It’s probably not news to you that viruses can cause cancer. In fact, viruses have been implicated in six different human cancers so far, such as the well-known relationship between human papillomavirus infection and cervical cancer. Still, it came as a bit of a surprise when a recent study drew a link between a cow virus and breast cancer.

Now, no one is saying (yet) that this virus causes breast cancer; this work is very much in its infancy and warrants further scrutiny. But if future, more in-depth studies bolster these findings and manage to establish a causal role for this virus, then it could have significant implications for breast cancer research and control, which presently focuses on treatment rather than prevention.

The virus in question is called bovine leukemia virus (BLV), which happens to be the commonest cancer-causing virus of cattle. Infected herds can be found all across the globe, and in the U.S. it’s been detected in 100% of large dairy operation herds and almost 40% of beef herds. But despite its potential to cause disease, it only leads to cancer in around 5% of those infected.

The virus predominantly targets blood cells, but it can also infect mammary cells within the udder and consequently can be detected in cow’s milk. Although pasteurization renders the virus harmless, scientists were worried that exposure to food products could lead to human infection, but it wasn’t until last year that a study was published proving it could actually be transmitted to humans.

Following on from an investigation that picked up antibodies against BLV in humans, the aforementioned study managed to find viral DNA in human breast tissue, prompting the same researchers to investigate possible links with breast cancer. To do this, scientists from the University of California, Berkeley, analyzed breast tissue samples from 239 donors, comparing those with the disease to controls from women with no history of breast cancer.

As described in PloS One, while they found BLV DNA in 29% of the normal controls, the rate was significantly higher in tissue from women with breast cancer, detected in 59% of the samples. Furthermore, they found that this apparent increased risk in the likelihood of breast cancer imposed by BLV was comparable to those of well-established risk factors, like certain lifestyle choices. The only factors that carried a significantly greater risk were genetics, age and high doses of radiation.

Now the link has been identified, scientists need to dig a bit deeper and look for potential transmission routes and evidence of a causal role, because at the moment we still can’t say this virus has the ability to cause breast cancer. But if it turns out it can, this could potentially be a good thing, as it may ultimately be possible to design preventative strategies like vaccines that could reduce breast cancer incidence.