Bird flu evolving into human virus
9 October 2007
The H5N1 avian flu virus, or 'bird flu' first appeared
in Hong Kong in 1997 and has been slowly evolving into a pathogen better
equipped to infect humans. A new study by a team of researchers from the US,
Japan and Vietnam, has identified that a single change in a protein will
enable it to infect the respiratory tract — a first step to spreading
quickly from person to person.
The final form of the virus, researchers
fear, will be a highly pathogenic strain of influenza that spreads easily
Reporting in the journal Public Library of Science
Pathogens (1), a team of researchers led by virologist
Yoshihiro Kawaoka of the University's School of Veterinary Medicine has
identified a single change in a viral protein that facilitates the virus'
ability to infect the cells of the upper respiratory system in mammals. By
adapting to the upper respiratory system, the virus is capable of infecting
a wider range of cell types and is more easily spread, potentially setting
the stage for a flu pandemic.
"The viruses that are in circulation now are
much more mammalian-like than the ones circulating in 1997," says Kawaoka,
an internationally recognized authority on influenza. "The viruses that are
circulating in Africa and Europe are the ones closest to becoming a human
As its name implies, bird flu first arises in chickens and other
birds. Humans and other animals in close contact with the birds may be
infected, and the virus begins to adapt to new host animals, a process that
may take years as small changes accumulate. Over time, an avian virus may
gather enough genetic change to spread easily, as experts believe was the
case with the 1918 Spanish flu, an event that killed at least 30 million
In the new study, which was conducted in mice, the
Wisconsin team identified a single change in a viral surface protein that
enabled the H5N1 virus to settle into the upper respiratory system, which
"may provide a platform for the adaptation of avian H5N1 viruses to humans
and for efficient person-to-person virus transmission."
undetermined changes are required for the virus to become a human pathogen
of pandemic proportions, Kawaoka explains, but establishing itself in the
upper respiratory system is necessary as that enables easy transmission of
the virus through coughing and sneezing.
To date, more than 250 H5N1 human
infections worldwide have been reported. Of those, more than 150 have been
fatal, but so far efficient human-to-human transmission has not occurred.
Most infections have occurred as a result of humans being in close contact
with birds such as chickens that have the virus.
According to Kawaoka, the
avian virus can be at home in the lungs of humans and other mammals as the
cells of the lower respiratory system have receptors that enable the virus
to establish itself. Temperatures in the lungs are also higher and thus more
amenable to the efficient growth of the virus.
The new study involved two
different viruses isolated from a single patient — one from the lungs, the
other from the upper respiratory system. The virus from the upper
respiratory system exhibited a single amino acid change in one of the key
proteins for amplification of influenza virus genes.
The single change
identified by the Wisconsin study, says Kawaoka, promotes better virus
replication at lower temperatures, such as those found in the upper
respiratory system, and in a wider range of cell types.
"This change is
needed, but not sufficient," Kawaoka explains. "There are other viral
factors needed to cause a viral pandemic strain of bird flu."
Kawaoka and other flu researchers are convinced it is only a matter of time,
as more humans and other animals are exposed to the virus, before H5N1 virus
takes those steps and evolves into a virus capable of causing a pandemic.
1. Hatta M, Hatta Y, Kim JH, Watanabe S,
Shinya K, et al. Growth of H5N1 Influenza A Viruses in the Upper
Respiratory Tracts of Mice. PLoS Pathog
The full paper is available at:
In addition to Kawaoka, authors of the study include
Masato Hatta, Yasuko Hatta, Jin Hyun Kim, Shinji Watanabe of the UW-Madison
School of Veterinary Medicine; Kyoko Shinya of Tottori University, Japan;
Tung Nguyen of the Vietnamese National Centre for Veterinary Diagnostics;
Phuong Song Lien of the Vietnam Veterinary Association; and Quynh Mai Le of
the Vietnamese National Institute of Hygiene and Epidemiology.
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