this process that makes the annual flu epidemics so common.

Now, there is another change that only type A influenza viruses can undergo. Called antigenic shift, it involves a more abrupt change of the virus. In fact, antigenic shift is made possible by a rare combination of two different strains of type A influenza viruses. The result is an entirely new subtype A flu virus, to which everybody is essentially susceptible, making influenza pandemics possible.

Although not entirely clear, it is postulated that the genetic reassortment underlying antigenic shift takes place in persons infected with the two subtypes of flu viruses at the same time. Another - more likely, in my opinion - possibility is that the shift takes place in intermediary hosts (animals). Pigs, for example, are known to possess in their trachea receptors for both avian and human influenza viruses. They are an ideal host for influenza genetic reassortment. It is no surprise experts think the Spanish flu had its origin in swine.

Avian / Bird Flu Transmission

I remember talking to a friend from Romania recently, after the first cases of avian flu were confirmed in the Romania Danube delta villages. He was wondering whether he should give up eating poultry to avoid becoming infected with bird flu... While giving up meat can decrease your risk of cancer and heart disease, bird flu is not transmitted by eating bird meat - unless, of course, one eats it raw.

The usual bird flu viruses don't cross the species barrier very easily. They are passed easily from bird to bird via surfaces contaminated with saliva, feces or nasal secretions from infected birds. I should mention here that bird flu, as other diseases, is partly the result of human greed combined with poverty: past avian flu outbreaks have originated in Southeast Asia, where agricultural practices allow humans, pigs and poultry to live in close proximity, facilitating virus reassortment.

Bird flu infections in humans usually happen in people working closely with infected birds. Infected humans do not transmit the bird flu virus to other humans, although there are a few, isolated reports of human-to-human transmission. As explained above, the fear is that a new subtype of influenza virus might develop in a host coinfected with both the bird and another strain of flu virus, and this new strain might be easily transmissible among humans. Should that happen, another influenza pandemic would inevitably follow...

To be continued...

Dr Gily

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