Exposure to the bovine leukemia virus from meat and dairy (or a blood transfusion from those who eat meat or dairy) is a risk factor for cancer.
In 2015, researchers in California found bovine leukemia virus (BLV) stitched into the DNA of human breast cancer tumors from mastectomies. The virus was found at much higher rates than in normal breast tissue obtained from breast reduction surgeries. Based on this difference, they calculated that as many as 37% of breast cancer cases may be attributable to exposure to BLV, likely through consuming milk or meat 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 the question: 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, they found 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%, indicating that this meat and dairy virus may be responsible for at least 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 compared infection rates in cancerous versus normal breast tissue found that the odds of detecting the virus in tumors were, 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 by 30%. If you take birth control pills for more than a dozen years, your risk may go up by 40%. If you’re obese when you’re older, your risk can go up by 60%. 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, as you can see below and at 2:16 in my video Breast Cancer and the Bovine Leukemia Virus in Meat and Dairy.
The only risk factors more potent than BLV infection were having the BRCA gene mutation, like Angelina Jolie has, or a high dose of ionizing radiation, like being in the wrong place at decidedly the wrong time, like Hiroshima and Nagasaki during World War II.
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, before the cancer diagnosis; they found it was present in some breast tissues 3 to 10 years before cancer was found. “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. While the review doesn’t provide absolute proof that BLV is a cause of breast cancer, based on the best available balance of evidence, BLV infection does indeed appear to be a risk factor for breast cancer.
The latest revelation is that BLV has 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, as well as from getting blood from someone who consumed meat or dairy. This could also mean that BLV could cause leukemia in people. It does in chimpanzees. Two infant chimps were fed milk from cows naturally infected with BLV, and 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 slaughtered and 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 whether this is a good news story or a bad news story.” If future studies show that BLV does cause breast cancer in people, there will be significant repercussions for the dairy and cattle industries. But that means there is something we can do about it. Perhaps action should be taken now to eradicate the infection from cattle, rather than waiting for a final verdict. Twenty-one nations have already eradicated BLV from their dairy cattle. In contrast, the BLV prevalence in the United States just keeps increasing. If 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, ear taggers, hoof knives, tattoo pliers, nose tongs, and other tools of the agribusiness trade. Though “in view of the emerging information about BLV in human breast cancer, it is prudent to encourage the 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.
Doctor’s Note
If you missed the previous video, see Bovine Leukemia Virus as a Cause of Breast Cancer.
Avoiding infectious risks like BLV is another advantage of making meat without animals. See my video, The Human Health Effects of Cultivated Meat: Food Safety.
Individual women who are lactose intolerant and 
So much so that as many as 37% of breast cancer cases 
