Bird flu could be “evolving under the radar” because of failures to monitor and control the spread of the virus, a leading pandemic scientist has warned.

Dr Thomas Peacock, a specialist in animal-to-human spread of viruses at The Pirbright Institute, said H5N1 could be transmitting undetected in the US because of “months of missing data” that leaves researchers, vets and authorities in the dark.

The strain is currently spreading between US dairy cows after crossing over from wild birds earlier in the year.

Four workers on cattle farms have also become infected and the Centers for Disease Control (CDC) recently reported the first human case with no known contact with animals.