We may have an explanation for the mysterious death of hundreds of bald eagles (Haliaeetus leucocephalus) across the south-eastern US. They may have ingested bromide-laced prey plucked from lakes, although the source of the bromide is unclear.

In 1994, dozens of bald eagles died in Arkansas. Since then, nearly 200 others – all living near artificial lakes from Texas to the Carolinas – have been diagnosed with vacuolar myelinopathy, which creates holes in the brain and spinal cord.

Eagles with the condition have crashed into cliffs or starved as they lost control of their wings and other bodily movements, says Timo Niedermeyer at Martin-Luther-University Halle-Wittenberg in Germany.

US researchers have long suspected a connection with water-thyme (Hydrilla verticillata), a fast-growing underwater weed, and the slimy blue-green algae – technically cyanobacteria – that covers it. They noticed that fish and water birds became weak and uncoordinated after consuming the weed and that eagles were eating this contaminated prey as it was easy to catch.