A case-control study has reportedly identified a “significant” association between bovine leukemia virus (BLV) infection and human breast cancer. Gertrude Case Buehring, et al., “Exposure to Bovine Leukemia Virus Is Associated with Breast Cancer: A Case-Control Study,” PLoS One, September 2015. After analyzing breast tissue specimens obtained from 239 donors for the presence of BLV, University of California, Berkeley, researchers apparently detected BLV “in the mammary epithelium of 59% of women diagnosed with breast cancer versus 29% of those with no history of breast cancer.” They further suggest that “as many as 37% of breast cancer cases may be attributable to BLV exposure,” with an odds ratio “comparable to that of commonly cited reproductive, hormone, and lifestyle risk factors for non-hereditary (sporadic) breast cancer.”

As explained in a concurrent press release, a 2014 study published in Emerging Infectious Diseases confirmed the presence of BLV in humans, though it is currently unknown how the virus passes between species. The study hypothesizes that possible transmission routes could include undercooked beef or raw cow’s milk, but there may also be a longstanding BLV reservoir in the human population due to millennia of cattle domestication.

The researchers note, however, that this case-control study does not prove the virus causes cancer. As the lead author elaborates, “We still need to confirm that the infection with the virus happened before, not after, breast cancer developed, and if so, how… 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.” See UC Berkeley Press Release, September 15, 2015.