Image caption Boas - one of the snakes affected by the new virus

The cause of a fatal illness that affects captive snakes has been identified, a study has shown.

The condition - called Inclusion Body Disease (IBD) - affects constrictor snakes including boas and pythons.

There is no treatment and symptoms include "stargazing" - a fixed upward stare - as well as breathing problems and general muscular paralysis.

It was long suspected that the disease was caused by a virus, but until recently its identity remained elusive.

The research is published in the open-access journal mBio.

In this breakthrough study, researchers from the University of California San Francisco analysed samples obtained from snakes diagnosed with IBD, using sensitive DNA sequencing techniques.

Media playback is unsupported on your device Media caption Dr Joseph De Risi explains why snakes have been tying themselves up in knots

In amongst some of the snake DNA was foreign genetic material - nucleic acid - that closely resembled that present in viruses belonging to a family called arenaviruses. This family includes Lassa Fever virus, which is associated with haemorrhagic fever in humans. However, there is no evidence that the newly discovered virus can pass from snakes to humans.

The scientists were also able to grow the virus from samples taken from one of the snakes.

Dr Mark Stenglein, who co-led the current study, said "we don't yet have formal evidence that these viruses cause the disease… although there is a good correlation [between disease and the presence of virus] … it is also possible that other viruses or pathogens cause a similar set of symptoms".

Arenaviruses can be divided into two main groups based on the location of the species they naturally infect - New World viruses originate from the Americas, whilst Old World viruses are found in Africa and Asia. Genetically, the newly discovered virus is distinct from these two groups.

Image caption There was a good correlation between the arenaviruses and the presence of disease

Commenting on the finding, the editor of the paper Michael Buchmeier, professor of infectious diseases at University of California Irvine, suggests that these snake viruses "may be representative of a predecessor of the Old World and New World branches of the [arenavirus] family".

The genetic analyses also revealed that one of the genes in the newly isolated virus group was more like that present in viruses belonging to a totally different family of haemorrhagic viruses called filoviruses. Ebolavirus belongs to this family.

The new discovery follows similar research published online in April 2012 in the journal Infection, Genetics and Evolution, which describes isolation of a novel virus from snakes - this time in Australia - that showed symptoms very similar to IBD. However, the virus isolated in this study belonged to a very different virus family known as paramyxoviruses.

Professor Jim Wellehan from the University of Florida College of Veterinary Medicine, who authored the paramyxovirus study, said: "The epidemiology of the paramyxoviruses is different [to IBD]. These [paramyxoviruses] are hot agents that snakes die quickly from, and it works fast. You have a room full of dead snakes in a week."

It is uncertain how the highly contagious IBD virus is spread. One possibility is that transmission occurs through inhalation - either directly from another infected snake or indirectly from contaminated bedding or following handling. Alternatively, mites - often found in colonies suffering from an IBD outbreak - might be implicated.

So far the disease seems to be restricted to captive snakes but some scientists are worried that the release of captive bred or rehabilitated snakes might unwittingly unleash this devastating virus into the wild. However, Dr Stenglein thinks differently: "These viruses have been infecting modern snakes and their ancestors for at least 35 million years. It is reasonable to assume wild snakes are also infected and are the reservoir."