After a small delay (explained in an editorial here), a long-awaited government-funded study on chronic fatigue syndrome (CFS) was released Monday — much to the relief of many of the estimated 4 million or so Americans who have this little-understood medical condition.

CFS patients (who are frequently told the condition is "in their head") have been waiting a long time to get some medical answers for their often-debilitating symptoms, which include prolonged joint and muscle pain, sore throat, tender lymph nodes, headaches, insomnia, and difficulties with memory and concentration.

The current study found that blood samples of people with CFS were much more likely than samples taken from people without the condition to contain gene sequences similar to those of a class of viruses that cause leukemia in mice (murine leukemia virus, or MLV). 

Specifically, the study found 32 of 37 (86.5 percent) of blood samples from CFS patients had genetic evidence of MLV compared to 3 of 44 (6.8 percent) of samples from people without the condition.

This finding adds support to the controversial findings of a study published last year, which linked a related virus, XMRV (xenotropic murine leukemia virus-releated virus) to chronic fatigue syndrome. Other groups of scientists, including one from the Centers for Disease Control and Prevention, had been unable to replicate that study’s findings, a factor that had cast doubts on its validity.

The current study did not find a link to XMRV, either. But it did find an association between MLVs and chronic fatigue.

“Our results clearly support the central argument … that MLV-releated viruses are associated with chronic fatigue syndrome and are present in some blood donors,” wrote the study's authors in their paper, which appears in the Proceedings of the National Academy of Sciences.

This finding doesn’t prove, however, that a virus causes CFS. It might, or it might not. The viral infection may result from a breakdown of the immune system. As the National Institutes of Health's Dr. Harvey Alter, the senior investigator of the study, told reporters in a conference call, “It’s an association, but that’s all it is.”

The New York Times notes that one of the reasons other studies were unable to find a link between CFS and retorviruses may be because of how the researchers chose chronic fatigue patients. The studies that did not find a link may have included people who didn't actually have CFS but instead had a psychiatric disorder, such as depression.

The next step will be to conduct clinical trials in which people with CFS are given antiviral medications to see if they improve. As a commentary that accompanied the study pointed out, the idea that a bacterium causes peptic ulcers (rather than stress and diet) only became accepted after antibiotics were shown they could cure people of peptic ulcer disease.

The New York Times and MedPage Today both have comprehensive summaries of the PNAS study, as does science reporter Mindy Kitei over at CFS Central.