The odds of a woman developing breast cancer is significantly higher if bovine leukemia virus is present, a new study suggests. These odds, researchers say, are far higher than with other common risk factors associated with breast cancer, such as obesity, alcohol consumption or the use of postmenopausal hormones.

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Researchers suggest a leukemia virus found in cattle may raise the risk for breast cancer in humans.

Scientists from the University of California-Berkeley (UC Berkeley), publishing in the journal PLOS ONE, looked for the presence of the bovine leukemia virus (BLV) in 239 women, comparing samples from those who had breast cancer with those who did not.

They identified BLV in 59% of samples from breast cancer patients, compared with 29% in the samples from those without breast cancer.

Until recently, science had been unsure if BLV could be found in the human population.

It was a team led by Prof. Gertrude Buehring, a professor of virology in the Division of Infectious Diseases and Vaccinology at UC Berkeley's School of Public Health, who overturned the long-held view that transmission of BLV to humans was not possible, after finding evidence of BLV in humans for the first time in 2014.

Prof. Buehring then led the team to research a possible link between BLV and breast cancer.

Fast facts about breast cancer
  • After increasing for 2 decades, breast cancer incidence rates in the US have been falling since 2000
  • Around 231,840 new cases of invasive breast cancer will be diagnosed in the US this year
  • Around 40,290 women in the US will die from breast cancer in 2015.

Learn more about breast cancer

BLV infects dairy and beef cattle, causing malignant lymphoma and lymphosarcoma in up to 5% of infected animals. A good indicator of exposure to BLV is presence of the virus antibodies in serum or milk. A baseline for the presence of BLV was established with the first study in the US in 1996, with results revealing that 89% of US dairy operations had BLV.

The most recent dairy study carried out in 2007 - involving over 82% of the entire US dairy herd from the nation's 17 major dairy-producing states - showed that nearly 84% of operations were positive for BLV, though only 7.5% of all operations had independently confirmed the presence of BLV.

"The tests we have now are more sensitive, but it was still hard to overturn the established dogma that BLV was not transmissible to humans, says Prof. Buehring. "As a result, there has been little incentive for the cattle industry to set up procedures to contain the spread of the virus."

BLV a significant risk factor for breast cancer

By showing a higher likelihood of the presence of BLV in breast cancer, this new research takes last year's findings by Prof. Buehring and her research team one step further.

Prof. Buehring says:

"This odds ratio is higher than any of the frequently publicized risk factors for breast cancer, such as obesity, alcohol consumption and use of postmenopausal hormones."

Prof. Buehring notes that the research does not show how the virus infects breast tissue, but it could be through unpasteurized milk, uncooked meat or human-to-human transmission.

It is an important first step, she says, with further research required to establish if BLV is present in breast tissue before cancers form. She stresses, however, that these latest findings do not prove that BLV causes breast cancer.

Earlier this month, Medical News Today reported that a Mediterranean diet with olive oil is linked to reduced risk of breast cancer.