The Link Between Bovine Leukemia and Breast Cancer

Contaminated dairy and beef with bovine leukemia virus may contribute to cancer development.

Contamination of dairy and beef with the bovine leukemia virus (BLV) could possibly be linked to breast cancer in humans, according to a study by University of California at Berkeley.

The study analyzed breast tissue from 239 women and compared samples from women who had breast cancer with women who had no history of the disease for the presence of BLV. The study found that 59% of samples that had breast cancer also had exposure to BLV, while 29% of tissue samples from women who had no history of the disease showed exposure to BLV.

“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,” said study lead author Gertrude Buehring, a professor of virology in the Division of Infectious Diseases and Vaccinology at UC Berkeley’s School of Public Health. “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 the blood and mammary tissues of beef and dairy cattle. The retrovirus is spread through infected blood and milk from cow to cow, but only causes disease in fewer than 5% of infected animals.

A 2007 US Department of Agriculture survey found that 100% of bulk milk tanks that had large herds of over 500 tested positive for BLV antibodies. This is not surprising due to the fact that an infected cow’s milk can be mixed in with the rest. Even smaller operations tested positive for BLV antibodies 83% of the time.

What was previously unknown to scientists was whether or not the virus could be spread to humans. This was confirmed, however, in a study led by Beuhring that was published in 2014 in Emerging Infectious Diseases. The paper changed a long-standing belief that the virus was not transmissible to humans.

“Studies done in the 1970s failed to detect evidence of human infection with BLV,” said Buehring. “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 new paper takes these findings to the next level, as it suggests the presence of the virus in breast cancer tissue indicates the virus may be responsible for the development of the disease. When the data was analyzed 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 publicized risk factors for breast cancer, such as obesity, alcohol consumption and use of post-menopausal hormones,” Buehring said.

Viral origins of cancer are not uncommon. Hepatitis B virus is known to cause liver cancer and the human papillomavirus can lead to cervical and anal cancers. Vaccines have been developed for both of these viruses and are used as a preventive measure to avoid the spread of disease and development of cancer.

“If BLV were proven to be a cause of breast cancer, it could change the way we currently look at breast cancer control,” Buehring said. “It could shift the emphasis to prevention of breast cancer, rather than trying to cure or control it after it has already occurred.”

Buehring emphasized that the study does not indicate how BLV infected the breast tissue samples. The virus could have come through the consumption of unpasteurized milk or undercooked meat, or it could have been transmitted by other humans.

Only further testing and research will determine the exact origins of the disease in breast tissue samples, and if BLV is indeed related to breast cancer or not.