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Bird Flu in Asymptomatic Ducks in Vietnam

Recombinomics Commentary
January 7, 2005

>>According to the Department of Animal Health, since December last year, the relapse of bird flu has been seen in 25 communes in11 localities, namely the southern city of Can Tho, the northern province of Nam Dinh and the nine southern provinces of Dong Thap,Tien Giang, Long An, Bac Lieu, Ca Mau, Hau Giang, Tra Vinh, Binh Phuoc and An Giang, killing and leading to the forced culling of some 28,700 fowls, mainly ducks and chickens…..
……The southern city of Can Tho, which finds that nearly half of samples from ducks raised in the locality are tested positive to the bird flu virus strain of H5<<

Nearly half of the ducks being positive for H5 is a very major concern. WHO had issued an alert on October 29, 2004 on H5N1 infections in asymptomatic ducks.  Lab experiments had shown that isolates from patients in Vietnam and Thailand could infect ducks asymptomatically.  The virus would grow to high titers in the intestine before excretion.  The excreted virus was unusually stable.

The asymptomatic ducks posed two types of problems.  Because the ducks were not sick, they were provocative targets for a second bird flu infection.  Dual infections leads to both reassortment (shuffling of genes0 and recombination (creation of new genes).  It is the creation of new genes that drives the current H5N1 pandemic and recombination provides the mechanism for creating a virus that can be efficiently transferred human to human while maintaining the high case fatality rate of about 70%.

The asymptomatic ducks also cause a detection problem. Since they are not sick, there presence is not notable, either when they are alive or after slaughter. Therefore the can transmit disease in backyard farms without being noted and they can enter the food chain.  Although cooking will kill the virus, hand to mouth contact while preparing a bird for cooking, or contamination of cutting utensils or food preparation surfaces can have live virus, which can then be transmitted to uncooked foods.

At least one of the recent cases does not have a history of contact with sick birds and a significant portion of last years cases also were not directly tied to sick or dying poultry.

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