Bovine Leukemia Virus and Human Breast Cancer Risk

Gertrude Buehring, Ph.D.
University of California, Berkeley

The purpose of the research proposed here is to determine whether: 1) humans can become infected with bovine leukemia virus; 2) such an infection occurs through consumption of foodstuffs from infected cows; 3) infection with BLV could lead to developing breast cancer. It is well established that breast cancer in the mouse is caused by a virus, the mouse mammary tumor virus, which is passed from mother to nursing babies via the milk. Attempts to find an analogous virus in human milk passed from mother to baby have failed. Since humans drink more cow's milk than they do human milk, perhaps cows would be a more promising target in the search for a milk-transmitted agent of human breast cancer.

Bovine leukemia virus (BLV) is a cancer-causing virus of cattle which can be passed from mother cow to calf via the milk. BLV commonly infects dairy and beef cattle and is found in the marketed milk and meat of these animals. Most infected cattle do not actually develop leukemia, but remain healthy and are not removed from the herd. Consumption of unpasteurized dairy products or undercooked beef could possibly allow transmission of infectious virus to humans. BLV infections are not limited to cattle. The virus can infect sheep and nonhuman primates experimentally and cause cancer in the sheep. In the laboratory it can infect the cells of many species including humans and other primates. We recently discovered that BLV may infect the breast cells of cows and cause these cells to behave more like cancer cells. This indicates that the tissue preference of this "leukemia" virus is not limited to blood cells.

We propose here a pilot study to examine the first aspect of the overall proposal, whether humans can become infected with BLV. Human breast tissues removed during surgery, breast tissue sections received from a pathologist, cells from milk and colostrum, and blood cells will be searched for evidence of different components of BLV using cellular and molecular techniques (immunocytochemistry, PCR, and in situ reverse transcriptase PCR). Human blood will be tested for antibodies to BLV. Our preliminary results suggest that the breast cells of some women are infected with BLV and that some humans have antibodies to BLV. The potential impact of the project is through prevention of these cancers by elimination of BLV in cattle, interception of its transmission from cattle to humans, or vaccination of humans. 

The purpose of the research proposed here is to determine whether 1) humans can become infected with bovine leukemia virus; 2) such an infection occurs through consumption of foodstuffs from infected cows; 3) infection with BLV could lead to developing breast cancer. This relates to BCRP Cycle II priorities of etiology and prevention. It is well established that breast cancer in the mouse is caused by a mammary tumor virus, which is passed from mother to nursing babies via the milk. Mouse breast cancer is proof that a naturally occurring cancer can be caused by a virus swallowed in a food source. Attempts to find an analogous virus in human milk passed from mother to baby have not been conclusive. Since humans drink more cows’ milk than they do human milk perhaps cows would be a more promising place to search for a milk-transmitted. agent of human breast cancer. Bovine leukemia virus (BLV) is a cancer-causing virus of cattle, which can be passed from mother cow to calf via the milk. BLV commonly infects dairy and beef cattle and is found in the marketed milk and meat of these animals. Most infected cattle do not actually develop leukemia, but remain healthy and are not removed from the herd. Consumption of unpasteurized dairy projects or undercooked beef could possibly allow transmission of infectious virus to humans. We recently discovered that BLV might infect the breast cells of cows and cause these cells to behave more like cancer cells. This indicates that the tissue preference of this "leukemia" virus is not limited to blood cells.

This research has addressed the first aspect of the overall proposal, whether humans can become infected with BLV. Human breast tissues removed during surgery, breast tissue sections received from a pathologist, and cells from milk and blood were searched for evidence of different components of BLV using cellular and molecular techniques (immunocytochemistry, PCR, and in situ hybridization). Human blood was tested for antibodies to BLV. We detected evidence of BLV DNA in blood cells from 9 of 22 human volunteers and in surgically removed breast tissues from 10 of 23 patients. We found evidence of BLV proteins in breast tissues from 8 of 26 patients. Antibodies to BLV were found in the serum of over half of the 100 human volunteers tested. These preliminary results suggest that humans may be infected with BLV. If subsequent studies confirm this and BLV is eventually shown to play a role in the development of breast cancer, prevention would be relatively simple: 1) by elimination of BLV in cattle (vaccination, isolation, or extermination); 2) by interception of transmission from cattle to humans (education to thoroughly cook beef and not consume raw milk products); or by 3) vaccination of humans.