Poultry farmers in the European Union should get ready to act against bird flu, stepping up surveillance and preparing for mass vaccinations or culls if the disease takes hold, the EU’s food safety agency said on Tuesday.

In a report for the European Commission, the European Food Safety Authority (EFSA) said the biggest risk of a major bird flu outbreak was if low pathogenic strains of the virus mutated into a more dangerous version and then spread throughout Europe.

The H5N1 strain emerged in Hong Kong in 1997, killing or forcing 1.5 million chickens, geese and ducks to be destroyed. The same strain has killed 63 people in Asia since late 2003.

While EU rules say such highly pathogenic strains are notified and controlled, that is not the case for the many low-pathogenic versions. EFSA said two of those strains, H5 and H7, should now also be controlled due to the risk of mutation.

The report also called for greater controls on the use of poultry feces as manure.

Estimates of the risk of a bird flu outbreak in the EU vary, but the U.N. Food and Agriculture Organization said last month migratory birds flying north next spring could bring the virus with them.

Earlier on Tuesday, EU Commissioner for Health and Consumer Protection Markos Kyprianou said the bloc’s 25 member states should take a coordinated approach to the disease and not impose piecemeal national measures.

“There is no need for extra measures besides increased surveillance of both domestic and wild birds,” he said.

But in addition to its estimate of the risks, EFSA also set out recommendations for what to do in the event of an outbreak, which might include the need to slaughter birds en masse.

For animal welfare reasons, birds should be gassed by carbon monoxide or by a mixture of inert gas with little oxygen, stunned or given a lethal injection. Chicks less than one week old can be dropped into a macerator, it said.

“Other methods such as: putting birds into plastic bags and burning them; gassing them with hydrogen cyanide…or injections with any chemical except barbiturates should not be used,” the report said.