Chronic fatigue syndrome, the debilitating condition once dismissed as "yuppie flu", has been linked to a virus that is also common in people with a certain type of prostate cancer.
It's still not clear if the virus, called XMRV, causes chronic fatigue syndrome (CFS), or is just more common in people with the disorder. But the discovery is sure to reignite the debate over whether CFS is fundamentally a psychological condition or a physiological one.
CFS is characterised by cramps, sleeplessness, weakness and headaches. It affects more than a million Americans and a quarter of a million Britons, yet its cause remains elusive.
Previously a number of viruses, including herpesviruses, enteroviruses and Epstein-Barr virus – which also causes glandular fever, or mononucleosis – have been suggested as triggers for CFS. But these have only been found in a small minority of people with the disorder.
A team led by Judy Mikovits at the Whittemore Peterson Institute in Reno, Nevada, decided to investigate whether XMRV (or xenotropic murine leukaemia virus-related virus, to give it its full name) might be linked to CFS after the virus was reported in 2006 to be present in the tumour tissue of patients with a hereditary form of prostate cancer.
It is still not clear what effect the virus has on people. But the fact that this type of prostate cancer and CFS have both been linked to changes in the same antiviral enzyme led Mikovits to wonder whether XMRV could playing a role in CFS too.
When her team analysed blood taken from 101 CFS patients, 68, or two thirds, tested positive for XMRV genes, compared with just eight out of 218 healthy controls. The next step will be working out whether XMRV causes CFS or just grows particularly well in people who have it.
Mikovits suspects that XMRV causes CFS. She says her team has found antibodies against XMRV in 95 per cent of the nearly 300 patients they have tested, but these results have yet to published in a journal. Antibodies are a more sensitive test than looking for viral genes, as they pick up people who have had XMRV in the past, not just those who still have it.
What's more, some characteristics of the virus match up with the syndrome's symptoms, she says. Viruses related to XMRV can cause blood vessels around the body to leak, a common symptom of CFS. Mikovits also notes that in mice, a protein that coats the shell of the virus causes the animals' nerves to degenerate. A class of immune cells called natural killer cells, which are thought to go wrong in CFS, are known to be susceptible to infection by the virus.
"XMRV infection of [natural killer] cells may affect their function," says Jonathan Kerr, a researcher at St George's, University of London, who was not involved in the study. "This does fit." He adds, however, that "an independent study to confirm these findings is very much needed".
That sentiment is echoed by John Coffin, a virologist at Tufts University in Boston. "This looks like a very, very interesting start," he says. "It's not impossible that this could cause a disease with neurological and immunological consequences, but we don't know for sure."
Wessely points out, however, that XMRV fails to account for the wide variety of other factors associated with the CFS, including childhood trauma and other infections such as viral meningitis. "Any model that is going to be satisfactory has to explain everything, not just little bits," he says.
If XMRV does turn out to contribute to CFS, this could point to new treatments. In the UK, patients are prescribed exercise and cognitive therapy, which seems to work for some patients, but not for most. Such failings underscore the need for therapies that go after the root cause of chronic fatigue syndrome – whatever it turns out to be.