The outbreak of avian flu that decimated farmed and wild birds around the world and in Germany in 2016 doesn't show signs of letting up in 2017. But experts disagree on the causes behind the outbreak.
Bird flu is back, and is spreading quickly. But don't worry, the virus isn't targeting humans - yet.
The H5N8 strain of bird flu reached Europe in October 2016, and since then has been detected in at least 14 countries including France, Denmark and Germany.
In Germany alone, authorities have recorded more than 30 separate incidents of bird flu, leading to the cull of tens of thousands of domesticated chickens, ducks and geese, among others.
In the wild, more than 500 birds are known to have been infected as well.
Wild ducks in Ireland and England, and an outbreak that killed 2,000 ducks in a French farm, are a few of the most recent cases sadly confirming that the outbreak has not ended in 2016 - on the contrary, it continues spreading.
Experts are desperately looking for the origin of the virus - without any official conclusion until now. Some argue that the natural migratory patterns of wild birds are to blame, while others insist industrial livestock farming and international trade are primarily responsible.
European Union and national authorities have adopted urgent protective measures for strict "biosecurity" in farms. But despite the efforts, new cases continue appearing daily.
In the worst-case scenario - a rare one, at that - a species jump could lead to the virus affecting humans.
With viruses, identifying the origin is the first step in controlling the spread. But it is exactly at this point where experts disagree. Vectors - that is, how the virus is transmitted - remain under discussion.
"Judging by history, it is most likely to spread through the poultry industry," Ariel Brunner, senior head of policy at BirdLife International, told DW. "But for the moment, we don't know."
Indeed, according to a statement from a scientific task force that includes the United Nations Environment Program/Convention on Migratory Species and the Food and Agriculture Organization (FAO), bird flu outbreaks are usually associated with the poultry trade and intensive domestic poultry production.
Lars Lachmann, a bird protection expert with the German Nature and Biodiversity Conservation Union (NABU), also points to industrial agriculture. Transmission most likely expanded via poultry from Asia to somewhere in Europe, and from there across cities and countries, he said.
But the German National Institute for Animal Health disagrees, and maintains that infected wild birds migrating from Asia to Europe are to blame, Franz Conraths - head of the Institute of Epidemiology at the Friedrich Loeffler Institute - told DW.
The first known victim of the ongoing outbreak was a wild swan in Hungary, found one week before the first outbreak in a farm.
Although this would confirm Conraths' hypothesis, Lachmann argues this might have been a coincidence, since it can take up to two weeks to detect the flu in domesticated ducks and geese.
Wild birds at risk
While there is no definitive conclusion, for Brunner it is hard to believe an ill bird would be able to fly the long distances on migratory routes - yet a chicken put on a plane in China and delivered to Nigeria can spread the virus in few days, he pointed out. In any case, wild birds are at risk.
Although a prolonged outbreak on poultry farms can have catastrophic economic consequences, mainly for poultry farmers, damage for biodiversity could reach even more frightening proportions.
"If it affects common species, that will be part of the natural cycle - but if it hits species already threatened by habitat destruction or hunting, it will be much more problematic," Brunner said.
Nearly a third of wild European birds species are in decline. Habitat loss, industrial agriculture including pesticides, and climate change are among factors already adding pressure to wild bird populations - viral outbreaks come on top of this.
Even more dramatically, Brunner pointed out that a random mutation could cause the virus to jump the species barrier as part of the natural process, infecting humans - as has happened in the past, for example with swine flu.
How to stop the spread
While the vectors remain unknown, the bird flu keeps spreading throughout Europe. The European Commission and national authorities have adopted urgent protective measures in an attempt to improve biosecurity, and strict emergency measures have been implemented in affected countries.
In Germany, for instance, animals in poultry farms must stay indoors to avoid contact with wild birds. Thousands have been culled. Although keeping the animals indoors is not a problem for industrial farms, it is for small and sustainability-oriented farms where animals are supposed to move freely.
Lachmann and his colleagues have supported the idea of keeping animals indoors - but only in high-risk areas, and for the minimum required time.
"Since we believe the transmission from wild birds is a less probable option, we agree on this measure," he said.
All experts agree now that the most important thing would be to prevent a future outbreak. For Brunner, that means by better controlling hygiene on farms, particularly industrial ones.
"These huge concentrations of species are the perfect breeding ground for diseases: The birds have lower immune systems than wild ones, and have no natural predators," he explained.
But until the origin of the virus has been identified, it seems impossible to wipe out the virus.
Lachmann says continuing to ignore the role of the poultry trade in the spread is a mistake.
Meanwhile, Conraths believes farmers should strengthen their protective measures to keep the animals far from wild birds - and wait it out.
"I am afraid we will have to live with situations like this," he said. "If the hypothesis is true and the virus travels with wild birds, we will have to adapt - this is a natural phenomenon."