Domestic ducks may be acting as a silent reservoir for the H5N1 Avian Influenza virus, which is highly pathogenic for chickens, and may thus have acquired an important new role in the transmission of the virus to other poultry and, possibly, to humans as well, three international agencies warned today.
The concern is greatest in rural areas of affected countries, where traditional free-ranging ducks, chickens and wildlife mingle, frequently sharing the same source of water, according to a joint statement by FAO, the World Health Organization (WHO) and the Office International des Epizooties (OIE).
Findings pointing to an altered role for domestic ducks join other recent evidence that the H5N1 virus circulating in parts of Asia has increased its virulence in chickens and mice (a laboratory model for mammals), and has expanded its host range to include mammals, such as certain members of the felidae family (e.g. cats and tigers), not previously considered susceptible to infection.
A new laboratory study of domestic ducks infected with several H5N1 viruses isolated in 2004 shows that, when compared with infections caused by viruses from 2003, domestic ducks are shedding more virus for longer periods and as before, are doing so without showing any sign of illness.
The study found that the quantities of virus excreted by healthy-looking ducks could approach those excreted by visibly diseased chickens, the agencies said. It was of public health concern that ducks might be infected and shed virus for long periods, yet give no warning signal in the form of visible signs and symptoms that alert officials and the public to take precautions, the statement added.
WHO, FAO and OIE said affected countries should be encouraged to include possible exposure to apparently healthy domestic ducks when assessing the risk of infection to humans and to issue appropriate advice for people living in affected areas. Such advice should cover the handling of domestic ducks, particularly at slaughter (e.g. scalding the ducks prior to plucking), and avoiding use of water that has been in contact with ducks for human consumption without being treated. WHO has prepared detailed guidelines.
WHO, FAO and OIE said that discovery of the altered role of domestic ducks in the transmission cycle of H5N1 needed to be addressed soon as it might complicate efforts to control the disease in poultry and to prevent further human cases. The three agencies therefore called for urgent animal surveillance research to establish how widespread the incidence of infection in ducks without symptoms has become. They also called for research on the effectiveness of current vaccines on duck populations.
The wider implications of the role played by domestic ducks are being jointly considered by FAO and OIE in formulating a long-term strategy for bringing the current Avian Flu outbreaks in poultry under control, the statement said.
In animals, an early detection and warning system remains the key to prevention and rapid response. Governments need to support their Veterinary Services to enable them to carry out effective control operations. FAO and OIE have recently issued comprehensive guidelines for responding to the outbreaks of H5N1 in poultry.
The statement said that regional collaboration in Avian Influenza surveillance in domestic animals and wildlife, reporting and control was also crucial. Such control needs to consider all suitable measures ranging from the safe and swift elimination of affected poultry, through strict biosecurity, movement and market control and if relevant, to vaccination and its close monitoring.
With the H5N1 virus now endemic in parts of Asia, changes in some traditional farming practices may be needed to decrease incidence of the disease, especially in rural areas, and thus reduce opportunities for community-wide exposure. Such changes may have to be significant and require careful assessment and implementation of recommended measures with respect to their social, economic, policy and institutional dimensions, today's statement said.