A virus previously thought to be linked to a baffling condition known as chronic fatigue syndrome is not the cause of the disease, scientists said on Monday after their study found previous research was contaminated in the lab.

Researchers from University College London, the Wellcome Trust Sanger Institute and Oxford University said cell samples from patients in earlier studies were contaminated with the virus, known as XMRV, which is found in the DNA of mice.

This suggests the patients were not infected with XMRV and it could not have triggered their illness, the scientists said.

The finding, published in the journal Retrovirology, is the latest to contradict a U.S. study from 2009 which suggested a link between XMRV and chronic fatigue syndrome (CFS) when the virus was found in the blood of 68 out of 101 CFS patients. The XMRV virus has also been identified in samples from certain prostate cancer patients.

“Our conclusion is quite simple: XMRV is not the cause of chronic fatigue syndrome,” said Greg Towers, of UCL, who worked on the latest study. “All our evidence shows that the sequences from the virus genome in cell culture have contaminated human chronic fatigue syndrome and prostate cancer samples.”

CFS is a debilitating condition of disabling physical and mental fatigue that does not improve with rest. It also known as myalgic encephalomyelitis (ME) and affects around 17 million people worldwide. There is no cure for CSF and scientists don’t know what causes it, but many sufferers say they think their illness started after a viral infection.

CELL LINE

Towers said it was vital to understand that this latest research did not suggest chronic fatigue syndrome is not caused by a virus of some sort. “We cannot answer that yet,” he said. “But we know it is not this virus causing it.”

The 2009 U.S. study that found a link had prompted hopes that CFS patients might benefit from a range of drugs designed to fight AIDS, cancer and inflammation.

But in January 2010, British researchers found no evidence of XMRV in 186 patients with chronic fatigue syndrome, and two separate studies published in February also failed to identify the virus in groups of ME patients.

Towers’ team said their study found that the XMRV found in the studies that linked it to CFS was from contamination by a laboratory cell line or mouse DNA. The sequences from the contaminated cell line and chronic fatigue patient samples were very similar, they said, and this is contrary to what scientists would expect from a virus if it were spreading in humans.

Tim Peto, a consultant in infectious diseases at Oxford University who was not involved in the research, said Monday’s findings meant “it now seems really very, very unlikely that XMRV is linked to chronic fatigue syndrome.”

“It came as a great surprise when XMRV was first suggested as being linked to chronic fatigue syndrome,” he said in an emailed statement. “There have now been a number of attempts which have failed to find the retrovirus in other samples, and this research suggests that in fact XMRV is probably a contamination from mouse DNA.”

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By Kate Kelland

LONDON