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Pandemic Bird Flu - Asymptomatic Chickens in Vietnam

Recombinomics Commentary

April 23, 2005

>>  Last year, U.S. researchers reported that ducks in Southeast Asia had begun carrying the bird flu virus without showing symptoms. Now, scientists in Vietnam have found numerous asymptomatic cases in the country's vast chicken population, according to Nguyen Tran Hien, director of the National Institute of Hygiene and Epidemiology.

"It seems that the virus may adapt in humans and in poultry a little bit. Therefore, the symptoms are not as severe as before," Hien said. "Also, the transmission may be faster and easier." <<

The report of H5N1 in asymptomatic chickens is cause for concern.  Last fall WHO issued a warning about H5N1 in asymptomatic ducks.  The warning was based on experiments in Hong Kong which showed that the H5N1 taken from fatal human cases could grow to high titers in asymptomatic ducks.  Moreover, the excreted virus was unusually stable.  The ability of H5N1 to kill humans, yet grow to high titer in ducks that appeared healthy, was a concern for many reasons.  It indicated that virus could accumulate because of increasing numbers of ducks spreading virus.  This may be related to a number of infections in Vietnam that were associated with swimming or bathing in canals in Vietnam.

The asymptomatic ducks also increased the likelihood of dual infections.  This could lead to further reassortment and recombination, which could increase genetic evolution of H5N1.  H5N1 growth in asymptomatic chickens, and humans with milder disease, are a major cause for concern.  The presence of the virus in more species increases the likelihood for more rapid evolution.  The virus in humans could recombine with highly infectious H3N2 to increase efficiency of transmission. 

Similarly, infection in asymptomatic chickens offers more targets for recombination.  The H5N1 human cases in the south appear to be much more severe than in the north.  Asymptomatic chickens offer yet another host for further recombination between these co-circulating strains.  A recent survey of poultry in the 11 provinces in the Mekong Delta indicated 71% of the ducks, and 21% of the chickens were H5N1 positive.

As the number of asymptomatic or mild cases of H5N1 infection increases, the opportunity for more genetic diversity increases.  This also increases the pandemic potential of H5N1.

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