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In 2015, researchers in California found bovine leukemia virus (BLV) stitched into the DNA of human breast cancer tumors from mastectomies at such higher rates than was found in normal breast tissue taken from breast reduction surgery that they calculated that as many as 37 percent of breast cancer cases may be attributable to bovine leukemia virus exposure, likely through the consumption of milk or meat obtained from infected animals.

In response, the milk and meat industries seemed more concerned about consumer confidence than consumer cancer. But scientifically, the research priority turned to: could the California results be replicated? The answer, it turns out, was yes. They were replicated among women in Iran. Replicated in Brazil. In Australia, the link was even stronger. In Texas, the same thing. Women diagnosed with breast cancer were found to be so much more likely to have bovine leukemia virus DNA in their breast tissue, compared with women without cancer, that the attributable risk was calculated at 51.82 percent––indicating that this meat and dairy virus could be responsible for at least one-half of the breast cancer cases among the women in Texas they studied.

All in all, six of the eight studies performed to date found the virus in human breast tissues, which suggests strongly that BLV does infect humans, and breasts can be targets of infection. Four of the five studies that were able to compare infection rates in cancerous tissue versus normal breast tissue found the odds of finding the virus in the tumors was, on average, four times higher.

How does that compare to other breast cancer risk factors? If you go on hormone replacement for five years, you can bump up your breast cancer risk 30 percent. If you take the pill for more than a dozen years, your risk may go up 40 percent. If you’re obese when you’re older, your risk can go up 60 percent. Having a first-degree relative with breast cancer may double your risk. But having your breast infected with bovine leukemia virus may quadruple your risk. The only risk factors more potent than BLV infection were having the BRCA gene mutations like Angelina Jolie has, or a high dose of ionizing radiation, like being in the wrong place at decidedly the wrong time.

Beyond confirmation, one study suggested that older patients had a greater likelihood of testing positive for bovine leukemia virus. That makes sense, if BLV is from exposure to dairy and meat. The older we get, the more meals we’ve had, and the more opportunities to become infected over time.

Researchers also discovered that the virus comes first, present in some breast tissues 3 to 10 years before cancer was diagnosed. “This argues against the idea of viral invasion of already malignant cells,” quashing the theory that maybe the virus is somehow just attracted to the cancer after the fact. Could this explain the consistent findings that breast cancer tissue is more likely to harbor infection? Again, the data showed no; the virus appeared to come first. This review doesn’t provide absolute proof that BLV is a cause of breast cancer, but based on the best available balance of evidence, BLV infection does indeed appear to be a breast cancer risk factor.

The latest revelation is that it’s now been found in human blood, too. This has a number of potential ramifications. Blood banks, for example, don’t screen for it. So, it’s possible you can get it from consuming meat or dairy, or getting blood from someone who consumed meat or dairy.

This could also mean that BLV could cause leukemia in people, too. It does in chimpanzees. Two infant chimps were fed milk from cows naturally infected with the BLV, and they both died of leukemia. We didn’t even know chimps could get leukemia.

This certainly suggests the possibility of transmission or induction of leukemia through the ingestion of milk from BLV-infected cows; or blood-borne spread could carry the virus to other organs. In cattle, the virus causes blood cancers, but this may be just because dairy cattle are turned into hamburger when they are still so young, so maybe they don’t have time for tumors to grow in other organs.

How concerned should we be about bovine leukemia virus? It is not clear yet whether this is a good news story or a bad news story. If subsequent studies show that BLV does cause breast cancer in humans, this will have significant repercussions for the dairy and cattle industries. But that means there is something we can do about it. Perhaps attempts should be made to eradicate the infection from cattle now, rather than wait for the final word. Twenty-one nations have eradicated BLV from their dairy cattle. In contrast, in the U.S. the BLV prevalence just keeps going up. If the industries are not going to step up and try to eliminate the disease, then the least they could do is eliminate some of the practices that spread the disease between animals.

BLV is spread via blood through contaminated needles, saw or gouge dehorners, tattoo pliers, ear taggers, hoof knives, nose tongs, and other tools of the agribusiness trade. Though in view of the emerging information about BLV in human breast cancer, it may be prudent to encourage the complete elimination of BLV in cattle, particularly in the dairy industry. The hope is that either way it may help reduce the scourge of breast cancer.

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