Infections from the bovine leukemia virus and breast cancer in humans have found to be linked, say researchers from the University of California, Berkeley.

For a study published in the journal PLOS ONE, researchers examined breast tissue from 239 women, testing for the bovine leukemia virus and comparing samples from women with breast cancer and those who have no history of the disease. Based on their findings, 59 percent of samples from women with breast cancer tested positive for the bovine leukemia virus while 29 percent of samples from those without cancer showed exposure to the virus.

Gertrude Buehring, lead author for the study, said that the link between the bovine leukemia virus and breast cancer came as a surprise because it was just last year when the virus was confirmed to be transmittable to humans. She does, however, reiterate that while their study showed an association between the virus and the cancer, it was not able to prove that the bovine leukemia virus causes breast cancer.

"We still need to confirm that the infection with the virus happened before, not after, breast cancer developed, and if so, how," she added.

The bovine leukemia virus infects beef cattle and dairy's mammary tissue and blood cells. It can be transmitted easily but only causes less than five percent of infected animals to get sick. A survey carried out by the U.S. Department of Agriculture in 2007 on bulk milk tanks showed that 100 percent of operations involving large herds of at least 500 cows tested positively for antibodies to the virus, but with smaller herds with less than 100 cows testing positive for the bovine leukemia virus 83 percent of the time, it showed that the infected cows can easily transmit the virus to others.

Studies carried out in the 1970s were not able to detect that people can get infected with the bovine leukemia virus. However, tests today are more advanced so not only were they able to determine that the virus can indeed transfer to humans, but they were also able to show that the presence of the virus makes it likely for an individual to develop breast cancer.

When data for the current study was analyzed, it was revealed that carrying the bovine leukemia virus gives a person a ratio of 3:1 chance of having breast cancer, with odds higher than more commonly known risk factors like obesity and alcohol consumption.

While the researchers were not able to identify how the bovine leukemia virus infected the tissue samples, they suggest that it is possible that the virus could have been acquired by consuming undercooked meat or unpasteurized milk.

The study was supported by the U.S. Department of Defense and the University of California Breast Cancer Research Program. Other authors include Gladys Block, Mark Hudes, Diana Jin, Hanne Jensen and Hua Min Shen.

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