Bovine leukemia virus (BLV or leukosis) is a bovine retrovirus that targets lymphocytes (white blood cells). It is present in many parts of the world, including Canada, the United States, and Eastern Europe. Infection with the virus usually doesn’t present with clinical signs, but around 30 per cent of the infected animals experience an abnormal increase in the number of lymphocytes in the blood. Anywhere from one to five per cent of infected cattle develop malignant (cancerous) tumours.
Once an animal is infected, they are lifetime carriers and the disease can be spread through contact with blood, milk, and in utero.
Most positive cows show no symptoms, and are generally considered to be as productive as uninfected cows. In later stages of disease, especially if tumours begin to form, cattle experience weight loss, decreased milk production, and sudden death. Leukosis tumours are often a leading cause of carcass condemnations.
Leukosis is traditionally associated with dairy animals, but beef cattle can be affected. In 1980, a national study determined that 40 per cent of dairy and 11 per cent of beef herds were infected across Canada. More recently, a 2010 M.Sc. project from the University of Saskatchewan reported an infection rate of 12 per cent in beef cattle herds across the country.
BLV is closely related to a virus that causes adult T-cell leukemia in humans, which has resulted in a great deal of research into a possible human health risk from animals infected with BLV. Most recently, a report in PLOS ONE titled, “Exposure to Bovine Leukemia Virus is Associated with Breast Cancer: A Case-Control Study,” indicated that the frequency of BLV DNA in breast tissue cells from women with breast cancer was 30 per cent higher than the normal controls. The authors of this study do not claim that BLV causes breast cancer, or even that it is definitively a significant risk factor for breast cancer, but they do raise the possibility of using BLV as a biomarker to identify higher-risk individuals.
Many media reports have spun this as a “link” between BLV and breast cancer, which is overstating the findings. This report in PLOS ONE was a case control study and case control studies are unable to establish causation. The authors themselves state: “Validation by other investigators is essential, and a prospective study showing that viral infection preceded detectable cancer development would be desirable to support the idea of a causal association of BLV with breast cancer.”
There is no way of knowing whether the subjects were infected with the virus before or after the onset of their breast cancer. The route of exposure to BLV is not clear — both pasteurization and proper cooking inactivate the virus, which renders it harmless. A large body of previous evidence has indicated little to no risk of transmission of BLV to humans, and in fact, this research group is the only one to have suggested otherwise.
Further, the methodology used in the PLOS ONE study to detect the BLV proviral DNA, in situ polymerase chain reaction (IS-PCR), is known to generate a high rate of false positives and background amplification. A report in the British Medical Bulletin in 2011 titled, “XMRV, prostate cancer and chronic fatigue syndrome,” reviews a similar situation where an endogenous retrovirus was implicated in chronic fatigue syndrome, but these results were unable to be replicated by other research groups.
Just as correlation does not equal causation, an association does not equal a link. We can’t say that a link between BLV and breast cancer is impossible, because it is impossible to prove a negative. But until another research group using different methodology replicates these results, we can feel safe being quite skeptical of any “link.”
Karin Schmid is a Beef Production Specialist with Alberta Beef Producers and Dr. Reynold Bergen is science director at the Beef Cattle Research Council.