New trouble ahead with bovine leukemia virus? Part 1

The headline “Virus in cattle linked to human breast cancer” is chilling.

For the first time, a research team at the University of California, Berkeley, established a link between infection with bovine leukemia virus (BLV) and human breast cancer. The study, published September 2015 in a respected, peer-reviewed journal PLOS ONE, will draw attention to a long-standing disease in beef and dairy cattle. For 45 years, enzootic bovine leukosis (EBL) — a retroviral disease known to be transmissible between cattle and closely related to T-cell leukemia virus in humans — stood on the doorstep of being declared a zoonoses and whether or not it might be associated with health conditions in humans.

What had been unclear until recently is whether the EBL virus could be found in humans, something confirmed in a study led by Dr. Gertrude Buehring published last year (2014) in the journal Emerging Infectious Diseases. The 2014 study, prompted by discovery of antibodies against BLV in humans, led Buehring’s team to examine the possibility of human infection with BLV. Because BLV in cattle is more abundant in mammary epithelium (lining of the mammary gland) than in lymphocytes (a type of white blood cell), human breast tissue was targeted. Study results overturned the long-held belief that BLV could not be transmitted to humans.

Study lead and author, Gertrude Buehring, a professor of virology in the Division of Infectious Diseases and Vaccinology at UC Berkeley’s School of Public Health is careful to point out: “Although the association between BLV infection and breast cancer was surprising to many previous reviewers of the study, it’s important to note that our results do not prove that the virus causes cancer.”

Dogma surrounding the disease enzootic bovine leukosis (EBL) meant little has been done to establish effective control programs. The wait is over. The news that EBL virus is shared between humans and cattle — probably transferred from cattle to humans in meat and milk — and that BLV’s presence has been linked to a higher prevalence of breast cancer in humans (cause still in question) is not good for either the beef or dairy industry. The meat and milk industries will now be forced to establish incentives in containing the spread of BLV virus, plus ways of eliminating it from cattle populations. Perceptions about food safety and any notion EBL might be associated with health risks in humans make it mandatory.

In the Berkeley study, researchers analyzed breast tissue for the presence of bovine leukemia virus (BLV) from 239 women, comparing samples from women who had breast cancer with women who had no history of the disease. They found 59 per cent of breast cancer samples had evidence of exposure to BLV, as determined by the presence of viral DNA. By contrast, 29 per cent of the tissue samples from women who never had breast cancer showed exposure to BLV.

The new paper takes earlier findings a step further by showing a higher likelihood of BLV in breast cancer tissue. Statistically, the odds of having breast cancer if BLV were present were 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,” said Buehring. Buehring emphasized that the study does not identify how the virus infects breast tissue. The virus could have come through the consumption of unpasteurized milk or undercooked meat, or transmitted by other humans. If BLV were proven to be a cause of breast cancer, it could change the way we currently look at breast cancer control by shifting the emphasis to prevention rather than trying to cure or control it after it has already occurred.

In the meantime, the beef and dairy industries should prepare for an onslaught of questions from the public and consumers about EBL control and eradication. The supercritical, those who go to any length to impute blame on modern food production systems for any reason, have a powerful tool. The kind of information revealed by the Berkeley study, applied inappropriately, stands to sully both industries.

Bovine leukemia virus (BLV) causes enzootic bovine leukosis (EBL) in cattle, an infectious disease causing fatal malignant cancer in a small percentage of animals (less than five per cent), yet responsible for significant economic loss in some herds. It’s insidious and often associated with reduction in overall herd productivity with no obvious clinical signs. EBL is recognized as an animal health problem worldwide. BLV-positive cattle remain a source of infection for herdmates and the virus is never eliminated. State-of-the-art immunologic methods made it possible to detect human infection with BLV and several good tests are available for use in cattle.

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. In the United States, up to 38 per cent of beef herds, 84 per cent of all dairy herds, and 100 per cent of large-scale dairy herds are infected (contain one or more positive animals). Several Canadian studies have shown similar figures. Up to 40 per cent of dairy cows and 10 per cent of beef cows are infected. There is no cure or treatment for BLV and because infected cattle can remain productive, they remain in herds and potentially spread virus to other animals. BLV is primarily transmitted through blood, most often during management practices like injections, dehorning, tattooing, tagging, and pregnancy checking. Biting flies may play a role in transmitting virus. Good tests for EBL exist.

A series of options to reduce the prevalence of EBL have been attempted by regulatory and industry groups in Europe and North America. Some countries have made efforts to eradicate the disease from beef and dairy herds. Control measures have primarily centred around: eradication of infected cattle, segregation of BLV-free animals and vaccination. Most have failed due to complacency, economic considerations, management restrictions and the lack of an effective vaccine.

Part 2 of this article will cover control strategies and where industry needs to go.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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