By Suzanne Vernon, PhD, Scientific Director
Researchers at the U.S. Centers for Disease Control and Prevention (CDC), along with collaborators in California and Germany, published a paper in the journal Retrovirology titled, “Absence of evidence of xenotropic murine leukemia virus-related virus infection in persons with chronic fatigue syndrome and healthy controls in the United States.” Blood samples from 51 people with CFS, 56 matched controls and 41 healthy blood donors were tested for antibodies to XMRV using a western blot assay and for XMRV DNA using a nested PCR assay. Three independent laboratories, including the retrovirus lab at CDC, Blood Systems Research Institute (BSRI) and the Robert Koch-Institute lab tested coded samples. There is no doubt of the technical competence of these laboratories to conduct these assays to detect XMRV antibodies and DNA. So why wasn’t XMRV detected?
At the 17th Conference on Retroviruses and Opportunistic Infections in February 2010, Qui, et al., reported that the rate of XMRV in U.S. blood donors was 0.1 percent, or 1 person out of 1000. There is a slim chance CDC would have detected XMRV DNA among just 41 healthy blood donor samples. So, no surprise there.
What about the CFS cases and controls? First, I would like to make a request of all authors of scientific papers – please provide a table that describes the subject and sample cohort! Combing back and forth in a paper to figure out who is who and what is what is frustrating! From what I can decipher, the samples were drawn from 18 people identified through a Georgia registry who met criteria described in the paper that is different from 1994 international CFS criteria. Eleven CFS cases and matched controls were identified from the Wichita studies, although it is not clear if these samples came from the longitudinal studies or the clinical study, and 22 CFS cases and controls from the Georgia community-based study. There is little indication that these three cohorts are comparable in regard to CFS definition, as each cohort was selected using different definition. The authors strenuously object to application of the Canadian case definition in other studies, stating that, “physical findings in persons meeting the Canadian definition may signal the presence of a neurological condition considered exclusionary for CFS.” Yet the physical findings listed are those commonly experienced by CFS patients, and one (tender lymphadenopathy) is a case-defining symptom of the 1994 criteria.
Further, the samples from these three study cohorts were collected using different types of tubes, each of which has a distinct way of being processed. As if this weren’t bad enough, none of the blood tubes used were of the same type used in the Lombardi study. (They used tubes containing sodium heparin that are intended for use with virus isolation). The blood tubes from the 18 Georgia registry patients are designed to collect whole blood and preserve nucleic acid; it is not clear where the plasma came from for these subjects since plasma cannot be obtained using these blood tube types. So the explanation for not finding XMRV in these samples is simple – this was a study designed to not detect XMRV using a hodge-podge sample set.
Detecting XMRV is hard. Replication of the Science paper will be hard because of the exacting methods required and because of the heterogeneity and complexity of CFS. Regardless of the outcome of any single study, it is critical that a valid replication study be designed and implemented by multiple laboratories, using standard and optimized techniques and testing split samples collected appropriately from adequate numbers of well-characterized cases and controls. Studies such as this one from Switzer, et al., continue to absorb time, divert precious resources and fuel controversy instead of consensus.
Absence of evidence of Xenotropic Murine Leukemia Virus-related virus infection in persons with Chronic Fatigue Syndrome and healthy controls in the United States. Switzer WM, Jia H, Hohn O, Zheng H, Tang S, Shankar A, Bannert N, Simmons G, Hendry RM, Falkenberg VR, Reeves WC, Heneine W. Retrovirology 1 July 2010.
Detection of an infectious retrovirus, XMRV, in blood cells of patients with chronic fatigue syndrome. Lombardi VC, Ruscetti FW, Gupta JD, Pfost MA, Hagen KS, Peterson DL, Ruscetti SK, Bagni RK, Petrow-Sadowski C, Gold B, Dean M, Silverman RH, Mikovits JA. Science 8 October 2009. 1179052.
XMRV: Examination of viral kinetics, tissue tropism, and serological markers of infection. Qiu X, Swanson P, Luk K-C, Das Gupta J, Onlamoon N, Silverman R, Villinger F, Devare S, Schochetman S and Hackett J. Abbott Diagnostics, Abbott Park, IL, US; Cleveland Clin, OH, US; and Yerkes Natl Primate Res Ctr, Emory Univ, Atlanta, GA, US. Paper #151 presented at the 17th Conference on Retroviruses and Opportunistic Infections, February 16-19, 2010.
For more information about the CDC’s cohorts from the Wichita, Georgia and registry studies visit: http://www.cdc.gov/cfs/publications/surveillance.htm
Suzanne Vernon, PhD, is the Association’s scientific director. She has nearly two decades of experience as a microbiologist.July 1, 2010