The H7N3 bird flu virus is suspected to have hit two more UK farms near Dereham, Norfolk.
Preliminary test results indicated the same avian influenza strain that caused the slaughter of 35,000 birds on a neighbouring farm last week, rather than H5N1 virus causing problems globally.
The two free-range flocks, comprising of more than 15,000 birds, have been culled.
UK Government agency Defra has enforced a restricted zone extending 1km from each of the infected premises.
“We still cannot say whether either of these two further farms are the index case, further premises may be involved. We are investigating whether there any links or movements between the two suspect farms and the confirmed infected premises. The working hypothesis remains that the most likely source of the virus is from another premises or from wild birds,” said Debby Reynolds, Defra chief veterinary officer.
The spread of the virus comes despite a Defra announcement on Friday (28 April) that tests indicated the Norfolk strain of the H7N3 virus had low pathogenicity.
The UK Veterinary Laboratories Agency (VLA) is further investigating the H7N3 vius, which last occurred in the UK in 1979, and Defra added that a high pathogenicity could not be completely ruled out yet.
“The State Veterinary Service is continuing to trace movements and contacts, the necessary surveillance and all appropriate worker protection measures have been put in place,” Reynolds added.