A mysterious "mad snake disease" that causes pythons to tie themselves up in knots may be caused by a rodent virus, scientists believe.
The fatal condition, called Inclusion Body Disease (IBD), strikes captive pythons and boa constrictors.
Snakes with the disease start to display strange behavioural traits, such as "stargazing" - staring upwards for long periods of time.
Other symptoms include appearing drunk and getting into a legless tangle.
"They tie themselves in a knot and they can't get out of it," said US expert Professor Michael Buchmeier, from the University of California at Irvine.
IBD gets its name from inclusions, or pockets of foreign material, found in the cells of affected animals.
Although the disease is known to be highly infectious, its cause has been a mystery until now.
A team led by Prof Buchmeier investigated an outbreak of IBD among snakes at the Steinhart Aquarium in San Francisco.
The researchers identified signs of an unusual virus in DNA samples from a number of affected boas and pythons.
Tests showed it belonged to a family of arenaviruses - a type of virus that normally infects rodents - never seen before. The virus did not fit neatly into either of the two known categories of New World and Old World arenaviruses.
"This is one of the most exciting things that has happened to us in virology in a very long time," said Prof Buchmeier. "The fact that we have apparently identified a whole new lineage of arenaviruses that may predate the New and Old World is very exciting."
He added that finding the virus in snakes was another surprise twist. Previously arenaviruses had only been known in mammals.
The research is published in mBio, the online journal of the American Society for Microbiology.
Arenaviruses usually infect mice and rats, but can cause haemorrhagic fever in humans.