Two new studies add to the mounting evidence suggesting chronic fatigue syndrome is not caused by the virus XMRV, as has been previously theorized.

In 2009, a group of scientists caused a stir when they reported that about two-thirds of patients with the mysterious disease had XMRV, a mouse retrovirus, in their blood. That study, published in the journal Science, led to hope that a cause of chronic fatigue syndrome had finally been found. However, further studies were unable to confirm those findings.

This week the same journal is publishing two papers and an editorial that question the validity of the 2009 study, saying that laboratory contamination may have been to blame.

In one of the new studies, researchers testedNew Test May Screen Donated Blood for Fatal Disease-Causing Proteins blood samples from 61 patients with chronic fatigue syndrome, including 43 samples from patients who had tested positive for the virus. The new study found no trace of the virus.

"There is no evidence of this mouse virus in human blood," study researcher Dr. Jay Levy, a professor at the University of California, San Francisco, said in a statement. Researchers need to continue to look for the real causes of chronic fatigue syndrome, he said.

The second study suggests the virus originally arose inside laboratories in the 1990s when two other mouse viruses joined together.

The two new studies make it almost impossible to deny that the 2009 finding was an error, said Mary Ann Fletcher, a professor at the University of Miami Miller School of Medicine, who has studied possible causes of chronic fatigue syndrome. "I don't think anybody's implying mal-intent," Fletcher said. "It's just a laboratory mistake."

However, others say it's too soon to make conclusions about the virus and chronic fatigue syndrome. The National Institutes of Health (NIH) is sponsoring several studies to carefully examine the link, and we should wait for those results, said Leonard Jason, a professor of psychology at DePaul University in Chicago who studies chronic fatigue syndrome.

"There's lot of controversy, [and] lots of circumstantial information," Jason said. "Let's wait for the science to be finished on this issue," he said. Those studies could be available in the year, he said.

It could be that another infectious agent is responsible, Fletcher said. For example, many in the field believe the Epstein-Barr virus may play a role in triggering the disorder. Research into possible infectious causes of chronic fatigue syndrome should continue, she said.

Chronic fatigue syndrome is characterized by extreme fatigue for at least six months that is not alleviated by rest, and cannot be explained by other conditions, according to the Centers for Disease Control and Prevention. It affects about 1 million to 4 million people in the United States, and 17 million worldwide, according to the NIH.

Pass it on: Two new studies call into question the link between chronic fatigue syndrome and XMRV.

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