A major study in 2009 that claimed a mouse virus was the cause of chronic fatigue syndrome was wrong, and its findings were likely based on contaminated lab samples, according to US researchers.

“There is no evidence of this mouse virus in human blood,” said Jay Levy of the University of California, San Francisco, senior author of the study to be published this week in the journal Science.

Instead, the mouse virus XMRV that was picked up in samples from chronic fatigue patients probably got there because “chemical reagents and cell lines used in the laboratory where it was identified were contaminated with the virus,” the university said in a statement.

The 2009 study was hailed as a breakthrough for the estimated one to four million Americans who suffer from the elusive but debilitating illness, and led to many being treated with antiretroviral drugs used against HIV/AIDS.

The study authors said experts need to keep searching for the cause of the disease, which can last for years and cause memory loss, muscle pain, extreme tiredness and possibly insomnia.

“Individuals with chronic fatigue syndrome need to know that taking antiretroviral therapies will not benefit them, and may do them serious harm,” said co-author Konstance Knox of the Wisconsin Virus Research Group in Milwaukee, Wisconsin.

“Physicians should not be prescribing antiviral compounds used in the treatment of HIV/AIDS to patients on the basis of a chronic fatigue syndrome diagnosis or an XMRV test result.”

The 2009 study was conducted by researchers in Nevada and Maryland, who found xenotropic murine leukemia virus-related virus (XMRV) in about two-thirds of blood samples taken from 101 patients with chronic fatigue syndrome.