[editing is mine]
Public release date: 9-Jul-2013
American Society for Microbiology
The H7N9 avian flu strain that emerged in China earlier this year
has subsided for now, but it would be a mistake to be reassured by this
apparent lull in infections. The virus has several highly unusual traits
that paint a disquieting picture of a pathogen that may yet lead to a
pandemic, according to lead scientists from the National Institute of
Allergy and Infectious Diseases. David Morens, Jeffery Taubenberger, and
Anthony Fauci, in a paper published in mBio®, the online
open-access journal of the American Society for Microbiology, describe
the history of H7 viruses in animal and human disease and point out that
H7 influenza has a tendency to become established in bird, horse, and
swine populations and may spillover repeatedly into humans.
"The evidence as a whole is complex and the implications of past
outbreaks for predicting the future course of the current H7N9 epizootic
[an epidemic among animals] are uncertain," write the authors.
The outbreak of H7N9 earlier this year led China to temporarily
close scores of live poultry markets in an effort to limit the spread of
the virus. Although this previously unrecognized strain of avian
influenza A has now been associated with 132 confirmed human infections
and 39 related deaths (as of June 14), the rate at which new cases are
recognized has dwindled in recent weeks.
In their minireview, Morens, Taubenberger and Fauci point out that
despite this apparent hiatus, viruses like H7N9, which have subtype 7
hemagglutinin, are a cause for heightened concern because of several
highly unusual characteristics. First, H7 viruses have repeatedly been
involved in numerous explosive poultry outbreaks including incidents in
New York, Canada, Mexico, the Netherlands, and Italy, and in almost all
of these cases the virus eventually spilled over into humans. Also, H7
viruses have the ability to mutate from a low pathogenicity form to a
high pathogenicity form in birds, a scenario that can lead to
large-scale culling and ultimately to human exposure to the virus among
poultry workers.
H7N9 also shares many characteristics with another influenza strain
that continues to spillover into humans: highly pathogenic avian
influenza H5N1. Among other commonalities, both viruses have a clinical
picture that includes bilateral pneumonia, acute respiratory distress
syndrome, and multi-organ failure, and it appears they are both
currently unable to easily infect most humans but cause severe disease
in individuals with uncharacterized genetic susceptibilities.
The fact that many H7 viruses tend to infect conjunctival cells is
also cause for concern. Some, but not all, cases of human H7 infection
feature prominent signs and symptoms in the eyes, including itching,
swelling, and tearing, that could enhance person-to-person spread in an
H7N9 outbreak.
The authors point out that many H7 viruses have adapted to infect
mammals, including horses and pigs, which raises the possibility that
H7N9 could adapt in a similar fashion. The possibility that H7N9 might
infect pigs is particularly troubling, as swine are considered a "mixing
vessel" for viruses - a breeding ground for novel viral reassortants
like the 2009 H1N1 pandemic influenza strain commonly known as "swine
flu".
The sum of these observations is this: we do not know what H7N9 will
do next. Although avian influenza viruses have not caused widespread
human transmission in 94 years of surveillance, there have been numerous
instances of avian influenza spillover and H7N9 "might arguably be more
likely than other avian viruses to become human-adapted," write the
authors.
Regardless of its future, H7N9 certainly holds lessons for
preventing human and animal pandemics. All the unknowns surrounding the
virus make a strong case for enhancing basic and applied research into
the evolution of influenza viruses and for better integration of
influenza virology within human and veterinary public health efforts.
"We have a unique opportunity to learn more of influenza's many
secrets, and thereby enhance our ability to prevent and control an
important disease that seems destined to appear again and again, in
multiple guises, far into the foreseeable future," write the authors.
http://www.eurekalert.org/pub_releases/2013-07/asfm-hih070313.php
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