World health experts have been working under a general assumption
that a feared reassortment between human and avian influenza viruses
- -- a scenario that could spark a pandemic -- might only occur during
a short winter interval, but researchers who recently examined virus
circulation patterns warn that time frames for co-infection are wider
and sometimes unpredictable.
Researchers Andrew W. Park, from the Institute of Research for
Development in Montpellier, France, and Kathryn Glass, from the
Australian National University in Canberra, report their findings in
the August issue of The Lancet Infectious Diseases [Park AW, Glass K.
Dynamic patterns of avian and human influenza in east and southeast
Asia. Lancet Infect Dis 2007 Aug;7(8):543].
Though much is known about human influenza patterns in temperate
zones such as the United States and Australia, little is known about
the seasonality of influenza A infections in tropical and subtropical
areas in Asia, where the H5N1 virus is more common, the authors
write. Reviewing the timing of influenza A infections, as well as
that of H5N1 poultry outbreaks and human infections, could reveal
useful information for epidemiologists and pandemic planners, they add.
Park and Glass examined the seasonality of the H5N1 virus using data
from the World Organisation for Animal Health (OIE) and the World
Health Organization (WHO) from 2003 onwards as well as articles from
the literature on animal outbreaks and human cases in Hong Kong
between 1997 and 2003.
They investigated the seasonality of the influenza A virus by
searching for published reports of confirmed reports in east and
southeast Asia between 1996 and 2005. The researchers also looked for
changes in patterns between host range and seasonality by conducting
a literature search that covered the period from 1996 to 2006.
After comparing the data, they found that human H5N1 cases generally
coincide with poultry infections, though human cases sometimes occur
before or shortly after poultry outbreaks. A review of H5N1 isolates
taken from aquatic and terrestrial poultry in mainland China from
July 2000 to January 2004 showed that a greater percentage of birds
were positive for the virus during winter months.
Similarly, the researchers found that the presence of H5N1 in
isolates from birds in live poultry markets in China showed a winter
pattern, but a number of outbreaks extended into summer months,
suggesting that risks are not confined to winter.
Surveillance data from the Pacific basin from 1954 to 1988 show a
marked variation in human influenza A activity, the authors say. They
found that while consistent seasonality of viral activity between
December and March occurs in Japan, patterns were not uniform across
the rest of the region.
"Periods of moderate to high activity typically last longer in
tropical and subtropical regions than in temperate regions, and they
occur more frequently than once a year," Park and Glass write. "It is
not prudent to assume there is a short period of risk of reassortment."
More influenza surveillance in east and southeast Asia would be
useful, especially where H5N1 has been confirmed, they added.
The existence of new host populations in which the H5N1 virus is
endemic raises the pandemic flu risk to humans, especially when there
is close contact between people and the infected animals, the
researchers point out.
Between 2003 and 2005, the H5N1 virus was found in several new host
species, they report, including tigers, leopards, pigs, raptors, and
domestic cats. But they say the greatest concerns are (1) the
frequency with which the virus is found in domestic ducks, because
the ducks have close contact with people, and (2) isolation of the
virus from pigs in China and Indonesia, because receptors in their
respiratory tracts make coinfection with human and avian strains and
thus generation of reassortant strains possible.
Though a review of the H5N1 viral circulation data suggest some
seasonal factors, outbreak patterns also appear to be affected by
interventions such as culling, the authors report. Migration and
movement of birds are also thought to play a role.
"Looking to the future, the suggestion that H5N1 virus has become
endemic in ducks is of great concern," the researchers write. Poultry
culls have successfully contained the spread of the disease in many
instances, but if the virus is endemic in some species, culling might
not be practical, and animal health officials may need to focus on
vaccination and intensive surveillance to contain the virus, they say.
Awareness of the seasonal risk of both human disease and viral
reassortment can help health officials design control measures to
help prevent an emerging pandemic strain, the authors conclude.
cidrap.umn.edu
- --
Communicated by:
ProMED-mail Rapporteur Mary Marshall
[Park and Glass show that outbreaks of seasonal human influenza in
tropical and sub-tropical zones of Asia are temporarily less
constrained than those in temperate zones (Australia, Europe and
North America). Hence, there may be greater opportunity for
interaction between human and avian viruses than previously assumed,
and the opportunity for generation of potentially pandemic viruses
does not necessarily decline during the summer months.
However, it should be remembered that opportunity for reassortment is
not the single determining factor in the generation of reassortants
with pandemic potential. The reassortant virus must possess a
phenotype that will endow it with some selective advantage. As these
authors hint, a greater hazard may be the increasing prevalence in
Asia of the H5N1 virus in flocks of domestic ducks. - Mod.CP] |
