The West Nile virus (WNV), a pathogen transmitted by mosquitoes that can kill people and birds, is still rampant in North America, wiping out millions of birds each year across the continent, according to a new study. The research raises concerns about the long-term impacts of the disease, particularly on endangered species and endangered.
"It really is a fantastic study, but sobering," said Peter Marra, an ornithologist at the National Zoological Park, Smithsonian Institution in Washington, DC, who has not participated. "When combined with other threats such as land use, climate change, cats and buildings, it is not surprising that we see these declines in many bird species across United States."
NileArrived in North America 16 years ago. Although it can infect and kill people, birds are the main host of the virus. But avian species in North America had never contracted WNV, and the novel pathogen spread across the continent in just 5 years, leaving millions of birds, finches jays to sparrows, dead in its wake. Previous studies have shown that factors such as climate and habitat affected how various bird species responded to the virus. For example, researchers have shown that urban birds seem more susceptible to the disease, although scientists do not yet know why.
To better understand these factors and to show the impact of WNV, a team of scientists analyzed 16 years of data from 1992 to 07 to more than 500 stations across the United States where birds are regularly imprisoned and banded. After 1999, these avian productivity Monitoring and Survivorship (MAPS) stations also began to check the birds for WNV, and so had the time records of where the disease has appeared at every station and in every case, and if the virus persists. Using this information, the scientists were able to determine if and how the virus first affected various bird populations, and if the numbers of birds have recovered or are still falling. (Scientists are not able to include MAPS data after 07 because they are not yet processed by volunteers and station personnel. However, researchers believe that the trends they have discovered not changed.) in total, they examined data over a quarter million birds of 49 species in 14 -in bird families, focusing on adults to more accurately determine the number of survivors.
Twenty-three of 49 species or 47% -were negatively affected by WNV, the report online today researchers in Proceedings of the National Academy of Sciences . They also found several striking models in the impact of the disease. Some first species suffered huge declines. Red-eyed Vireo ( Vireo olivaceus ) populations, for example, fell by about 29% when they first encountered the disease. Before WNV arrived, red-eyed vireo, a species that is widely distributed across the continent, had an estimated population of 130 million birds. Ultimately, nearly 37 million of these deaths probably vireo virus, scientists say. But the populations of this species are supported mass mortality in the first year of WNV hit. Their numbers subsequently recovered, which is the model that environmentalists disease waiting to see.
"It's like after a flu epidemic," said Ryan Harrigan, biologist of infectious diseases at the University of California, Los Angeles, and an author of the study. "Everyone builds immunity, and the impact tends to decline." Eleven of the 23 species affected in the study have experienced this type of recovery.
But the other 12 species are not so lucky. Their populations are still declining disease. "Seeing the persistence of the disease was a shock," says Harrigan. "We do not know why these species are unable to recover. It is alarming." For example, populations of vireo warbling ( Vireo gilvus ) fell 8.7% only when they have contracted WNV. But instead of recovering, populations of this species continued to decline annually by roughly the same percentage, a trend that scientists expect continued. "It is not just that he was a decrease of 8.7% a year; it is down 8.7% this population less next year and the year after, so the effect is compounded, "says Harrigan.
He and his colleagues believe that the WNV has killed more than 15 million of these vireo, nearly a third of their original population of 49 million. the thrush Swaainson ( Catharus ustulatus ), purple finch ( Carpodacus purpureus ), tufted titmouse (bicolor Baeolophus ), and Wrentit ( Chamaea fasciata ) were also affected.
Harjeet Singh 
to try to understand why some species are better than others to overcome the disease, the researchers compared the habitats "birds, to see if people in more urban landscapes had larger declines previous studies have suggested. Again, the results were mixed. Adult populations of 10 species, including the Spotted Towhee ( Pipilo maculatus ) and Song Sparrows ( Melospiza melodia ) have done very well in these habitats, perhaps because they have more food resources through human bird feeders provided. But 11 other species in those lost and probable urban areas continue to lose significant numbers of their adults.
The scientists also constructed a family tree of all species in the study to see if the closely related birds responded similarly to the virus. Again, the pattern was mixed. New sparrows and their close relatives, and generally vireos died off in the first year after contracting the virus, while the finches generally known persistent declines. Scientists say they do not yet know why some species are better than others in overcoming the disease.
"Is it because of differences in their immune systems or their habitats or the amount of exposure to [WNV]?" Says Harrigan. He and his colleagues are now looking more closely areas they have identified as those where some avian species are still dying of the disease to see if they can identify the reason.
other scientists expect that the study will lead similar investigations. "as with all good studies, it generates many new questions," says Staffan Bensch, an ecologist from animals to Lund University in Sweden, who wonders if the virus has benefited some songbird species by killing their avian predators, jays and crows.
Another mystery: WNV did not affect at least three species, the black-capped chickadee ( Parus atricapillus ), American robin ( Turdus migratorius ), and the wren house ( Troglodytes aedon ). "These species have gone straight through without any problems," says Harrigan. "How a species has no effect on this disease?"
Answering these questions will require more long-term data Type collected in MAPS stations, scientists say. "It will not be the last time we see a disease like this," warns Harrigan. And although the study did not focus on endangered species, all scientists fear the virus may be causing them further damage. "what is the effect on birds with smaller populations and ranges?" he said. Like many things about WNV, scientists not yet know. They only know that the disease and its effects on birds and North American people-are here to stay. Understanding more about how the virus affects birds and help them overcome this, so enjoy human, too, scientists say.
0 Komentar