BLV is a virus that invades cells in cows. Scientists discovered it in 1969 by isolating white blood cells called lymphocytes in cattle. BLV is a retrovirus, which means that it replicates in cells through a process called reverse transcription.

BLV is commonly found in cows in the United States. However, only about 5 percent of animals infected with BLV show leukemia. About 30 percent of infected cows have a condition called lymphoproliferation, in which white blood cells are produced in high numbers.

Researchers discovered that BLV has oncogenic properties. This means it can contribute to cancers in cows. Because cows frequently interact with humans, there has recently been interest in understanding if BLV can affect humans. Additionally, BLV is closely related to human T-lymphotropic virus type 1, suggesting that it can also affect human health.

Scientists initially thought that BLV couldn’t transmit from cattle to humans. This thinking was based on 10 studies conducted in the 1970s that didn’t detect BLV infection in humans. As a result, cattle farmers assumed they didn’t have to worry about the transfer of BLV from cow to human.

Due to more sensitive and accurate tests, scientists have learned that BLV can indeed infect humans. A 2003 study published in AIDS Research and Human Retroviruses showed that many of the 257 participants had antibodies to BLV. This suggests that BLV can transmit from cows to humans, though it doesn’t necessarily mean it’s harmful to human health.

In a more recent study, published in 2014 by the Centers for Disease Control and Prevention in its Emerging Infectious Diseases journal, researchers from the University of California, Berkeley, discovered BLV in human breast tissue. In cows, BLV has cancer properties.

In a 2015 study published in PLOS One, this group decided to investigate if there is an association between BLV and breast cancer. To test their hypothesis, the team of researchers collected breast tissue from a database called the Cooperative Human Tissue Network. They looked at 218 tissues samples from consenting donors. About 114 of these subjects had breast cancer, while 104 had no breast cancer. Both groups had similar risk factors related to breast cancer, as well as similar demographics, including:

  • age
  • sex
  • location

When analyzing the data, researchers discovered DNA from BLV in about 59 percent of the women with breast cancer. In contrast, only 29 percent of those in the control group had DNA from BLV. This difference was statistically significant. Researchers concluded, “These findings have the potential for primary and secondary prevention of breast cancer.” They emphasized that the results weren’t conclusive and more research needs to be done.

However, two follow-up studies published in 2016 failed to find a similar pattern of increased rates of BLV DNA in women with breast cancer. One study performed in Chinese women used a similar methodology as the 2015 study. But researchers couldn’t detect any BLV DNA in breast cancer tissue or blood from women with breast cancer. Another study using deep sequencing to comb through DNA found in over 50 breast cancer samples also failed to detect any evidence of DNA from BLV in the breast cancer tissues.

In humans, there aren’t many known signs of BLV. This is largely due to the lack of research on BLV infection in humans.

That said, doctors can identify BLV infection by testing for antibodies that specifically target the virus. They use a process called immunoblotting to do this by searching for the p24 capsid protein, which acts as an antigen, or a substance that triggers the production of antibodies.

Because BLV is a bovine-based virus, it has symptoms that are more notable in cows, such as swollen lymph nodes and conjunctivitis.

Researchers aren’t exactly sure how BLV is transmitted from bovine to human. Some suspect that human consumption of beef and dairy products may facilitate the transfer, but more evidence is needed to support this.

If you’re concerned about BLV and other cow-to-human viruses, consider eliminating beef from your diet and reducing or eliminating your consumption of dairy products. This may reduce your exposure to cow products that could potentially contain the virus, although rates of transmission still aren’t established.

Researchers have now seen that BLV can be transmitted from cows to humans. Research indicates a potential association between breast cancer and BLV. Although there are no known symptoms of BLV infection in humans, its presence can be detected by determining the presence of antibodies to the virus.

BLV is a virus commonly found in cows. The virus doesn’t cause serious problems in most cows — it’s thought to cause leukemia in a very small percentage of cases.

BLV can transmit from cows to humans. The way this happens isn’t well understood. A recent research study showed a positive association between BLV in human breast tissue and the incidence of human breast cancer, although other studies performed since haven’t been able to reproduce these findings. More research is needed to investigate the nature of the relationship between BLV and breast cancer.

If you think you might have breast cancer, talk to your doctor about your risk factors and specific symptoms. Consider getting a mammogram. According to the National Breast Cancer Foundation, symptoms of breast cancer include:

  • a lump in your breast
  • a change in skin texture of expanded pores near your breast
  • sudden swelling around the breast and nipple area
  • sudden changes in the size of your breast
  • clear or bloody discharge from your nipple