Wild bumblebees around the world are likely contracting deadly diseases from their honeybee cousins, according to a new study.

Wild and captive bee populations are in decline in America, Asia, and Europe, for reasons scientists can't understand.

Researchers said they found evidence that backs a theory that bumblebees are being affected by viruses or parasites from honeybee hives, according to the study.

"Wild populations of bumblebees appear to be in significant decline across Europe, North America, South America and also in Asia," said study author Mark Brown of the University of London, according to the Associated Press. He said his study confirmed that a major source of the decline was "the spillover of parasites and pathogens and disease" from managed honeybee hives.

The team conducted a three-phase experiment and the results were published in the journal Nature.

For the first experiment, the team exposed bumblebees in a lab to two pathogens, the Nosema ceranae parasite, and the deformed wing virus, to see if they can catch viruses already known to affect honeybees.

Researchers then set out to the countryside, collecting honeybees and bumblebees from places around Britain and tested them for infection, according to AFP.

"Honeybees and bumblebees have very similar levels of those pathogens at the same site, so that means there is some connection between honeybees and bumblebees at those sites that is highly indicative of a spillover," said Matthias Fuerst of the Royal Holloway University of London, according to AFP.

Researchers determined in the final step that honeybees and bumblebees collected from the same location had more "closely-related strains" of the same virus than bees from other locations, which they believe to be a key indication of inter-species infection, according to AFP.

Even though the researchers couldn't prove that the pathogens crossed from honeybees to bumblebees, instead of the other way round, they did say this is the "most logical conclusion."

More honeybees than bumblebees were infected, and infected honeybees had higher virus levels than infected bumblebees.

"The prevalence of the virus is much higher in honeybees so the logic for making that prediction is clear, but we don't have absolutely definitive evidence that the flow is in that direction," said Fuerst.

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