Two years ago, Vincent Lombardi at the Whittemore Peterson Institute in Reno, Nevada, reported a possible cause of the tiredness and muscle pain of Chronic Fatigue Syndrome (CFS) after discovering a mouse virus called xenotropic murine leukaemia virus-related virus (XMRV) in blood samples from 68 of 101 people with CFS compared with just eight of 218 samples from healthy volunteers.
Now Jay Levy's group at the University of California, San Francisco have evaluated blood samples from 61 patients with CFS from a single clinical practice, 43 of whom had previously been identified as XMRV-positive. Their analysis included polymerase chain reaction and reverse transcription polymerase chain reaction procedures for detection of viral nucleic acids and assays for detection of infectious virus and virus-specific antibodies. They found no evidence of XMRV or other Mouse Leukemia Viruses in these blood samples. In addition, they found that these gammaretroviruses were strongly (X-MLV) or partially (XMRV) susceptible to inactivation by sera from CFS patients and healthy controls, which suggested that establishment of a successful MLV infection in humans would be unlikely. Consistent with previous reports, they detected MLV sequences in commercial laboratory reagents. Their results indicate that previous evidence linking XMRV and MLVs to CFS is likely attributable to laboratory contamination.
In another study, Vinay Pathak at the National Cancer Institute in Frederick, Maryland, suggested that XMRV originated in lab mice between 1993 and 1996 – after many of the people in Lombardi's study were diagnosed with CFS – and so cannot be the cause. Pathak said that researchers grew cancers in mice without immune systems in order to make prostate tumour tissue for study. The tumour cells picked up two leukaemia viruses which combined to form XMRV. The strains in Lombardi's samples are so similar to this “recombinant” virus that it is unlikely to have another source.