We assessed the prevalence of XMRV in a cohort of 293 American patients with CFS or chronic conditions associated with immune activation and/or immune deficiency and did not detect XMRV in any participant sample. A previous report showed that XMRV DNA could be detected in patient samples after a single round of 45-cycle PCR [7
]. Our XMRV amplification strategy used similar amounts of input PBMC DNA and identical primer sets as were used in previous reports, and we increased the sensitivity of our methods by adding a nested PCR amplification that used two additional previously published XMRV primer sets [4
]. These negative findings demonstrate that XMRV was not associated with any specific group that we investigated; the choice of PCR primers did not affect XMRV prevalence estimates. XMRV DNA could be present at levels below our detection threshold. However, we used PCR methodology that was comparable to previously published methods that detected XMRV DNA in CFS and healthy control subjects [7
The upper limit of the 95% confidence interval around our CFS participant XMRV prevalence estimate (0%) was 9.5%. This result is similar to reports from Europe and suggests a far lower rate of XMRV infection, if any, in patients with CFS compared to the initial report [7
]. Regional differences in XMRV prevalence among CFS patients could reflect geographical clustering of XMRV infection, and weakens the epidemiological link between XMRV infection and CFS.
To further characterize our CFS cohort and provide a basis of comparison to other CFS groups described in the literature, we administered a 43-item CFS questionnaire. Although our questionnaire may be confounded by recall bias, a majority of our CFS participants reported ongoing symptoms at the time of study entry. These symptoms had been present for an average of 12 years and were debilitating enough to cause the majority of participants to stop working.
Endogenous or latent viruses can become activated in patients with altered immune function. We explored the effect that immune activation or suppression could have on XMRV prevalence by including participants with rheumatoid arthritis, HIV infection (both treatment naïve and virologically suppressed), and hematopoietic stem-cell and solid organ transplant. We did not identify an association between XMRV prevalence and immune status. Healthy individuals were not included per se in this study and preclude us from drawing conclusions about the prevalence of XMRV in the general population. Our rheumatoid arthritis age- and gender- matched hospital cohort may contain healthy patients that presented for routine clinical care, but we would reasonably expect greater morbidity in this control group, relative to the population-at-large in Boston. We did detect a mouse endogenous retroviral sequence in PBMC DNA from one participant, but could not replicate this finding. Mouse endogenous retroviral sequences are not present in the human genome; reagent testing did not identify the source of this contaminating sequence.
In summary, we found no evidence of XMRV infection in a cohort of patients cared for at Boston-area hospitals and no association of XMRV with either chronic fatigue syndrome or chronic conditions with altered immune function. Further research should be performed to define the demographic and geographic distribution of XMRV and to clarify its relationship with chronic fatigue syndrome.