A mysterious plague that has killed off millions of starfish along the US Pacific Coast since 2013 is now believed to be a virus that causes the creatures to melt, US researchers said Monday.

Known as densovirus, the microorganism has been found in diseased and dead starfish, and is the likely culprit for the massive upsurge in deaths, said the report in the Proceedings of the National Academy of Sciences.

The virus causes the limbs of starfish, or sea stars, to pull apart and their skin to waste away, and has been wreaking havoc on populations from Baja California to southern Alaska.

Ian Hewson of Cornell University led the genomic analysis on sea star associated densovirus (SSaDV), a type of parvovirus commonly found in invertebrates.

"There are 10 million viruses in a drop of seawater, so discovering the virus associated with a marine disease can be like looking for a needle in a haystack," said Hewson, a professor of microbiology.

"Not only is this an important discovery of a virus involved in a mass mortality of marine invertebrates, but this is also the first virus described in a sea star."

Researchers found the virus present at low levels in museum samples of sea stars collected in 1942, 1980, 1987 and 1991.

Overpopulation, pollution or mutations in the virus could have contributed to its sudden surge to epidemic proportions, the study found.

Densovirus has also turned up in water filters from public aquariums, sea urchins and brittle stars.

More research is needed to find out what triggers outbreaks, said co-author Drew Harvell, a professor of ecology and evolutionary biology.

"It's the experiment of the century for marine ecologists," said Harvell.

"It is happening at such a large scale to the most important predators of the tidal and sub-tidal zones. Their disappearance is an experiment in ecological upheaval the likes of which we've never seen."

The study was funded by the National Science Foundation (NSF) and Cornell University's David R. Atkinson Center for a Sustainable Future.