dennis
Have binoculars. Will travel.
A friend, who is employed by the State of New Jersey in the field of mosquito research, has sent this article:
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 <[email protected]> for bringing this article to our attention.
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 <[email protected]> for bringing this article to our attention.
