Researchers Find Breast Cancer May Be Connected to Cattle Virus

The bovine leukemia virus, or BLV, which is transmitted between cattle herds may play a key role in the development of breast cancer.

The bovine leukemia virus, or BLV, which is transmitted between cattle herds may play a key role in the development of breast cancer. Researchers with the University of California Berkeley found earlier this month that BLV exposure may be the cause for as many as 37% of breast cancer cases studied in the report.

The study, which gathered breast tissue samples from 239 donors between 2002 and 2008, discovered that 59% of women diagnosed with breast cancer had BLV in their mammary epithelium compared with 29% of women with no history of breast cancer. Researchers also identified different levels of BLV DNA among different race groups. Women of Caucasian ancestry had a higher association between BLV and breast cancer than women of African heritage.1

While the connection between BLV and breast cancer has been proven significant, the way humans contract BLV is yet to be known. BLV is most commonly traced to cattle and is transmitted through infected blood or milk; however, less than 5% of animals become ill from BLV.2According to the report, “Approximately 38% of beef herds, 84% of diary herds, and 100% of large-scale dairy operation herds in the USA are infected with BLV.”

The researchers noted that BLV infects the mammary epithelial of cows, a main source of nutrients for millions of people.

“Most humans in Western cultures consume more cow’s milk products in a lifetime than they do human milk, which led us to investigate whether a bovine virus might be an initiating agent for breast cancer,” the authors wrote in their report.

The direct connection between BLV and breast cancer is tricky to trace because pasteurization and cooking meat at high temperatures generally kills any bacteria or virus in the substance. The report authors suggest that transmission of BLV from cow to human is plausible, as people have likely consumed undercooked meat or raw milk.1

Finding a direct link to the cause of breast cancer through any means has not been an easy task; the only animal experiment to prove breast cancer etiology was a mother mouse transmitting a retrovirus, or mouse mammary tumor virus (MMTV), to her baby through milk.3

Researchers of the study used the In Situ Polymerase Chain Reaction (IS-PCR) method to detect the BLV DNA in the specimens that were collected from the Cooperative Human Tissue Network. The UC Berkeley team discovered that their results were neither impacted by age or length of storage of the sample. Additionally, 82 of 114 cases showed measurements of estrogen receptor (ER) and/or progesterone receptor (PR)—of these two groups, BLV DNA was found more frequently in ER-positive samples than PR-negative samples.1

Though the results of the study indicate a positive step toward uncovering the mysteries of breast cancer, the authors assert that much more work is needed to demonstrate whether or not BLV precedes the cancer development, in order to correlate causation. At this point, direct causation cannot be proven.

“If the virus is indeed proven to be causative, vaccination at an early age likely would prevent the disease,” said study author Hanne M. Jensen, MD, professor in the Department of Medical Pathology and Laboratory Medicine of UC Davis Health System. “At this time, cervical cancer from the human papillomavirus can be vaccinated against.”

However, lead investigator Gertrude Buehring, professor of virology in the Division of Infectious Diseases and Vaccinology at UC Berkeley’s School of Public Health, said in a statement that the odds ratio for BLV is higher than any other more commonly believed risk factor, some of which include obesity, alcohol consumption, and postmenopausal horomones.2Further research on BLV may lead to further advancements in the study of breast cancer and tackle questions researchers continue to face regarding breast cancer etiology.

“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 in a statement.2“It could shift the emphasis to prevention of breast cancer, rather than trying to cure or control it after it has already occurred.”


  1. Buehring GC, Shen HM, Jensen HM, et al. Exposure to bovine leukemia virus is associated with breast cancer: A case-control study. PLoS ONE. 2015;10(9):e0134304.
  2. Yang S. Virus in cattle linked to human breast cancer. Berkeley News. 2015. Accessed September 18, 2015.
  3. Robert-Guroff M, Buehring G. In pursuit of a human breast cancer virus, from mouse to human. Humana Press. 2000:475-487. doi:10.1007/978-1-59259-024-7_27.
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