Migratory birds may be spreading viruses that cause bird flu around the world, say scientists.
Outbreaks in poultry may become more common in the future, especially in ill-prepared countries, they warn.
A severe strain found at a duck farm in the UK last year may have been carried by wild birds out of Russia, according to Dutch researchers.
The virus is a low human health risk, but wild birds on long migratory routes should be monitored, they say.
H5N8 is a strain of bird flu that appeared late last year in Russia, East Asia, North America and four European countries, including the UK.
The infections led to millions of poultry being culled.
In the UK, ducks were affected at a farm in Driffield, Yorkshire. Another outbreak of bird flu in Hampshire this week is of the H7N7 form of the virus, which is less severe.
Scientists at the Erasmus Medical Center in Rotterdam, Netherlands, say the presence of the H5 virus in a migratory bird in Russia and other detections in wild birds and poultry is “worrisome”.
“More poultry outbreaks could occur in the future, especially in countries that are ill-prepared,” a team led by Dr Ron Fouchier wrote in the journal, Science.
“Despite the currently low public health risk, the outbreaks should be monitored closely, given that several animal species are susceptible and that influenza viruses are generally unpredictable.”