Warned that bird flu is spreading "efficiently" in rodents, amid growing fears that it could lead to a new pandemic.
Experts described the discovery as "extremely worrying", claiming it shows the pathogen is one step away from circulating in humans.
This is the first known study to clearly establish that individual mammals can not only contract the disease, but also spread it.
However, deaths of infected mammals such as mink, foxes, raccoons and bears indicated that this was possible.
H5N1 - the strain of bird flu that caused the current global outbreak, and is considered the largest ever - is not easily transmitted between humans.
Some experts fear that mutations in the virus that make transmission easier from mammals to mammals could change that.
Globally, fewer than 900 human cases of H5N1 virus have been recorded, which kills nearly 50% of the people it infects.
The virus is usually caught through close contact with an infected bird, whether it is alive or dead.
New research, which has not yet been peer-reviewed, found that H5N1 can "cause fatal disease in multiple species of mammals".
Canadian researchers, including some government health agencies, infected the ferrets with one of four strains of the H5N1 virus.
Rodents were chosen for the study because they have a similar respiratory structure to humans, which gives experts an idea of how the virus interacts in humans.
They found that "direct contact" with a single strain of H5N1 virus isolated from an infected bird resulted in a "fatal outcome". It raises the possibility that the strain may have evolved "certain adaptations that allow for a higher degree of replication, pathogenicity, and transmissibility".
And they warned that if such a strain made the jump to humans, the consequences could be dire.
"Because there is little or no population-wide immunity to H5, if an H5N1 isolate capable of somewhat persistent transmission makes the jump to humans, this would likely represent a devastating infection in the population," they wrote.
John Fulton, a pharmaceutical industry consultant and founder of BioNiagara, said H5N1 is a threat "100 times worse than Covid".
"This discovery is very worrying, and governments must take immediate action by seeking and mobilizing all high-potential production capacities for vaccines and treatments to prevent and treat H5N1 avian influenza," he added.
And vaccine makers GSK, Moderna and CSL Seqirus have begun developing new human doses to target the fast-spreading strain of the virus.
Others, such as Sanofi, have generic H5N1 vaccines in stock that can be adapted to the strain currently circulating.
Like other forms of influenza, people can become infected if the virus gets into their eyes, nose, or mouth, or they breathe it in.
But in the case of bird flu, this usually happens in people who spend a lot of time with infected creatures.
A series of human avian influenza cases emerged in early 2023.
Earlier this year, a Cambodian man and his daughter were diagnosed with the H5N1 virus.
Their cases have sparked international concern, with many experts fearing that the infection was evidence that the virus had mutated to better infect people.
Further testing found that the Cambodian family did not have the H5N1 strain that is spreading rapidly among the world's wild birds - but instead, a known variant circulating locally in the Prei Veng district where they resided.
There has been only one case of a British person with H5N1 since the ongoing outbreak began in October 2021.
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