New York: Researchers have established for the first time a link between the bovine leukemia virus (BLV) and human breast cancer.
Bovine leukemia virus infects blood cells and mammary tissue of dairy and beef cattle.
The retrovirus is easily transmitted among cattle primarily through infected blood and milk, but it only causes disease in fewer than five percent of infected animals.
The analysis of breast tissue from 239 women showed a higher likelihood of the presence of BLV in breast cancer tissue.
When the data was analysed statistically, the odds of having breast cancer if BLV were present was 3.1 times greater than if BLV was absent.
"This odds ratio is higher than any of the frequently publicised risk factors for breast cancer, such as obesity, alcohol consumption and use of post-menopausal hormones," said study lead author Gertrude Buehring, professor at University of California, Berkeley's School of Public Health.
They found that 59 percent of breast cancer samples had evidence of exposure to BLV.
By contrast, 29 percent of the tissue samples from women who never had breast cancer showed exposure to BLV.
"It is important to note that our results do not prove that the virus causes cancer," Buehring pointed out.
"However, this is the most important first step. We still need to confirm that the infection with the virus happened before, not after, breast cancer developed, and if so, how," Buehring noted.
Buehring emphasised that this study does not identify how the virus infected the breast tissue samples in their study. The virus could have come through the consumption of unpasteurised milk or undercooked meat, or it could have been transmitted by other humans.
The study was published online in the journal PLOS ONE.