Poultry farmers across the West Country have been told they must follow strict rules to stop the spread of bird flu.
The virus was found in 17 wild birds on the Fleet Nature Reserve near Weymouth.
The bird flu prevention zone has been extended to cover whole of England.
DEFRA says keepers must minimise movement in and out of enclosures, keep them clean and disinfect footwear.
Keepers with more than 500 birds must also restrict the number of people entering enclosures.
This is the first time avian flu has been identified in the UK this winter and while the disease does not represent a threat to the public, it is highly infectious and deadly to birds.
The biosecurity measures include:
Ensure the areas where birds are kept are unattractive to wild birds, for example by netting ponds, and by removing wild bird food sources;
Feed and water your birds in enclosed areas to discourage wild birds;
Minimise movement in and out of bird enclosures;
Clean and disinfect footwear and keep areas where birds live clean and tidy;
Reduce any existing contamination by cleansing and disinfecting concrete areas, and fencing off wet or boggy areas.
DEFRA says the prevention zone will be in place until further notice and will be kept under "regular review".
It says there are no plans to carry out any culls or put movement restrictions in place.