* Virus found in 67 percent of chronic fatigue patients

* Findings show link to CSF, not proof of causation

* Discovery a major step toward treatment options

WASHINGTON, Oct 8 (Reuters) - A virus linked to prostate cancer also appears to play a role in chronic fatigue syndrome, according to research that could lead to the first drug treatments for a mysterious disorder that affects 17 million people worldwide.

Researchers found the virus, known as XMRV, in the blood of 68 out of 101 chronic fatigue syndrome patients. The same virus showed up in only 8 of 218 healthy people, they reported on Thursday in the journal Science.

Judy Mikovits of the Whittemore Peterson Institute in Nevada and colleagues at the National Cancer Institute and the Cleveland Clinic emphasized that the finding only shows a link between the virus and chronic fatigue syndrome, or CFS, and does not prove that the pathogen causes the disorder.

Much more study would be necessary to show a direct link, but Mikovits said the study offers hope that CFS sufferers might gain relief from a cocktail of drugs designed to fight AIDS, cancer and inflammation.

"You can imagine a number of combination therapies that could be quite effective and could at least be used in clinical trials right away," Mikovits said in a telephone interview.

She said AIDS drugs such as non-nucleoside reverse transcriptase inhibitors and integrase inhibitors as well as nonsteroidal anti-inflammatory drugs and cancer-fighting proteasome inhibitors could be tested as potential treatments for CFS.

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makes a cancer drug called Velcade that is a proteasome inhibitor, although there are no reports that it has been tested against XMRV.

INCAPACITATING FATIGUE

CFS impairs the immune system and causes incapacitating fatigue, according to the U.S. Centers for Disease Control and Prevention. Sufferers can also experience memory loss, problems with concentration, joint and muscle pain, headaches, tender lymph nodes and sore throats.

Symptoms last at least six months and can be as disabling as multiple sclerosis or rheumatoid arthritis, the CDC said.

But Mikovits said there is currently no treatment for CFS aside from cognitive behavioral therapy to help patients cope with the disorder's crippling effects.

The XMRV virus is a retrovirus, like the HIV virus that causes AIDS. As with all viruses, a retrovirus copies its genetic code into the DNA of its host but uses RNA -- a working form of DNA -- instead of using DNA to do so.

Known formally as xenotropic murine leukemia virus-related virus, XMRV has also been found in some prostate tumors and is also known to cause leukemia and tumors in animals. [ID:nN07209255]

Mikovits' team said further research must now determine whether XMRV directly causes CFS, is just a passenger virus in the suppressed immune systems of sufferers or a pathogen that acts in concert with other viruses that have been implicated in the disorder by previous research.

"Conceivably these viruses could be co-factors in pathogenesis, as is the case for HIV-mediated disease, where co-infecting pathogens play an important role," the report said.

Because 3.7 percent of the healthy test population tested positive for XMRV, the researchers said several million otherwise healthy people in the United States could be infected with it. (Editing by Maggie Fox and Vicki Allen)