Murine leukemia viruses (MLV), a family of retroviruses known to cause cancer in mice, may be linked to chronic fatigue syndrome (CFS), a study shows.

The full name of the virus is xenotropic murine leukemia virus-related virus. It is part of a family of viruses known as murine leukemia viruses (MLV), which is a type of retrovirus known to cause cancer in mice.

The new study, published in the Proceedings of the National Academy of Sciences, conflicts with some earlier studies. Several U.S. studies, including a recent report from the CDC and research done in the U.K. and the Netherlands, found no evidence of MLV in the blood of people with CFS. One recent study, however, found evidence of an MLV-related virus called XMRV in blood cells of patients with CFS.

The new study shows that 86.5% of 37 people with CFS had evidence of murine leukemia virus in their blood, as did 6.8% healthy blood donors.

“There is a dramatic association with CFS, [but] we have not determined causality for this agent,” said Harvey Alter, MD, chief of clinical studies and associate director for research in the department of transfusion medicine at the National Institutes of Health (NIH) Clinical Center in Bethesda, Md., at a news conference. “Other labs have not found this virus, so a dilemma at present is how to reconcile that some labs find the association and others do not.”

“We think it is in the patient populations, not the lab testing [contamination causing a false-positive lab result], but the latter has not been completely ruled out,” he says.

More Questions Than Answers?

Steve Monroe, PhD, director of the division of high-consequence pathogens and pathology at the CDC, tells WebMD that the new study “raises as many questions as it answers and there are still a lot of things about this virus that we don’t know.”

Andrew L. Mason, an associate professor of medicine at the University of Alberta in Edmonton, Alberta, Canada, says it’s time to act, not point fingers.

There have been several studies showing the presence of this virus in the blood of people with chronic fatigue syndrome and prostate cancer, but other studies have not found it.

“We don’t know why that is,” he says. “It is baffling, and we need to sort it out rather than ignore it. It’s there. Does it cause disease? We don’t know, but it’s there and that needs to be investigated.”

“There is only one way to prove or disprove XMRV’s role and that is to do a proper study with antiviral drugs,” Mason says. In an editorial accompanying the new study, he suggests studies that compare antiviral drugs with placebo or dummy pills on viral load and CFS symptoms in affected individuals are now feasible.

Such drugs are used to treat HIV, the virus that causes AIDS, and can cause several side effects.

“The drugs are well-tolerated, and we would be justified ethically to see if they work,” he tells WebMD. He likens this situation to the now Nobel-prize-winning research that first tested antibiotics to determine if some ulcers are caused by the bacterium H. pylori.

“We didn’t know that H. pylori was causative until they tried using medicine and it worked,” says Donnica Moore, MD, a women’s health expert and president of Sapphire Women’s Health Group in Far Hills. N.J.

The issue is a personal one for Moore, whose son was diagnosed with CFS about six years ago. As a result, she has become an outspoken advocate for research into the cause and cures for CFS. Moore is also a spokeswoman for the Whitemore Peterson Institute for Neuro-Immune Disease, the group that published a previous report linking XMRV and CFS.

“I hope this study puts to rest any question about the validity of the [findings] and will allow science to move on toward diagnostic, treatments, and causality studies,” she says.