Experts are calling for stricter measures and more testing to prevent the bird flu virus from jumping to human beings following the discovery of a cow and two cats that tested positive for the virus on a Frisian dairy farm.

An analysis of the milk of the cow was carried out last week when the two cats showed symptoms, and revealed it had recovered from a bout of bird flu.

According to experts, the spread of bird flu among mammals increases the risk for humans, particularly in such densely populated countries as the Netherlands where animals and people live in close proximity.

The findings indicate for the first time that Dutch dairy farms are susceptible to the virus, bird flu expert Thijs Kuiken said. “Until now, we only knew of cows in the United States that had the virus and on a much larger scale, with 1,084 farms in 19 states,” he told the Financieele Dagblad.

Lab experiments had already shown that variant of the virus was present in European countries but it was assumed that aspects of US dairy farm practices, such as the size of the farms and frequent transports from farm to farm were to blame for the outbreaks.

One of the reasons the Netherlands has become the second country to have discovered the virus in a cow is a greater emphasis on testing cows on farms close to poultry farms following the American outbreaks, Wageningen University researcher Wim van der Poel told the paper.

The term bird flu has become obsolete, Van der Poel said. Seals, foxes, ferrets and cats are just some of the mammals that can carry the virus. “If more species are contaminated the exposure of people to the virus also increases. The virus will also adapt as infections spread. That is worrying and a good reason to monitor infection in mammals,” he said.

Some 40 of the 71 cases of bird flu in humans in the United States were a result of cow to human contact, Kuiken said. “I am not expecting those numbers in the Netherlands but cows pose an extra risk,” he said. “Brussels must also get is skates on about allowing possible vaccines. That would greatly limit circulation.”

Bird flu culls in the most recent outbreak numbered have involved killing some 1.7 million birds to date.