The short version is the study analyzed breast tissue from 239 women, and found evidence of BLV exposure in 59% of the samples from women with breast cancer, compared to only 29% of samples from women without a history of breast cancer. As the authors point out, this is an important finding but only the first step – for a start, they “still need to confirm that the infection with the virus happened before, not after, breast cancer developed, and if so, how.”
There are various examples of cancer-causing viruses in the human and animal worlds. For example, hepatitis B virus in people can cause liver cancer, and human papillomavirus can cause cervical cancer. In sheep and goats, there is a virus called nasal adenocarcinoma viurs which causes “contagious” nasal tumors. However, perhaps partly because viruses tend to be more species-specific than bacteria, there are no known zoonotic viruses that have been linked to cancer in humans – yet.
Bovine leukemia virus is a well known retrovirus that naturally infects cattle. Most cattle that become infected do not show any signs of disease, but less than 5% will eventually develop cancer (lymphosarcoma). The virus is transmitted between cattle primarily by blood contamination (e.g. reusing needles on multiple cattle, contaminated ear tagging equipment, blood on rectal sleeves). A number of studies have been done in the past to evaluate if BLV is a risk to people, but none produced any conclusive evidence that the virus is hazardous to humans. Until a recent paper published in 2014, it was thought that BLV virus could not be transmitted to people at all. Several European countries have managed to successfully eradicate the disease, but test-and-slaughter programs are often not practical when infection is common (estimated prevalence in US dairy cattle is 44%, and 10% in US beef cattle). Instead, the emphasis is placed on preventing transmission within the herd, and infected cattle are then gradually eliminated through normal culling for other reasons.
If more evidence of a link between BLV and disease in humans is found, the cattle industry will then have considerably more motivation to eliminate this virus from their herds (which would benefit the cows as well of course). However, as the authors of the study point out in the news release, “our results do not prove that the virus causes cancer.” While we await further research, consider it yet another good reason to ensure meat is always cooked properly and milk is pasteurized before consumption – viruses like BLV and other zoonotic bacteria can’t take the heat!