Canadian Blood Services banned donations from anyone with chronic fatigue syndrome as a precaution in April. (George Osodi/Associated Press)

A U.S. government study has uncovered a family of mouse viruses in some people with chronic fatigue syndrome, raising more questions about whether an infection may play a role in the complicated illness.

The study does not prove that having any of the viruses causes harm, stressed co-author Dr. Harvey Alter of the National Institutes of Health.

But it strengthens suspicions, and the government has additional research underway to determine whether the link is real.

Meanwhile, a group of French and Canadian scientists said it is time to test whether antiviral medications like those used against HIV might treat at least some people with chronic fatigue.

The virus connection first made headlines last fall when Nevada researchers reported finding a specific type, named XMRV, in the blood of two-thirds of the 101 chronic fatigue patients they tested. Several other studies, including one from the Centers for Disease Control and Prevention, failed to find XMRV virus in patients, making researchers wonder if this might be a false alarm.

Monday's study, published in Proceedings of the National Academy of Sciences, thickens the plot. This time, NIH and U.S. Food and Drug Administration scientists examined the blood of 37 chronic fatigue patients and again did not find XMRV but instead found a group of closely related bugs named MLV-related viruses in 86 per cent of the cases.

Testing of 44 healthy blood donors, in contrast, found evidence of those viruses in nearly seven per cent.

Syndrome lacks test, treatment

Various viruses have been linked to chronic fatigue over the years only to be ruled out as potential culprits in the mysterious illness thought to afflict about one million Americans.

It is characterized by at least six months of severe fatigue, impaired memory and other symptoms, but there is no test for it and no specific treatment.

These MLV, or "murine leukemia-related viruses," are known to cause some cancers in mice, and the XMRV relative has been found in some human prostate tumors, too.

But there is no easy way to test for it, meaning studies of a link at this point must be in research labs, not doctors' offices, FDA and NIH researchers said.

No one knows how people become infected, but Alter said a major study is underway to see if evidence exists of transmission through blood.

Federal regulations require that blood donors be in good health, said FDA's Dr. Hira Nakhasi.

In April, Canadian Blood Services announced a ban on donations from anyone with chronic fatigue syndrome as a precaution.