Researchers with University of California, Berkeley, are exploring a link between a common bovine virus and breast cancer in women.
In a study analyzing 239 tissue samples from women diagnosed with breast cancer, scientists found 59 per cent had been exposed to the bovine leukemia virus (BLV) compared to 29 per cent of tissue samples from women without.
While stressing that the results do not prove that BLV infection causes breast cancer, lead researcher Gertrude Buehring, a professor of virology in the Division of Infectious Diseases and Vaccinology at UC Berkeley’s School of Public Health, said the presence of BLV in humans could be a significant indicator of risk.
“As many as 37 per cent of breast cancer cases may be attributable to BLV exposure,” the paper published in the online journal PLOS ONE 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,” said Buehring.
Bovine leukemia virus infects dairy and beef cattle’s blood cells and mammary tissue. The retrovirus is easily transmitted among cattle primarily through infected blood and milk, but causes disease in fewer than five per cent of infected animals.
A 2007 U.S. Department of Agriculture survey of bulk milk tanks found that 100 per cent of dairy operations with large herds of 500 or more cows tested positive for BLV antibodies. Even dairy operations with small herds of fewer than 100 cows tested positive for BLV 83 per cent of the time.
The PLOS ONE paper noted that pasteurization renders the virus non-infectious, as does thorough cooking of beef. However, it said the virus, which is readily transmitted from cow to calf, may have become established in the human population before pasteurization became common in the 1920s and may still be entering the population through consumption of raw milk and undercooked beef.
It was believed until recently, based on studies done in the 1970s, that the BLV could not be transmitted to humans, but research published by Buehring last year overturned those results.
“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,” she said.
There is precedence for viral origins of cancer. Hepatitis B virus is known to cause liver cancer, and the human papillomavirus can lead to cervical and anal cancers. Notably, vaccines have been developed for both those viruses and are routinely used to prevent the cancers associated with them.
Buehring emphasized 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 unpasteurized milk or undercooked meat, or it could have been transmitted by other humans.
As well, she said researchers must still confirm that infection with the virus happened before, not after, the breast cancer developed.
If BLV were substantiated as a risk factor for breast cancer, its detection in breast fluid cells or tissues might serve as a biomarker to identify women at higher risk for developing breast cancer.