Ducks may be silent carriers of H5N1 avian flu

Oct 29, 2004 (CIDRAP News) – Healthy-looking ducks may be acting as a "silent" reservoir for the deadly H5N1 influenza, the World Health Organization (WHO) announced today.

A new laboratory study of domestic ducks infected with several 2004 H5N1 viruses shows that ducks shed more of this virus compared with ducks infected with H5N1 viruses from 2003, researchers in Tennessee found. Ducks also shed the 2004 viruses from 1 to 7 days longer.

Most infected ducks don’t even appear sick, WHO announced in a press release.

The H5N1 viruses from 2004 appear more environmentally stable as well: early results indicate they can survive at 37°C for 6 days, compared with 2 days for viruses from the 1997 outbreak, WHO wrote.

The studies were finished last week by WHO collaborators at St. Jude Children’s Research Hospital in Memphis, Tenn. The WHO said the findings were being announced in advance of study publication because of their significance for human health.

The amount of virus shed in the respiratory droplets and feces of apparently healthy ducks is almost as much as the amount excreted by obviously ill chickens. The findings suggest that ducks may act as a silent reservoir for H5N1 virus, which causes widespread illness and death in chickens, WHO wrote.

The findings could have important public health ramifications as governments wrestle with how to stop the spread of avian flu to humans. Avian flu has killed 32 people in Thailand and Vietnam since late January.

WHO said these findings add to recent indications that domestic ducks have played a central role in the march of H5N1 avian flu in Asia, where free-ranging chickens and ducks often flock together and share water.

The findings augment other recent evidence that the current H5N1 virus in Asia has increased its pathogenicity in chickens and in laboratory mice, which serve as a research model for mammals. The virus has also expanded into tigers and housecats, which once were not considered susceptible to infection, WHO noted.

WHO recommended that people in affected countries not keep ducks as pets or allow them in houses and that water supplies for human use should not be drawn from open ponds used by domestic ducks. The agency also said those investigating human cases of avian flu should include exposure to apparently healthy domestic ducks as a risk factor for infection.

In other developments today, Thai authorities reported the spread of H5N1 flu to another 60 royal Bengal tigers at a zoo and breeding facility not far from Bangkok. The tigers were being monitored and treated with antiviral drugs, but the governor of the Chonburi province where Sriracha Tiger Zoo is located said the 60 sick tigers may have to be euthanized, according to a report by Agence France-Presse (AFP).

Previously, 83 tigers at the zoo died or were euthanized as a result of avian flu. The outbreak is believed to have stemmed from tigers being fed infected raw chicken carcasses.

Thai Prime Minister Thaksin Shinawatra earlier had declared that he wanted to eradicate avian flu in Thailand by November. Recent reports suggest Thailand has little chance of achieving that goal. A report to the World Organization for Animal Health (OIE) by the director general of Thailand’s livestock department said that active surveillance revealed 166 new outbreaks of avian flu during October.

Thai officials announced this week that they would use several measures to better control the disease. Efforts will focus on active surveillance, data analysis, and research, according to a Vietnam News Agency report. Steps include increased monitoring and specialized care of people potentially ill with avian flu, adequate supplies of antiviral medications, and extra training for volunteers, according to China’s Xinhua news agency.

See also:

Oct 29 WHO news release

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