Swine flu H1N1 virus more virulent than previously thought
23 July 2009
A new, highly detailed study of the H1N1 flu virus shows that the
pathogen is more virulent than previously thought.
Writing in a fast-tracked report published in the journal Nature,
an international team of researchers led by University of
Wisconsin-Madison virologist Yoshihiro Kawaoka provides a detailed
portrait of the pandemic virus and its pathogenic qualities.
In contrast with run-of-the-mill seasonal flu viruses, the H1N1 virus
exhibits an ability to infect cells deep in the lungs, where it can
cause pneumonia and, in severe cases, death. Seasonal viruses typically
infect only cells in the upper respiratory system.
“There is a misunderstanding about this virus,” says Kawaoka, a
professor of pathobiological sciences at the UW-Madison School of
Veterinary Medicine and a leading authority on influenza. “People think
this pathogen may be similar to seasonal influenza. This study shows
that is not the case. There is clear evidence the virus is different
than seasonal influenza.”
The pandemic H1N1 flu virus (red) has been shown to
be more virulent than scientists previously believed. The filamentous
shape of the virus, which in this image have recently budded from
infected cells, is also unusual.
Photo: courtesy Yoshihiro Kawaoka, University of Wisconsin–Madison
The ability to infect the lungs, notes Kawaoka, is a quality
frighteningly similar to those of other pandemic viruses, notably the
1918 virus, which killed tens of millions of people at the tail end of
World War I. There are likely other similarities to the 1918 virus, says
Kawaoka, as the study also showed that people born before 1918 harbor
antibodies that protect against the new H1N1 virus.
And it is possible, he adds, that the virus could become even more
pathogenic as the current pandemic runs its course and the virus evolves
to acquire new features. It is now flu season in the world’s southern
hemisphere, and the virus is expected to return in force to the northern
hemisphere during the fall and winter flu season.
To assess the pathogenic nature of the H1N1 virus, Kawaoka and his
colleagues infected different groups of mice, ferrets and non-human
primates — all widely accepted models for studies of influenza — with
the pandemic virus and a seasonal flu virus. They found that the H1N1
virus replicates much more efficiently in the respiratory system than
seasonal flu and causes severe lesions in the lungs similar to those
caused by other more virulent types of pandemic flu.
“When we conducted the experiments in ferrets and monkeys, the
seasonal virus did not replicate in the lungs,” Kawaoka explains. “The
H1N1 virus replicates significantly better in the lungs.”
The new study was conducted with samples of the virus obtained from
patients in California, Wisconsin, the Netherlands and Japan.
The Nature report also assessed the immune response of
different groups to the new virus. The most intriguing finding,
according to Kawaoka, is that those people exposed to the 1918 virus,
all of whom are now in advanced old age, have antibodies that neutralize
the H1N1 virus. “The people who have high antibody titers are the people
born before 1918,” he notes.
Kawaoka says that while finding the H1N1 virus to be a more serious
pathogen than previously reported is worrisome, the new study also
indicates that existing and experimental antiviral drugs can form an
effective first line of defense against the virus and slow its spread.
There are currently three approved antiviral compounds, according to
Kawaoka, whose team tested the efficacy of two of those compounds and
the two experimental antiviral drugs in mice. “The existing and
experimental drugs work well in animal models, suggesting they will work
in humans,” Kawaoka says.
Antiviral drugs are viewed as a first line of defense, as the
development and production of mass quantities of vaccines take months at
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