Dutch scientists have determined bird flu can spread more easily among humans than was previously thought, researchers said on Tuesday.

Albert Osterhaus, a professor at Rotterdam’s Erasmus University, said the finding was based on a study into the spread of the potentially fatal disease among humans after an outbreak of the disease in the Netherlands in 2003.

He said researchers who tested medical workers and friends and family of 90 people who contracted the highly infectious avian influenza last year found hundreds of undiagnosed cases.

“We discovered some 500 more people were infected directly from their relatives or colleagues during the outbreak in 2003,” Osterhaus said, noting that none of the additional people had shown bird flu symptoms.

The study - which was carried out with the National Institute of Health - concluded more vigilance against the disease was needed as both the strain that appeared in the Netherlands and another in Asia were very infectious.

Local poultry farms are being tested regularly since the bird flu outbreak last year led to the slaughter of a quarter of all Dutch poultry at a cost of hundreds of millions of euros.

The Netherlands culled 30.7 million birds at some 1,300 farms to contain the outbreak, which was first discovered in March 2003 and caused the death of a veterinarian who took samples at an infected farm but was not vaccinated.