Scientists have uncovered a strong link between an unusual virus and chronic fatigue syndrome, which affects more than 1 million people in the United States.
Researchers found that two-thirds of people with chronic fatigue are infected with a retrovirus called XMRV, according to a new study in the journal Science Express. XMRV has also been found in the tumors of some prostate cancer patients.
Scientists say it's too soon to say whether XMRV actually causes chronic fatigue.
People with the syndrome feel tired even after a good night's sleep. Many also have debilitating pain in their muscles or joints, trouble concentrating and immune problems.
The new study compared blood samples from 101 chronic fatigue patients with samples from 218 healthy people. About 67 percent of the sick people had XMRV, compared with fewer than 4 percent of healthy people.
Understanding The Retrovirus
XMRV and other retroviruses are known to infect immune cells. XMRV has been found in some prostate tumors. It's also related to a retrovirus that causes cancer in animals.
The best-known retrovirus is HIV. But scientists say XMRV is simpler, and not a close relative.
In people, XMRV could explain "the entire spectrum of symptoms that have come to be known as chronic fatigue syndrome," says Judy Mikovits, one of the study's authors and research director of the Whittemore Peterson Institute at the University of Nevada, Reno.
But scientists have pointed to viruses as a cause of chronic fatigue before and been wrong.
"This is a very striking initial finding, but it is only an initial finding," says John Coffin, a molecular biologist from Tufts University who was not involved in the study. He co-authored a companion piece about the finding in Science Express.
Researchers have suspected for a long time that chronic fatigue might be caused by a virus.
One reason for that suspicion is that many people get the condition after a flu-like illness, says Mikovits. "They get very bad flu-like symptoms and essentially never recover," she says.
Most viruses don't survive long in the body. But retroviruses are one type that lingers. HIV, for example, is a retrovirus that infects people for a lifetime.
So when scientists found XMRV in people a couple of years ago, Mikovits thought there might be a connection to chronic fatigue, which also tends to last a lifetime.
She was in a position to find out. After many years at the National Institutes of Health, she'd come to the Whittemore Peterson Institute, a place founded to help people with chronic fatigue.
Mikovits worked with a team that began checking patients' blood for XMRV.
"We simply did a screen of the sickest of the sick of our patients because we figured that would be where we would find the most virus," she says. "And, lo and behold, there it was."
Tests also showed the virus was infectious and was provoking an immune response in people with chronic fatigue.
Isolating The Cause
The finding is "the best news ever" for people with chronic fatigue, says Annette Whittemore, one of the founders of the Whittemore Peterson Institute. "We've always known there was something out there. Now we see its face," she says.
Whittemore has an adult daughter who has had chronic fatigue since she was 12. She created the institute with Dr. Daniel Peterson, one of the first doctors to identify people with the condition that later became known as chronic fatigue syndrome.
Even though XMRV has not yet been shown to cause chronic fatigue syndrome, it has characteristics that make it a likely suspect, experts say.
For one thing, it's a retrovirus.
"Retroviruses in general give rise to infections that persist indefinitely," says Coffin. Most other viruses are eliminated from the body.
Coffin says he's concerned by the finding that nearly 4 percent of healthy people carried XMRV. That would mean 10 million people in the U.S. are infected.
Coffin says scientists need to find out whether the virus is causing health problems other than chronic fatigue in any of these people.
For people who have chronic fatigue, the XMRV finding could lead to the first treatments.
Antiviral drugs developed for people with HIV may also work against XMRV, Mikovits says. The institute plans to begin testing that idea soon.
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