A retrovirus first seen in prostate cancer patients three years ago has now been discovered in the blood of people suffering from chronic fatigue syndrome (CFS), Vincent Lombardi and colleagues report¹ today in Science. The virus can be passed on from person to person and may be linked with other health conditions, experts say.

 

“We have discovered a highly significant association between the XMRV [xenotropic murine leukemia virus-related virus] retrovirus and CFS,” write Lombardi, from the Whittemore Peterson Institute in Nevada, USA, and colleagues. Two-thirds of 101 patients included in the study had the viral DNA in their blood, compared with just 3.7% of healthy controls.

 

The virus could play a part in causing CFS, suggest the authors. Like HIV and other retroviruses, it could be responsible for immune and neurological effects seen in infected people. They noted signs of an immune response specific to XMRV in the patients they studied.

 

But the finding raises more questions than it answers, they caution. The virus may well have nothing to do with causing CFS.

 

Writing² in an associated article, John Coffin and Jonathan Stoye call for more research to pin down the role of this virus in CFS and other diseases. “Closely related viruses cause a variety of major diseases, including cancer, in many other mammals,” they write. “Further study may reveal that XMRV is a cause of more than one well-known ‘old’ disease, with potentially important implications for diagnosis, prevention, and therapy.”

 

The study comes on the heels of recent research suggesting a link between prostate cancer and XMRV. The viruses found in the two sets of patients are effectively identical genetically, according to Lombardi and colleagues.

 

“Both conditions are associated with dysfunction of [the antiviral enzyme] RnaseL, which is an important element of intracellular anti-viral immunity,” explains Yasuhiro Takeuchi, from the Division of Infection and Immunity at University College London in the UK. “This could help XMRV infection in these patients.”

 

Cell-culture experiments conducted by Lombardi and colleagues suggest that XMRV can spread between people through transmission of either infected cells or viral particles alone. “Given that infectious virus is present in plasma and blood cells, blood-borne transmission is a possibility,” write Coffin and Stoye.

 

The authors note that some 3.7% of healthy controls tested positive for the virus. In the earlier study of prostate cancer patients, this figure was 5% in the control group. Putting these together, Coffin and Stoye suggest that “perhaps 10 million people in the United States and hundreds of millions worldwide are infected with a virus whose pathogenic potential for humans is still unknown.”

 

The form of XMRV now seen in humans can be traced back to a mouse virus, the xenotropic murine leukemia virus (MLV), explain Coffin and Stoye. This is called an “endogenous” virus because it infects reproductive cells and can therefore be passed on to offspring. Their similarity leaves “little doubt” that XMRV emerged by cross-species transmission, they say, and this probably happened outside the laboratory.

 

Both MLV and XMRV belong to the group of gammaretroviruses, notes Takeuchi, which are known to cause cancer, immunological and neurological diseases in animals.

 

The experts are mystified as to why the virus is coming to light now. Stoye suggests one explanation could be the use of improved methods to identify novel viruses in tissues taken from different sources. “[This] reflects a continuing trend over the past 40 years for identifying novel viruses more and more quickly,” he notes.

 

“I expect our knowledge of XMRV in human health will expand very fast,” says Takeuchi. “I think many researchers are starting or have started to look [for it]”.