Although the topic is in its infancy, it is plausible that viruses could contribute to breast cancer risk. Links between viral infection and cancer have been established for a number of viruses, among them hepatitis B and hepatitis C (liver cancer), human papilloma virus (HPV) (cervical cancer), human T lymphotrophic virus type 1 (HTLV-1) (T-cell leukemia), and Epstein-Barr (B and T cell lymphomas, Hodgkin’s disease, and other cancers).

It can take years or even decades for infection with an oncogenic virus to result in cancer, and only a minority of those infected will actually progress to cancer. This is because viruses do not appear to cause cancer directly, but may induce a tissue microenvironment that is supportive of cancer initiation or growth. For example, in some cases, viral gene products in tumor cells can help them maintain their excessive proliferation. As such, a relevant viral infection can be thought of as a cancer risk factor rather than a trigger that inevitably causes cancer. Exposure to HPV, Epstein-Barr, human cytomegalovirus (HCMV), measles, mouse mammary tumor virus or the closely-related human mammary tumor virus, and bovine leukemia virus (BLV) all have been proposed as contributing to breast cancer risk, although the findings are controversial. Now a new study has reported that women with BLV in their breast tissue could be three times as likely to have breast cancer as women without BLV.

Bovine leukemia virus (BLV)

BLV is a virus closely related to HTLV-1 that infects the majority of U.S. beef and dairy cattle herds — one study estimated that 83% of dairy herds are infected with BLV. Western Europe is the only global region with strong and successful anti-BLV programs. BLV does not normally cause obvious illness in cattle and the majority of infected cattle appear to be carriers with the virus in a latent state. However, BLV depresses the immune system and produces a benign mononucleosis-like condition in at least 30% of infected animals. Only up to 5% of animals infected with BLV eventually develop leukemia (enzootic bovine leukosis) and only obviously ill animals are culled and removed from human consumption. Beef labelled "organic" is not necessarily BLV-free. There is no efficient vaccine that can protect cattle against BLV infection. The majority of U.S. producers do not screen for BLV or actively try to reduce BLV incidence within their herds. This could change since BLV infection has been shown to reduce milk production and decrease longevity.

BLV is transmitted in cattle through the transfer of infected cells, especially in blood or milk. Cattle management procedures such as dehorning, ear tattooing, and reuse of infected needles are some common modes of transmission. Transfer of infected blood also appears to occur in regions with a high density of insects that feed on blood. In addition, prolonged direct contact between infected and healthy animals appears to be a risk factor for BLV transmission. BLV is transmitted to calves both in the womb and after birth (via colostrum and milk), although maternal antibodies present in the colostrum offer some protection to calves.

The majority of people in the U.S. carry BLV antibodies in their blood, indicating exposure to the virus. Although BLV-infected cells have been found in beef and dairy products, heat and pasteurization destroy the virus. The presence of BLV antibodies does not necessarily mean active infection with BLV; the antibodies could be a response to heat-denatured BLV antigens consumed in food. Raw milk and uncooked beef are possible sources of actual BLV infection, however. It is not clear to what extent handling raw beef during cooking (such as in making meatballs or preparing steak for grilling) could transmit the virus. However, it makes sense to use the same precautions in handling raw beef that are currently recommended for raw chicken. Tasting raw ground beef mixtures during cooking preparation or consuming beef "rare" or as beef tartare could introduce risk of BLV infection. The authors of the study described below also raise the possibility that human-to-human transmission of BLV may be occurring.

Latest research finds BLV in breast tissue associated with breast cancer

The case-control study referenced at the beginning of this news story was designed to investigate whether the presence of BLV in human breast tissue is associated with breast cancer. A number of breast cancer risk factors have been well documented, including age, parity and age of first birth, hormone exposures, radiation exposure, genetic mutations, and various aspects of lifestyle. However, the agents that induce cellular changes from normal to malignant are not well understood.

The authors previously detected BLV in human breast epithelial cells (the type of cell in which breast cancer typically develops). To conduct the present study, the authors used archival breast tissue from 239 donors. Medical records and examination of tissues by an anatomical pathologist were used to categorize the tissue as coming from women with breast cancer, having precancerous changes, or normal. Exposure to BLV was determined by in situ-PCR detection of a biomarker, BLV DNA, in the breast tissue.

Breast tissue from women with breast cancer was twice as likely to contain BLV DNA as normal tissue (59% compared to 29%). In addition, in women with precancerous breast changes, the rate of BLV (38%) was inbetween that of normal women and women with invasive breast cancer. The authors calculated that women with BLV in their breast epithelial tissue were three times as likely to have breast cancer as women without BLV. BLV was found more often in estrogen receptor positive (ER+) (68%) than in ER- (50%) samples and more often in progesterone receptor positive (PR+) (74%) than in PR- (51%) samples. Further analysis showed that the BLV did not appear to be in its usual latent state in 6% of the women, indicating that viral replication could be occurring. The authors conclude that the presence of BLV was significantly associated with breast cancer among the specimens in this study. The level of association is comparable to those of well-established breast cancer risk factors related to reproductive history, hormones, and lifestyle. The association is exceeded only by breast cancer risk factors related to genetics (familial breast cancer), high dose ionizing radiation, and age, according to the authors.

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