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Industry: Email Alert RSS FeedWhen flu flies the coop: a pandemic threatens
Science News, Sept 10, 2005 by Ben Harder
Aside from playing a role in the geographic spread of H5N1, infections in waterfowl have revealed some novel and troubling aspects of the virus.
First, avian-influenza viruses, including the H5N1 virus collected in 1997, generally aren't harmful to wild bird species. Avian-flu viruses, says Webster, have "lived with the aquatic birds for probably millions of years in perfect harmony. It's only when they come into domestic poultry that they kill."
But in late 2002, the H5N1 virus became highly pathogenic to waterfowl. An apparently new strain killed wild ducks, geese, swans, and flamingos in one outbreak in Hong Kong. "That is a very, very unusual feature," says Webster.
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Second, he says, the virus has since evolved in a worrisome direction. In laboratory experiments in mallard ducks, it rapidly shifted from being potentially fatal to causing only asymptomatic infections. Nevertheless, it remained highly virulent to domestic chickens and, presumably, to people. A resilient wild waterfowl, such as the mallard, could therefore become a permanent biological reservoir for a strain of avian flu with pandemic-causing potential.
In the study, Webster and other St. Jude researchers, along with collaborators in China, Indonesia, Thailand, and Vietnam, tested 11 virus samples that had been isolated in 2003 or 2004 from infected birds and people. They also included three strains from 1997 through 2001.
At specialized facilities designed to handle highly hazardous biological materials, the scientists exposed healthy mallards to H5N1 virus and then housed the infected birds with other, unexposed ducks. The viruses spread readily among the animals. The viruses isolated after 2002 killed a quarter of the ducks. The older viruses weren't lethal to those animals.
The researchers then tested whether the lethal strains' virulence had shifted during the first experiment. They isolated H5N1 virus from four ducks that had survived an infection with an H5N1 sample that had killed at least one other bird in the same cage. When exposed to any one of the four new samples, fresh ducks showed no sign of illness, suggesting that the viruses had evolved in the course of a single infection to be relatively harmless to other members of the species.
However, when researchers tested two of the same new samples on chickens, all of the birds died. That indicates that the viruses' lethality to other species was unmitigated, Webster and his colleagues say in the July 26 Proceedings of the National Academy of Sciences.
Taken together, Webster says, the study findings hint that large numbers of wild ducks and other waterfowl may now be carrying dangerous strains of H5N1 without showing symptoms. Those birds could be flying below the radar of public health programs that investigate possible avian-flu outbreaks only when they receive reports of dead or dying birds.
This so-called passive surveillance, as opposed to active testing of apparently healthy birds, is the most that may be feasible in many regions of Asia where financial resources are scarce.
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