Bird Host Preference--Report from ESA

Ecological Society of America, Conference Report (Day 1, August 4, 2003)
Provided via BioMedNet (http://news.bmn.com/conferences/):

WEST NILE VIRUS, FRAGMENTATION & THE BROWN-HEADED COWBIRD
Investigator: Thomas Unnasch

4 August 2003

by Laura Spinney

US researchers have found that the mosquitoes that transmit West Nile
virus (WNV) to birds are quite particular about the species they feed
on. One of their favorites, the brown-headed cowbird, happens to be
increasing in numbers and pushing westwards through the US as a
result of the fragmentation of its habitat by humans - showing how we
might be driving new epidemics towards ourselves.

The primary hosts of WNV are birds. Mosquitoes that normally feed
only on birds maintain a cycle of infection within them, and the
virus only breaks out of that cycle to infect other species when
"bridge vectors" - mosquitoes that bite both humans and birds - come
into contact with an infected bird.

For that reason, the degree of contact between bird and mosquito - or
horse and mosquito in the case of another, far more vicious
neurological disease called Eastern equine encephalomyelitis (EEE) -
is thought to be a major factor determining whether the virus crosses
the species barrier. When contact is high, the virus amplifies itself
more quickly and there is a higher chance that the bridge vector will
come into contact with it.

To investigate how the degree of contact affects viral levels, and
hence the risk of infection for humans, Thomas Unnasch of the
University of Alabama at Birmingham and colleagues analyzed the
stomach contents of bird-biting mosquitoes in three US states: New
Jersey, New York and Tennessee. They used a reverse
transcriptase-polymerase chain reaction to detect the presence of
WNV, and another sensitive assay to determine the species of origin
of the mosquitoes' bloodmeals.

At all the test sites, they found that of the 24 bird species the
mosquitoes fed on, three accounted for more than 50% of the blood
they had ingested. Of these, the most notable was the brown-headed
cowbird.

Similarly, in Tuskagee National Forest, Alabama, which saw an
epidemic of EEE in 2001, the first year of the study, the mosquitoes
favored two species: the American robin and the brown-headed cowbird,
with the cowbird accounting for more than 40% of their bloodmeals.

The American crow seemed not to be to the mosquitoes' liking at any
of the sites, although American crows are regarded as "sentinels" for
the arrival of WNV because they are highly susceptible to it and die
off quickly once infected.

In both the EEE and WNV studies, the researchers were surprised to
find that the birds the mosquitoes preferred to bite were not endemic
to the swamps they themselves inhabited. The birds' usual habitats
were grasslands or higher altitude ecosystems.

According to Unnasch, that suggests the mosquitoes' habitat could be
larger than was previously thought, and they might forage outside
swamp areas before returning to them to digest their meals and lay
their eggs. At the same time, forest clearance could be enabling
grassland-dwelling cowbirds to stray closer to mosquito-ridden areas.
"What we are seeing is a consistent pattern of these arboviral
vectors targeting just a very few species," he says.

His team also found that in July and August a higher proportion of
the mosquitoes' bloodmeals came from hosts other than birds. Before
that July drop-off, however, birds were their main targets.

Unnasch thinks that the mosquitoes might be zeroing in on fledglings
of certain species. He suggests that young birds are a dead-end
population. Because they are virally naïve, they die quickly and
provide no reservoir for transmission to other birds or humans.

However, fledglings abound early in the season and transmission
generally peaks in late summer, he says, so more research is needed
to explain the delayed, late summer peak.

"This to me is really interesting because of the increases we have
seen in brown-headed cowbirds with the fragmentation of the
landscape," said Sharon Collinge of the Department of Environmental,
Population and Organismic Biology and the Environmental Studies
Program at the University of Colorado, Boulder. "They've moved
westwards and they tend to forage more around forest edges."

--
Thanks to Carolee Caffrey <ccaffrey@audubon.org> for bringing this
article to our attention.
--
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