Bird flu can’t spread easily in humans: expert

By Patricia Reaney

LONDON (Reuters) – Scientists said on Wednesday they may
have uncovered why the H5N1 avian flu that is so lethal in
birds has not been able to spread easily among humans.

It is because bird flu viruses attach to receptors, or
molecules on cells, in different regions of the respiratory
system from human influenza viruses.

Receptors act like doorways that allow the virus to enter
the cell, multiply and infect other cells. Humans have
receptors for avian viruses, including H5N1, but they are found
deep within the lungs.

Cells in the upper airway in humans lack the receptors
targeted by avian flu viruses, which limit their ability to
spread from person to person.

“For the viruses to be transmitted efficiently, they have
to multiply in the upper portion of the respiratory system so
that they can be transmitted by coughing and sneezing,” said Dr
Yoshihiro Kawaoka, a virologist at the University of
Wisconsin-Madison, who led the research team.

The H5N1 bird flu virus has killed 103 people and infected
184 since late 2003. People infected with the virus, which has
spread from Asia to Europe, the Middle East and Africa, have
had close contact with diseased birds.

Scientists fear the virus could mutate into a pandemic
strain that could become highly infectious and capable of
killing many millions of people.

“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,” Kawaoka and his
team said in a report in the journal Nature.

MULTIPLE MUTATIONS

Professor John Oxford, a virologist at St Bartholomew’s and
the Royal London Hospital, Queen Mary’s School of Medicine and
Dentistry in London, said the research provides a reasonable
explanation for the small number of human cases.

“This gives some meaning to the great conundrum at the
moment, the fortunate conundrum of why this virus is not going
from human to human,” Oxford said in an interview

“It looks like a deep scientific study into that and we
need more of those studies.”

Kawaoka and a team of researchers in Japan infected human
tissue with bird flu viruses. Their findings suggest that
strains of H5N1 circulating in birds would have to undergo
several key genetic changes to become easily transmissible in
humans.

“The virus has to mutate to recognize human virus receptors
which are in the throat,” Kawaoka told Reuters.

“Certainly, multiple mutations need to be accumulated for
the H5N1 virus to become a pandemic strain.”

Scientists do not know how far the H5N1 has mutated to
become a pandemic strain but Kawaoka’s findings show the
importance of looking for changes in virus recognition of human
receptors.

The changes or mutations must occur in the hemagglutinin
protein, the “H” in the virus designation, for avian H5N1
viruses to recognize human receptors, according to the
researchers.

“Identification of H5N1 viruses with the ability to
recognize human receptors would bring us one step closer to a
pandemic strain,” Kawaoka added.