Mysterious disease causes mass deaths among small birds in Sweden

A newly identified illness known as “blåmessjukan” (blue tit disease) has led to a surge in deaths among small birds in Sweden this spring, with blue tits the hardest hit, according to public broadcaster SVT Nyheter.

Reports of sick and dead birds have risen sharply since February, according to the Swedish National Veterinary Institute (SVA). The disease, which causes symptoms resembling severe pneumonia, remains poorly understood, though transmission appears to occur when birds gather at feeders.

“We don’t know exactly why this bacterium has started spreading among blue tits, but we do know they infect each other when they congregate at feeders and sit close together,” said Karin Olofsson-Sannö, deputy state veterinarian at SVA. “It’s a relatively newly discovered disease, so knowledge about it is still limited.”

To curb the outbreak, SVA is urging the public to stop feeding small birds—particularly blue tits—hoping to reduce transmission at shared feeding sites. Olofsson-Sannö noted that natural food sources are now available, making supplementary feeding unnecessary.

“At this time of year, they can manage on their own. There’s food for them in nature, so we should avoid gathering them together,” she said.

The disease currently poses no known risk to humans, though SVA is monitoring its genetic characteristics. The last comparable outbreak in Sweden occurred in 2020.