Flu viruses can quickly swap genes -study

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) – Strains of the influenza virus are
constantly swapping genes among themselves and giving rise to
new, dangerous strains at a rate faster than previously
believed, U.S. researchers reported on Monday.

They found that slightly mutated influenza A strains in New
York that circulated between 1999 and 2004 gave rise to the
so-called Fujian strain that caused a troublesome outbreak in
the 2003-2004 flu season.

Such events probably are what lead to the occasional
pandemics of flu that can kill millions of people, David Lipman
and colleagues at the National Institutes of Health found.

They hope their findings, published in the journal Public
Library of Science Biology, will help scientists better predict
which viral strains will attack during upcoming flu seasons and
design better vaccines.

Influenza viruses are notorious for trading genes back and
forth and mutating. Scientists previously believed that the
gene swapping occurred gradually but the new study shows that
several genes can be exchanged at once, causing sudden changes
in important characteristics of the virus.

This is why a new flu epidemic sweeps the world every year,
killing between 250,000 and 500,000 globally and 36,000 people
in the United States alone every year.

Each year, experts must predict which strains will be most
common and design a new vaccine to fight them. Some years, such
as in 2003-2004, the vaccine does not include the most common
strain.

Lipman and colleagues sequenced the genomes of 156
influenza A viruses, named H3N2, that were collected by New
York State public health officials between 1999 and 2004.

“We found that there are co-circulating minor variants that
are not infecting many people,” Lipman said in a statement.
“One of these can cause the next major epidemic.”

They found “at least four reassortment events occurred
among human viruses during the period 1999-2004″ — meaning
there was an exchange of genes four different times.

LURKING UNDER THE RADAR

The newly mixed viruses, previously unnoticed because of
their low virulence, suddenly became capable of infecting
thousands of people.

This suggests that scientists need to study circulating flu
viruses more carefully because important mutations can occur
suddenly and without warning, the researchers said.

Experts say a new and deadly flu pandemic is certain to
come but it is impossible to predict when. The H5N1 avian flu
virus, which arrived in Asia in late 2003, has so far killed
more than 50 people in the region including Vietnam, Thailand
and Cambodia.

It does not easily pass from person to person yet but
health officials say it can acquire this ability at any time
and if it does, it could kill millions.

A second study, published in the Proceedings of the
National Academy of Sciences, found that an early wave of the
1918 “Spanish Flu” pandemic may have hit New York City several
months before a big epidemic exploded globally.

The 1918-1919 pandemic was the worst in recorded history,
killing as many as 40 million people.

An outbreak at the end of the previous flu season may have
killed 3,000 children and young adults, Donald Olson of the New
York City Department of Health and colleagues found.

“The historical lesson from 20th-century influenza
pandemics is that they occur in multiple waves,” Olson said in
a statement.