Climate change could increase levels of
avian influenza in wild birds
Rising sea levels,
melting glaciers, more intense rainstorms and more frequent heat waves may come
to mind, among other planetary woes, when we mention climate change. Now, two University of Michigan researchers
say an increased risk of avian influenza transmission in wild birds ranks among
those risks.
Population ecologists
Pejman Rohani and Victoria Brown used a mathematical model to explore the
consequences of altered interactions between an important species of migratory
shorebird and horseshoe crabs at Delaware Bay as a result of climate change.
They found that climate
change could upset the carefully choreographed interactions between ruddy
turnstone shorebirds and the horseshoe crabs that provide the bulk of their
food during the birds' annual stopover at Delaware Bay, a major estuary of the
Delaware River bordered by New Jersey to the north and Delaware to the south.
Climate change-caused
disruptions to the well-timed interplay between the birds and crabs could lead
to an increase in the avian influenza infection rate among ruddy turnstones and
resident ducks of Delaware Bay, the researchers found.
Delaware Bay lies at a
crossroads for many bird species traveling between continents, an increase in
the avian infection rate there could conceivably help spread novel subtypes of
the influenza virus among North American wild bird populations, according to
Rohani and Brown.
"We're not suggesting that our findings
necessarily indicate an increased risk to human health," said Rohani,
a professor of ecology and evolutionary biology, a professor of complex systems
and a professor of epidemiology at the School of Public Health.
"But every single pandemic influenza virus
that has been studied has included gene segments from avian influenza viruses.
So from that perspective, understanding avian influenza transmission in its
natural reservoir is, in itself, very important."
Avian influenza refers
to infection with bird flu Type A viruses. Those viruses occur naturally among
wild aquatic birds worldwide and can infect domestic poultry and other bird and
animal species.
Avian flu viruses do
not normally infect humans. However, sporadic human infections with avian flu
viruses have occurred. Since 2003, for example, more than 600 cases—including
more than 300 deaths—of human infection with highly pathogenic avian influenza
A H5N1 have been reported worldwide, according to the World Health
Organization.
Delaware Bay, known as
a hot spot for avian influenza virus, hosts many resident bird species as well
as the hundreds of thousands of migratory birds that gather to feed on
horseshoe crab eggs. Infection levels in
ruddy turnstones, which stop at Delaware Bay each May during their northbound
migration to breeding grounds in the Arctic after wintering in South America, have
been found to be exceptionally high.
The birds time their
arrival at Delaware Bay to coincide with the availability of the horseshoe crab
eggs. Brown and Rohani wondered what would happen to influenza levels in
Delaware Bay birds if climate change altered the timing of the ruddy
turnstone's migratory flight to Delaware Bay or affected the timing of
horseshoe crab spawning.
Their mathematical
model looked at virus infection rates in ruddy turnstones and two species of
duck—mallards, which winter at the bay, and American black ducks, which live
there year-round.
The researchers found
that if ruddy turnstones reached Delaware Bay either several weeks earlier or
later than their current May arrival date, influenza infection rates in the
species increased significantly, driving up the infection rates—also called
prevalence levels—in the resident ducks as well.
"If the ruddy turnstones arrive either
earlier or later than they do now, then their arrival coincides with higher
viral prevalence in the resident ducks," said Brown, a postdoctoral
research fellow in the Department of Ecology and Evolutionary Biology and the
Center for the Study of Complex Systems. "And because these birds are interacting with a greater number of
infected resident ducks, prevalence levels in ruddy turnstones are boosted.
"There's a feedback mechanism at work as
well. Higher prevalence levels in the ruddy turnstones may, in turn, impact the
prevalence levels in the resident ducks, driving them even higher."
If the timing of the
horseshoe crab spawning season at Delaware Bay changed significantly due to
climate change, ruddy turnstone populations would drop significantly due to a
loss of food, and the influenza infection rate would decrease sharply as well,
the researchers found.
The Western Hemisphere
Shorebird Reserve Network declared Delaware Bay a site of hemispheric importance
in 1986.
Sites of hemispheric
importance act as staging, nesting or breeding grounds for at least 500,000
shorebirds annually, or at least 30 percent of the biogeographic population of
any species.
SOURCE: [ Findings were published
online Aug. 29 in the journal Biology Letters. ]
