Bovine leukemia virus (BLV) is a virus that is very commonly found in cows. About 89 percent of dairy farms and 38 percent of beef farms in the United States have cows that are infected with BLV. Cattle in other parts of the world are also frequently affected. About 5 percent of cows with a BLV infection develop leukemia or lymphoma.

Recently, evidence of the bovine leukemia virus has been found in humans. Scientists are now studying the connection between BLV and human cancers. So far, BLV has been linked to breast cancer, but not to human leukemia.

Viruses and Cancer

A virus is a tiny microorganism that needs to infect other cells in order to survive. Viruses contain a few genes, stored on DNA or RNA molecules, and a few proteins. When they infect other cells, they usually damage or kill the cell.

Some types of viruses can cause cancer. About 15 percent of all cases of cancer in humans are linked to viral infections. One of the most common examples of a cancer-causing virus is human papillomavirus (HPV). HPV is primarily spread through sexual activity. It is the main cause of cervical cancer, and it can also cause cancer of the penis, vagina, anus, mouth, or throat. Getting an HPV vaccine can help prevent HPV infection. Vaccination can drastically reduce the number of cases of virus-associated cancers.

Viruses and Blood Cancers

Researchers have identified several viruses that can cause leukemia or lymphoma. For example, human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL). ATLL is a rare cancer of the lymphocytes (white blood cells that fight infection). This condition can either be a leukemia or a lymphoma, depending on whether the cancerous lymphocytes appear in the blood or in the immune system.

Several additional viruses have also been linked to developing lymphoma, including:

• Esptein-Barr virus (or EBV)
• Human herpesvirus 8 (or HHV-8)
• Human immunodeficiency virus (or HIV)
• Hepatitis C

Not everyone who is infected with these viruses will develop cancer. However, these viruses will increase a person’s risk of developing certain types of lymphoma. Currently, there are no vaccines that can protect against these viruses.

What Is Bovine Leukemia Virus?

BLV is a virus that primarily infects cow lymphocytes. A small portion of BLV-positive cows develop tumors called lymphosarcoma in the heart, uterus, stomach, spinal canal, or eye. Infected cattle may also develop high levels of white blood cells.

BLV is very similar to HTLV-1. Both HTLV-1 and BLV are retroviruses — viruses that carry their genes on pieces of RNA. They are also both considered to be deltaretroviruses, a subtype of retroviruses that infect cells of the immune system. HTLV-1 and BLV both make a protein called Tax protein, which is responsible for turning on viral genes after the virus infects a cell. Tax protein also causes T-cell proliferation (very fast growth) and blocks the cells from fixing damage. These effects can make a T cell turn cancerous.

Does BLV Infect Humans?

Because BLV causes cancer in cows, and because BLV is similar to a virus that causes human cancers, researchers have begun studying whether BLV can infect and cause cancer in humans. Scientists started by looking at breast tissue, since BLV is known to infect mammary cells in cows. One study analyzed more than 200 human breast tissue samples and found 44 percent contained bovine leukemia virus DNA. Past studies have also shown that nerve cells can be infected with BLV. It appears that BLV can indeed infect human cells.

Scientists aren’t exactly sure how BLV might be transmitted from cows to humans. It is possible that bovine leukemia virus particles might be present in meat or milk from cows, and that humans might become infected when they ingest these products. However, this has not yet been confirmed. Researchers are still studying the transmission of BLV.

Can BLV Cause Cancer in Humans?

Over the past decade, some studies have begun to show a link between BLV and breast cancer.

For example:

• Research by American scientists found BLV in 59 percent of breast cancer tissue and in only 29 percent of healthy breast tissue.
• A Brazilian study found BLV infection in 30.5 percent of breast cancer cases and 13.9 percent of healthy cases.
• A study of Australian women found that 80 percent of women with breast cancer and 41 percent of women without breast cancer were infected with BLV.

These studies reported very different rates of BLV infection. This may be due to different rates of cow milk and meat consumption in different parts of the world. However, these studies all showed that women with breast cancer are more likely to have BLV infections than women without breast cancer.

These analyses are not conclusive. Other studies have not found any BLV infections in breast tissue or have found lower rates of BLV infection in women with cancer compared with healthy women. Additionally, many people with BLV infections do not develop cancer, and not everyone with breast cancer has BLV. The link between bovine leukemia virus infection and breast cancer remains somewhat murky.

One other study found evidence of BLV in samples from a specific type of lung cancer. Researchers found that 80 percent of cases of squamous cell carcinoma of the lungs were infected with BLV. However, this study was very small, only looking at 10 samples total, and the results have not yet been repeated. More research is needed to understand how BLV and lung cancer may be connected.

Research in this field is ongoing. It is not yet clear whether or not BLV can cause cancer.

Does BLV Cause Leukemia or Lymphoma in Humans?

BLV causes blood cancers in cows, so it’s possible that the virus could cause the same diseases in humans. Recently, scientists discovered that BLV could infect human blood cells. In a study of 95 women, researchers found that 38 percent had BLV-infected white blood cells.

Only one study has looked for bovine leukemia virus in human leukemia samples. Researchers examined hundreds of cases of leukemia, including acute lymphoblastic leukemia, acute myeloid leukemia, and chronic myelogenous leukemia. The study authors did not find any evidence of BLV in these cases.

It is important to note that as technology improves, researchers come up with better ways to detect viral infections. So far, no studies have found evidence that BLV is one of the causes of leukemia. However, it is possible that this will change in the future.

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