A virus previously suspected in chronic fatigue syndrome is not the cause of the disease, say researchers who found samples were contaminated in the original study.

British researchers reported Monday that samples from patients in an earlier study thought to contain the XMRV virus were in fact contaminated by cells or mouse DNA rather than infection. Sequences from the genome of XMRV likely tainted human chronic fatigue syndrome samples, British researchers say. (University College London/AJC1/Flickr)

"Our conclusion is quite simple: XMRV is not the cause of chronic fatigue syndrome," said Professor Greg Towers, a Wellcome Trust Senior Research Fellow at University College London.

"All our evidence shows that the sequences from the virus genome in cell culture have contaminated human chronic fatigue syndrome and prostate cancer samples."

Chronic fatigue syndrome is characterized by at least six months of severe fatigue, impaired memory and other symptoms, but there's no test for it — doctors rule out other possible causes — and no specific treatment.

Chronic fatigue syndrome could still have a viral cause, the researchers noted, saying they are unable to answer that question.

Writing in the journal Retrovirology, the study's authors proposed using more rigorous methods to prevent contamination of cell and DNA samples.

Scientists need robust guidelines when ascribing a cause to a disease, said study author Professor Paul Kellam, virus genomics group leader at Wellcome Trust Sanger Institute. Sueraja Narasimhan worked with DNA samples at the U.S. Centers for Disease Control and Prevention's chronic fatigue syndrome lab in Atlanta in 2006. British researchers are now calling for more rigorous methods to prevent contamination in labs following a number of failed attempts to find XMRV in chronic fatigue samples. (John Bazemore/Associated Press)

Towers and his co-authors found that sequences from the contaminated cell line and chronic fatigue patient samples were extremely similar — the opposite of the pattern of evolution they would expect as a viral infection spreads in a human population, the researchers said.

The findings mean it now seems highly unlikely that XMRV is linked to chronic fatigue syndrome, said Tim Peto, a consultant in infectious diseases at Oxford University who was not involved in the research.

"It came as a great surprise when XMRV was first suggested as being linked to chronic fatigue syndrome," Peto said in an emailed statement to Reuters.

"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."

Chronic fatigue syndrome is also known as myalgic encephalomyelitis or ME.

With files from The Associated Press