Dutch authorities have detected a low-pathogenic H7 bird flu strain at a poultry farm and stepped up measures to prevent a major outbreak in one of the world´s top poultry exporter, the farm ministry said on Tuesday.

The Netherlands, Europe´s second biggest poultry producer after France, was hit by an H7N7 avian flu strain in 2003, which led to the culling of some 30 million birds, about a third of its poultry flock.

The new cases of low-pathogenic H7 bird flu were found at a farm with some 25,000 birds in the central region of the Gelderse Vallei, the ministry said.

"This strain is much less dangerous than the strain that hit the Netherlands in 2003," it said in a statement. "But because it might mutate to a more aggressive form...we have closed the farm.

"It´s an H7 virus but we cannot say yet which strain exactly," a ministry spokeswoman said. The outbreak had been detected as a result of regular monitoring carried out after the 2003 debacle.

Birds at the infected farm would most probably have to be culled and the authorities had begun an investigation into the causes of the outbreak and stepped up monitoring, the ministry said.

Tests made at five commercial farms surrounding the infected one had proved negative, it said.

H7 bird flu in its highly pathogenic form can kill large numbers of birds and can occasionally infect people, although it is rarely fatal in humans.

The 2003 outbreak of H7N7 in the Netherlands infected around 90 people, including a veterinarian who died.

The Netherlands has never reported a case of the highly pathogenic H5N1 avian flu strain found in several other European Union countries.

Dutch veterinarians and scientists believe the 2003 outbreak of the different H7 strain was caused by wild birds that infected outdoor poultry in the central Netherlands, then spread to the south and into Germany and Belgium where it raged on a lesser scale.

Some Dutch scientists say the country is exposed constantly to the spread of low pathogenic viruses by wild birds.