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What are XMRV and MLVs?

XMRV refers to a recently discovered retrovirus called xenotropic murine leukemia virus-related virus. It  was first identified in 2006 in samples from men with prostate cancer. XMRV is closely related to a group of retroviruses called murine leukema viruses (MLVs), which are known to cause cancer in certain mice.

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What is chronic fatigue syndrome?

Chronic fatigue syndrome, or CFS, is a debilitating and complex disorder characterized by profound fatigue that is not improved by bed rest and that may be worsened by physical or mental activity. Persons with CFS most often function at a substantially lower level of activity than they were capable of before the onset of illness. In addition to these key defining characteristics, patients report various nonspecific symptoms, including weakness, muscle pain, impaired memory and/or mental concentration, insomnia, and post-exertional fatigue lasting more than 24 hours. In some cases, CFS can persist for years. The cause or causes of CFS have not been identified and no specific diagnostic tests are available. Moreover, since many illnesses have incapacitating fatigue as a symptom, care must be taken to exclude other known and often treatable conditions before a diagnosis of CFS is made. For more information, see CDC's CFS Web page.

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Have XMRV and MLVs been associated with human disease and illness, including CFS?

As noted above, XMRV was first identified in 2006 in patients with prostate cancer.  A study published by Lombardi and colleagues (Science, 2009;326:585) reported finding evidence of XMRV in about two-thirds of CFS patients and nearly 4% of healthy persons.  However, several other recent studies found no evidence of XMRV in persons with CFS and in controls; these studies include a report published by CDC investigators and colleagues in July 2010 (see update).  More recently, researchers from the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and Harvard Medical School reported evidence of MLVs in about 87% of CFS patients and 7% of healthy blood donors (Proceedings of the National Academy of Sciences,  2010).

The reporting of different research findings from different studies is not uncommon.  The different  findings may be due to a variety of factors, such as differences in the study populations.  At the present time, the potential role of XMRV and MLVs in causing diseases such as prostate cancer and CFS and the frequency of XMRV and MLV infection among healthy persons are unknown. If it is determined that XMRV and MLVs have a role in causing disease and illness in humans, prevention recommendations can be made.

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How are XMRV and MLVs transmitted? Are certain individuals more likely to be infected with XMRV and MLVs?

The manner in which XMRV and MLVs are transmitted is unknown. It is unclear whether certain individuals are more likely to be infected with XMRV and MLVs. Studies of these viruses in humans have been under way for only a short time, and therefore these and similar questions have not been answered.

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If researchers find that XMRV and MLVs are found in a majority of patients who have chronic fatigue syndrome, does this mean that CFS may be contagious?

One study that has been conducted on CFS and close contacts suggests there is no evidence that CFS is contagious or spread person to person. Occurrence of CFS is not more common in family members of patients with CFS, and none of the features typical of contagious disease have been associated with CFS. These features include seasonal or regional occurrence, travel history, occupation, exposure to animals, injection drug use, and sexual behavior.

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Given that recent scientific papers have reported finding XMRV and MLVs in healthy people, can these viruses be transmitted through blood transfusion or organ/tissue transplantation?

XMRV and MLVs, like all retroviruses, are presumed to be transmitted by blood and tissues. Therefore, it is also presumed that XMRV and MLVs can be transmitted through blood transfusion. Since it is possible that XMRV and MLVs might infect many types of human cells, including some blood cell types, the safety of blood could be a concern if XMRV and MLV infection is confirmed to cause human illness and disease.

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What is being done to evaluate the risk of XMRV and MLVs?

The Department of Health and Human Services (HHS) is conducting studies to determine the prevalence of XMRV and MLVs in the blood donor population. Additional research is needed to determine if XMRV and MLVs cause illness and disease. If it is determined that XMRV and MLVs may have a role in causing illness and disease, prevention recommendations can be made.

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Who has collections of blood samples for this research?

The National Institutes of Health (NIH) currently funds the Retrovirus Epidemiology Donor Study-II (REDS-II), which focuses on addressing critical questions in blood safety. The REDS-II laboratory sample collection, called RADAR, has specimens from blood donors and matched recipients from various regions of the country. These matched specimens can be useful in testing for the transmissibility of "new" or "emerging" infectious agents, such as XMRV.

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If researchers have tests for XMRV and MLVs, why can’t they be used for screening of blood donors?

Although specialized research assays have been developed by some laboratories for use in XMRV and MLV studies, no standardized, validated tests are available for XMRV and MLVs. Screening tests require additional validation, for use in a large population, compared with diagnostic tests.  CDC, NIH, and FDA, together with several nonfederal laboratories, are participating in an XMRV/MLV assay comparability study that is intended to help build a consensus among researchers on how to standardize these testing methods.

While it is presumed that XMRV and MLVs can be transmitted through blood transfusion, no such transmission has been identified, and there are no known cases of XMRV or MLV infection or related illness in transfusion recipients. Therefore, there is currently no requirement for testing of the blood supply for the presence of XMRV or MLVs.

If it is determined that XMRV or MLVs may have a role in causing disease and illness, prevention recommendations can be made. Such a risk, if it exists, could be decreased by developing and using blood donor screening assays or other measures. The use of a donor screening assay by blood establishments would require FDA approval of the test.

There are many steps between what is currently known about XMRV and MLVs and the release of an FDA-approved test, if such a test were warranted. The test components and procedures must be standardized, and the test performance assessed in research studies before licensure (approval) by the FDA. However, if the risk is demonstrated, these steps could be taken more quickly, as occurred for screening approval of West Nile virus.

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Should an individual with diagnosed chronic fatigue syndrome donate blood?

At the present time, there is no FDA guidance to defer donors who have CFS in the United States. However, FDA regulations require that a donor should be in good health. Medical directors at blood collection centers should exercise judgment in determining whether individuals with a history of CFS are in good health at the time of donation.

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Should an individual who has been diagnosed, treated and currently in remission from prostate cancer donate blood?

There is no known association of prostate cancer with history of transfusion. In general, FDA has not recommended deferral of donors who have a history of cancer due to lack of such as association.

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Should an individual diagnosed with chronic fatigue syndrome or in remission from prostate cancer be an organ or tissue donor?

At the present time, there are no U.S. recommendations to defer organ and tissue donations from individuals diagnosed with chronic fatigue syndrome or who are in remission from prostate cancer.

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