The study was completed by examining the tissue in 239 female breasts which were then compared to women with or without breast cancer; the scientists looked for bovine leukemia virus (BLV). Researchers uncovered that 59 percent of breast cancer samples contained BLV. On the other hand, 29 percent of breast tissue without breast cancer also revealed exposure to BLV.
Lead author Gertrude Buehring, Ph.D., said, “The association between BLV infection and breast cancer was surprising to many previous reviewers of the study, but it’s important to note that our results do not prove that the virus causes cancer. 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.”
BLV infects dairy and blood cells and mammary tissue of cattle. Although BLV can easily be transmitted between cattle, it only leads to disease in less than five percent of the animals.
In a 2007 survey conducted by the U.S. Department of Agriculture, all dairy operations with herds of 500 or more tested positive for BLV in bulk milk tanks. Even smaller herds of 100 or less tested positive for BLV in 83 percent of cases.
Previous research on whether or not BLV could be spread to humans determined that is was not transmissible. Buehring added, “Studies done in the 1970s failed to detect evidence of human infection with 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. As a result, there has been little incentive for the cattle industry to set up procedures to contain the spread of the virus.
The results of the new study reveal that when BLV is present in breast tissue, the risk of breast cancer increases 3.1 times. Buehring said, “This odds ratio is higher than any of the frequently publicized risk factors for breast cancer, such as obesity, alcohol consumption and use of post-menopausal hormones.”
Buehring acknowledges the study does not reveal how BLV affects breast tissue but merely that there is a link present. If future research uncovers the role of BLV in the formation of breast cancer, it could change the way we look at breast cancer prevention.
The findings were published in PLOS ONE.