[Sreekar]

How humans catch bird flu: Study
(source:The Times of India: Breaking news, views, reviews, cricket from across India.)

Scientists on Sunday (06/01/2008) said they had figured out how influenza viruses carried by birds latch on to humans, a discovery that may open the way to a vaccine against not just deadly avian flu but against all flu types.

There are many strains of flu virus, but only a few have succeeded in crossing the species barrier from animals to humans.

Strains known as H1 and H3 are the most common, and are especially efficient in attacking cells in the upper reaches of the respiratory system. Variants of the H5 virus, by contrast, usually remain confined to wild or domesticated fowl.

But when they do infect humans it is often with lethal results, as immune systems are unable to recognise and counter the novel pathogen.

Of 348 confirmed cases of H5N1 avian flu in the last five years, 216 -- more than 60 percent -- have died as a result, according to the World Health Organisation (WHO).

What health officials fear most is the emergence of a new H5 strain that can easily "jump" from birds to humans, potentially unleashing a pandemic on the scale of the "Spanish flu" of 1918-19 that killed tens of millions of people.

The findings, published in the British journal Nature, overhaul scientific understanding of how viruses attach themselves to cells inside human lungs.

Researchers have long known that whether an influenza strain infects humans depends on the ability of a protein on the surface of the virus, called hemagglutinin, to bind to a sugar receptor in the respiratory tract.

In humans, these receptors are known as alpha 2-6, whereas their counterparts in birds are known as alpha 2-3.

Up to now, scientists believed it was a genetic switch in the virus that allowed it to bind to human rather than bird receptors, thus making the much-feared "species jump" possible.

But the study, led by Massachusetts Institute of Technology (MIT) professor Ram Sasisekharan, says that the big factor is the shape of the sugar receptors in human lung cells.

The human alpha 2-6 receptors come in two shapes, one broadly resembling an umbrella, and the other a cone. To infect humans, flu viruses must bind to the umbrella-shaped receptors, the researchers found.

"This work enables researchers to look at flu viruses in an entirely new way," said Jeremy Berg, director of the National Institute for General Medical Sciences, which funded the research.

At the very least, the new discovery will help scientists rapidly identify strains that may develop the capacity to attack human respiratory systems.

"Now that we know what we are looking for, this could help us not only monitor the bird flu virus, but it can aid in the development of potentially improved therapeutic interventions for both avian and seasonal flu," said Sasisekharan.

Some 500,000 people around the world die every year from seasonal influenza, in which a strain mutates slightly from previous strains.

A virus that would cause a pandemic, though, would be genetically so new that immune systems and vaccines would not be primed to recognise it. The "Spanish flu" virus killed as many as 50 million people, although the toll is widely disputed.

-Thanks much
Sreekar

[nirmalaguru]

H5N1 VIRUS

Of all influenza viruses that circulate in birds, the H5N1 virus is of greatest present concern for human health for two main reasons.
First, the H5N1 virus has caused by far the greatest number of human cases of very severe disease and the greatest number of deaths. It has crossed the species barrier to infect humans on at least three occasions in recent years: in Hong Kong in 1997 (18 cases with six deaths), in Hong Kong in 2003 (two cases with one death) and in the current outbreaks that began in December 2003 and were first recognized in January 2004.

A second implication for human health, of far greater concern, is the risk that the H5N1 virus – if given enough opportunities – will develop the characteristics it needs to start another influenza pandemic. The virus has met all prerequisites for the start of a pandemic save one: an ability to spread efficiently and sustainably among humans. While H5N1 is presently the virus of greatest concern, the possibility that other avian influenza viruses, known to infect humans, might cause a pandemic cannot be ruled out.

The virus can improve its transmissibility among humans via two principal mechanisms.

The first is a “reassortment” event, in which genetic material is exchanged between human and avian viruses during co-infection of a human or pig. Reassortment could result in a fully transmissible pandemic virus, announced by a sudden surge of cases with explosive spread.

The second mechanism is a more gradual process of adaptive mutation, whereby the capability of the virus to bind to human cells increases during subsequent infections of humans. Adaptive mutation, expressed initially as small clusters of human cases with some evidence of human-to-human transmission, would probably give the world some time to take defensive action, if detected sufficiently early.

Source of Infection - All evidence to date indicates that close contact with dead or sick birds is the principal source of human infection with the H5N1 virus. Especially risky behaviours identified include the slaughtering, defeathering, butchering and preparation for consumption of infected birds. In a few cases, exposure to chicken faeces when children played in an area frequented by free-ranging poultry is thought to have been the source of infection.

Symptoms in Human infected - Initial symptoms include a high fever, usually with a temperature higher than 38oC, and influenza-like symptoms. Diarrhoea, vomiting, abdominal pain, chest pain, and bleeding from the nose and gums have also been reported as early symptoms in some patients. Watery diarrhoea without blood appears to be more common in H5N1 avian influenza than in normal seasonal influenza. The spectrum of clinical symptoms may, however, be broader, and not all confirmed patients have presented with respiratory symptoms.

H5N1 avian influenza in humans is still a rare disease, but a severe one that must be closely watched and studied, particularly because of the potential of this virus to evolve in ways that could start a pandemic.

Source - World Health Organisation(WHO - Media Centre)
For more details : WHO | Avian influenza (" bird flu") - Fact sheet