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Date: 7 Aug 2003
Posted by: Environmental Risk Analysis Program <envrisk@cornell.edu>

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 na•ve, 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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