U.S. researchers say they have linked a second type of mouse virus to the baffling condition called chronic fatigue syndrome.

The findings will likely raise more questions about whether an infection might play a role in the condition.

The researchers say they found evidence of XMRV, a member of murine leukemia virus retrovirus family, which causes cancer in mice in a significant number of chronic fatigue patients.

At least one of four different MLV-like viruses were found in 32 of 37 of patients with chronic fatigue syndrome, or 86 per cent, compared with just three of 44, or 6.8 per cent, of otherwise healthy blood donors. The full results appear in the Proceedings of the National Academy of Sciences.

Lead researcher, Dr. Harvey Alter of the National Institutes of Health, says it remains unclear is the virus might be causing disease, is somehow a byproduct of the illness, or is completely unrelated.

"There is a dramatic association with CFS, [but] we have not determined causality for this agent," Dr. Alter, told a news conference this week.

"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."

It's also possible that chronic fatigue syndrome represents several different diseases, some of which are caused by the viruses, Alter said.

The findings are similar to those made in another study by researchers at the Whitemore Peterson Institute in Reno, Nev. But they also contradict several other studies, including a recent one from the U.S. CDC, as well as research done in the U.K. and the Netherlands. Those studies found no evidence XMRV or other MLV viruses in people with CFS.

Over the years, various viruses have been linked to chronic fatigue syndrome but none has emerged as a potential culprit of the condition, which causes unrelenting fatiguein patients.

There is currently no specific treatment for CFS and no easy way to test for it.

In an editorial accompanying the new study, a prominent Canadian scientist says it's time to test whether antiviral medications (like those used against HIV) can treat at least some people with chronic fatigue syndrome.

"There is only one way to prove or disprove XMRV's role and that is to do a proper study with antiviral drugs," Andrew L. Mason, an associate professor of medicine at the University of Alberta in Edmonton, told WebMD.