Avian influenza, the virus that has led to the deaths of millions of birds and more than 200 people since 2003, may be more prevalent than previously thought in Europe as it goes undetected in waterfowl.

Germany's discovery of the fatal H5N1 strain in healthy ducks and geese two months ago may be a sign that domestic animals are harboring bird flu without getting sick, increasing the threat to human health, the Rome-based Food and Agriculture Organization of the United Nations said in an e-mailed release.

The spread of the disease by birds that don't succumb to it has caused entrenched infection in nations including Indonesia, the country with the most human H5N1 cases, and is thwarting efforts to eradicate the virus. Europe's Black Sea area, where chicken and waterfowl populations are similar to those in Asia, may become a reservoir for H5N1 because birds migrating from Siberia spend the winter there, the FAO said today.

"It seems that a new chapter in the evolution of avian influenza may be unfolding silently in the heart of Europe,'' said Joseph Domenech, the organization's chief veterinary officer, in the statement.

The UN agency called for more surveillance in domestic duck and geese populations in countries that don't have sufficient measures in place, and said Europe should prepare for more outbreaks.

The FAO estimates the number of domestic ducks in the Ukraine at around 20 million, and said around 8 million ducks and geese populate the Danube delta in Romania.

"These figures compare easily with chicken and waterfowl densities in Asia, where the virus continues to circulate among chickens but has found a niche in countries with tens of millions of domestic ducks and geese,'' the FAO's senior animal health officer Jan Slingenbergh said.

All countries bordering the Black Sea have reported outbreaks of bird flu, as open poultry systems and a lack of separation between domestic and wild birds nourish the spread of the disease.