Scientists and poultry farmers are mystified by the shock discovery the H5N1 virus at Holton.

Scientists and poultry farmers are mystified by the shock discovery the H5N1 virus at Holton.

The virus, which killed about one third of the 55-day-old turkeys, must have originated from an outside source.

What is puzzling is that Matthews has a vertically integrated operation, producing its own turkey eggs, which are reared on its other farms. It does not bring eggs in from outside.

The infected turkeys, weighing about 3.5kg to 4kg each, were all about eight weeks old. Tests are being carried out but the virus does not seem to have spread to the other 21 sheds.

The culled birds will be incinerated at a specialist plant in Staffordshire.

"The virus is very easily killed with disinfectant," said Norfolk farmer Nigel Joice, chairman of the NFU's regional poultry board.

How the virus arrived inside a sealed unit, fitted with sophisticated ventilation systems is another puzzle.