German officials battling the current H5N1 avian flu outbreak have admitted that infected ducks may have entered the food chain.

Tests on 18 deep frozen ducks at a processing plant at the centre of the current outbreak were positive for the virus, according to a state official. These 18 ducks had been set aside as a routine batch sample at the plant.

Roland Eichhorn of the Bavarian Consumer Affairs Ministry could not rule out the possibility that consumers had unwittingly eaten infected meat.

But if this was the case, the health risk would be low, he asserted. "This type of duck is casseroled and then the meat poses no danger to the consumer."

The discovery came as officials completed the slaughter of 205,000 ducks on two further units 87 miles (140km) north of Munich after birds were found to have antibodies indicating exposure to the virus.

The head of Bavaria's state office for health and food safety, Volker Hingst, said the slaughter was "a purely precautionary measure," taken after "laboratory indications of H5N1" were found. The birds were not visibly sick, he added.

Both farms had links with the original farm in the current outbreak, which came to light on 25 August when an abnormally high mortality rate was detected in a flock of nearly 170,000 ducks. More than 400 birds died in a short period.

Investigations into the source of the virus are ongoing but straw contaminated with the H5N1 virus was the likely source of an outbreak. Ottmar Fick, the chief veterinarian in the Erlangen district of northern Bavaria said: "It remained unclear how the straw, which was stored on the farm, became infected, although wild birds were a possible source."

Adding weight to that theory are the large numbers of wild birds seen in central and southern Germany in recent months, including wild ducks near Munich.