The security zone now spreads from the south to Gramsbergen in the northeast, near the German border. It contains the cities of The Hague and Rotterdam but excludes the capital, Amsterdam.
Exports of ruminants -- animals which chew the cud -- are banned from the zone except if an importing country permits them after carrying out individual health checks. Germany and France have refused to import any animal from the zone.
The disease has hit livestock in The Netherlands, Germany, Belgium and northern France in the past year after previously being confined to countries on the Mediterranean basin.
It causes high fevers, swollen heads and a tell-tale blue tongue in sheep. It can be transmitted to cows and goats without causing sickness but poses no risk of being transmitted to humans.