World Flu Pandemic... not this time around ! ! ?

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23 » 2006

Bird flu infection too deep to spread easily, studies say

SHARON KIRKEY CanWest News Service

Thursday, March 23, 2006

Bird flu might never get the traction it needs to touch off a feared

pandemic, suggests a sudden rush of studies that show why the disease doesn't

spread easily from human to human.

The disease - which scientists have suggested could mutate into a strain that

could become highly infectious and capable of killing many millions of people -

appears to lodge in an unusual part of the human respiratory tract, a part where

we don't normally develop flu infections unless they're serious, two teams of

researchers are reporting today.

Using human tissue samples, scientists have found that the H5N1 bird flu that

has reached three continents prefers to latch onto and invade only cells deep

down in the lungs.

As a result, it can't be spread by sneezing and coughing, they say.

That's unlike human flu, which infects cells in the nose, the sinuses, the

throat, trachea and bronchi - the main air passages to the lungs.

The finding appears to solve a medical riddle: Why have there been so few

confirmed human cases of bird flu, even in countries with massive outbreaks in

chicken and ducks?

So far, H5N1 bird flu has infected at least 183 people in eight countries,

killing 103 of them. The vast majority came from close contact with infected

birds. Five young people who died from bird flu in Azerbaijan might have fallen

sick after plucking feathers from the carcasses of swans that had been dead, but

not buried, for weeks, the World Health Organization says.

" Our findings provide a rational explanation for why H5N1 viruses rarely

infect and spread from human to human although they can replicate efficiently in

the lungs, " University of Wisconsin-Madison researchers report today in the

journal Nature.

" No one knows whether the virus will evolve into a pandemic strain, but flu

viruses constantly change, " Yoshihiro Kawaoka, the group's leader, said in a

press release issued with the study.

" Certainly multiple mutations need to be accumulated for the H5N1 virus to

become a pandemic strain. " He said samples taken from people infected with bird

flu can be searched for molecular markers that can warn if the virus is changing

into a human strain.

A Dutch group is reporting similar findings in a study published online by the

journal Science.

If H5N1 bird flu is ever to become a human strain capable of unleashing a

global pandemic, it would have to acquire the ability to reproduce in the upper

respiratory tract, where it could be easily spread via virus-infected droplets

that are coughed or sneezed. That might be happening: The H5N1 bird flu that

caused an outbreak in humans in Vietnam in 2003 and 2004 was different from the

1997 strain that hit Hong Kong.

Some of the 2004 versions had mutations that allowed binding to receptors, or

molecules on the surface of cells, higher up in the airways.

" Those mutations were also seen in some of the kids who died in Turkey, " said

Earl Brown, a University of Ottawa virologist who has been tracking the

evolution of H5N1 viruses. Four children died in an outbreak in Turkey in

January.

Luckily, those viruses were buried with the people they killed.

It was already known that bird flu and human flu bind to different receptors

on the cells in the body, like a key fitting into a lock. If a virus can't get

into a cell, it can't churn out copies and infect other cells.

" What's new and what's interesting is that they looked at where those

receptors are distributed throughout the respiratory tract, " said Offit,

chief of infectious diseases at the Children's Hospital of Philadelphia and an

adviser to the U.S. government on vaccine policy.

The Dutch team found H5N1 virus attached abundantly to cells lining the

alveoli, the microscopic air sacs deep within the lungs, and the bronchioles,

tiny tube-shaped ducts connected to the alveoli.

The higher up the respiratory tract, the fewer receptors there were for bird

flu.

That means " no matter how hard you cough, you're not getting (bird flu) virus

that is that far down up into your trachea, " Offit said.

skirkey@...

© The Gazette (Montreal) 2006