In a report published today, Defra confirmed two further cases of low-pathogenic H7N3 avian influenza in free-range poultry in Norfolk. Both flocks were owned by the same farmer and comprised 8,000 chickens each. They have now been culled.

It is clear that the infection at these two farms pre-dates that at the intensive broiler plant where the virus was first diagnosed on 27 April. All three farms are within 3 km of each other.

The free range flocks were housed, fed and watered in barns but allowed outside access during daylight. This means that wild birds cannot be ruled out as the origin of the original infection.

However, it is not clear how the infection entered the intensive broiler production plant, which operated biosecurity measures to prevent infection getting in. There is some evidence that the virus has increased in its pathogenicity (its ability to cause serious disease) to poultry, as it has spread amongst the domestic flocks, though further test results are awaited to confirm this.

One worker at the broiler plant has developed conjunctivitis as a result of contracting the virus. Defra states that the virus in its current form has only extremely limited implications for human health.

Defra are maintaining livestock movement controls within 1 km or each of the farms and continuing to conduct surveillance of poultry flocks in the area.

It is believed that most highly pathogenic avian influenzas evolve in domestic poultry from low pathogenic strains introduced from wild birds. The pattern of infection in this case seems to fit this theory.

No further cases of highly pathogenic H5N1 have been discovered in the UK since the single whooper swan in Fife on 5 April.

There are no restrictions on access to the countryside or feeding garden birds as a result of this recent development, and no reason for alarm.