The same virus has previously been identified in at least a quarter of prostate tumors, particularly those that are very aggressive, and has also been linked to certain types of cancers of the blood.
Cancer Institute called together a group of experts in August to consider its potential effect on public health.
"We are in the very early days," said Stuart Le Grice, director of the National Cancer Institute's Center of Excellence in HIV/AIDS and Cancer Virology, who organized the meeting but was not involved in the new study. "The data need to be confirmed and repeated. . . . We need to know that it is a cause and not just a passenger. In a sense, we are at the same stage as we were when HIV was first discovered. Hopefully, we can take advantage of what we learned from working with it."
Le Grice emphasized, however, that traces of the virus had been found in blood samples preserved for 25 years. "This is not associated with a new and spreading disease. We are not on the verge of an epidemic," he said.
Chronic fatigue syndrome, which affects at least 1 million Americans and more than 17 million people worldwide, is characterized by debilitating fatigue, chronic pain and depression, as well as other symptoms. Many doctors have argued that it is not a real disorder because there have previously been no biochemical markers that characterize it. The only effective treatments are behavioral changes and antidepressants, and they are of limited benefit.
Chronic fatigue syndrome has been theoretically linked to a variety of other viruses, including Epstein-Barr virus and human herpesvirus 6, but none have been found in a significant proportion of patients. Today's findings could explain why.
Like HIV, which causes a constellation of symptoms, XMRV is a retrovirus; retroviruses are known to suppress the immune system, making it easier for other viruses to establish a beachhead.
Dr. William C. Reeves, who heads chronic fatigue syndrome research at the Centers for Disease Control and Prevention, cautioned against racing to conclusions based on the findings, even though he characterized them as promising.
"It is almost unheard of to find an association of this magnitude in any study of an infectious agent and a well-defined disease, much less an [ill-defined] illness like chronic fatigue syndrome," he said in an e-mail. It is extremely difficult to prove causation with a ubiquitous virus like XMRV, and it "is even more difficult in the case of CFS, which represents a clinically and epidemiologically complex illness," he said.
The new study was organized by Judy A. Mikovits, director of research at the Whittemore Peterson Institute for Neuro Immune Disease, a small, 3-year-old institute on the campus of the University of Nevada, Reno.
Others involved in the study included cancer biologist Robert H. Silverman of the Cleveland Clinic Lerner Research Institute, who discovered XMRV three years ago and was the first to link it to prostate cancer, and Francis Ruscetti of the laboratory of experimental immunology at the National Cancer Institute, where Mikovits worked for 20 years.
The team reported in the online version of the journal Science that they found the virus in 68 of 101 blood samples from patients with the syndrome, but in only eight of 218 healthy patients.
In a telephone interview, Mikovits said they had also found antibodies against the virus in 95% of the chronic fatigue syndrome patients. Experts noted that no test was perfect at identifying all cases of an infection, and the antibody tests Mikovits used were being refined.
"My gut feeling is it's not a carrier virus," she said. "It's a human retrovirus, just like HIV, which is why all those other pathogens are not able to be controlled." The close association with chronic fatigue syndrome is important, she added, because "never before has there even been a biomarker in this disease."
The team concluded that the virus is not transmitted through the air. It is found in saliva and blood products, and the implication is that it is sexually transmitted, "but that has not been proven," Le Grice said.
Unfortunately, Reeves said, the major flaw of the study is that there is not enough information about how subjects were selected to rule out any bias in choosing them.