The groundbreaking study was the work of scientists from the Whittemore Peterson Institute (WPI), located at the University of Nevada, Reno, the National Cancer Institute (NCI), part of the National Institutes of Health, and the Cleveland Clinic, and was published online on 8 October in the journal Science.
CFS is a debilitating disease that affects millions of people worldwide, including an estimated 1 to 4 million Americans, according to the US Centers for Disease Control and Prevention (CDC).
The illness is hard to diagnose and the mystery surrounding the condition leads some people to suspect that patients are somehow either imagining their symptoms or malingering, or that they have some underlying psychiatric problem.
According to the CDC, for a diagnosis of CFS to be valid, the patient must be showing at least four of the following symptoms: chronic fatigue that lasts for six months or more (without other medical conditions), sore throat, tender lymph nodes, pain in the muscles and joints, problems with memory and concentration, unusual headaches, poor or unrefreshing sleep, and feeling extremely tired after exertion.
XMRV is a retrovirus, a family of viruses that includes HIV and HTLV-1, known to cause cancer and immune deficiencies in humans.
Retroviruses carry their genetic information in RNA rather than DNA, and this is converted to DNA when they invade host cells. Once they invade the host cell they merge their genetic material with the host's and become an integral part of the host's genome for life.Many retroviruses are harmless and even considered "junk" DNA, while others, such as HIV, lead to life-threatening and debilitating diseases like AIDS and can pass onto to the next generation.
This study showed XMRV can be found in human blood cells and is infectious, although it is transmitted through body fluids and is not airborne.
The study authors have stressed that their study only establishes a link between the XMRV retrovirus and CFS, they have not proved that it causes the disease. This means it is possible that it is a side-effect of CFS or is caused by some other condition that coincides with the disease.
However, lead investigator Dr Judy Mikovits, who is director of research for WPI, told the media recently that:
"Since the original Science paper was submitted, we have continued to refine our test for XMRV and have surprisingly found that 95 percent ME/CFS samples tested positive for XMRV antibodies in the plasma."
"This finding clearly points to the retrovirus as a significant contributing factor in this illness."
Co-author Dr. Robert H Silverman, a professor in the Department of Cancer Biology at the Cleveland Clinic Lerner Research Institute, and who originally discovered XMRV in prostate cancer tissue of men with a specific genetic immune system defect, said:
"The discovery of XMRV in two major diseases, prostate cancer and now chronic fatigue syndrome, is very exciting. If cause-and-effect is established, there would be a new opportunity for prevention and treatment of these diseases."
For the study, the researchers decided to to test the blood of patients with CFS for the presence of XMRV, because like the prostate cancer patients that Silverman examined, patients with CFS also have an immune system defect.
Of the 101 patients with CFS that they tested, they found 68 of them (67 per cent) had XMRV in their blood.
In contrast, only 8 (3.7 per cent) of 218 healthy people they tested had blood showing signs of XMRV.
The blood came from banked blood samples donated from several medical practices throughout the United States.
The researchers found that blood cells containing XMRV expressed XMRV proteins at high levels and produced infectious viral particles. They also found that XMRV can be transmitted between blood cells.
This is the first study to directly isolate infectious XMRV from humans.
The researchers also confirmed their findings by using transmission electron microscopy to observe XMRV particles in patient samples.
They are currently developing a clinically validated test to detect XMRV antibodies in the blood of patients with ME/CFS and other human diseases.
Annette Whittemore, founder and president of WPI and mother of an ME/CFS patient told the press that:
"This is the breakthrough that we have been hoping for. Now we have scientific proof that this infectious agent is a significant factor in ME/CFS."
"Patients and their doctors will soon have a blood test to verify their diagnosis and provide the answers that they've been seeking," she added.
Co-author Dr Francis Ruscetti, of the Laboratory of Experimental Immunology at NCI, said:
"These compelling data allow the development of a hypothesis concerning a cause of this complex and misunderstood disease, since retroviruses are a known cause of neurodegenerative diseases and cancer in man."
Retroviruses like XMRV have also been shown to trigger latent viruses. This could explain why so many different viruses, such as Epstein-Barr virus, which was causally linked to Burkitt's and other lymphomas in the 1970s, have been associated with CFS, according to a statement from the NIH.
Dr William Schaffner, professor of infectious diseases at Vanderbilt University in Nashville, Tennessee, told the New York Times this was an exciting discovery that made sense and he suspects it will lead to an "avalanche of subsequent studies".
He said CFS frustrates doctors because it is poorly understood, many patients are ill with it for years and years and there is no effective treatment.
"In interacting with patients with chronic fatigue syndrome, you get the distinct impression that there's got to be something there," said Schaffner.
"Detection of an Infectious Retrovirus, XMRV, in Blood Cells of Patients with Chronic Fatigue Syndrome."
Vincent C. Lombardi, Francis W. Ruscetti, Jaydip Das Gupta, Max A. Pfost, Kathryn S. Hagen, Daniel L. Peterson, Sandra K. Ruscetti, Rachel K. Bagni, Cari Petrow-Sadowski, Bert Gold, Michael Dean, Robert H. Silverman, and Judy A. Mikovits.
Science, Published online 8 October 2009.
Sources: Whittemore Peterson Institute, NIH, New York Times.