Dairy Industry Responds to Bovine Leukemia Virus in Breast Cancer

What was the response to the revelation that as many as 37 percent of breast cancer cases may be attributed to exposure to bovine leukemia virus (BLV), a cancer-causing cow virus found in the milk of nearly every dairy herd in the United States? I discuss this issue in my video Industry Response to Bovine Leukemia Virus in Breast Cancer. The industry pointed out that some women without breast cancer harbored the virus, too. Indeed, BLV was found in the tissues of 29 percent of women who didn’t have breast cancer, a finding the researchers replied “is not surprising considering the long latency period of breast cancer…” In other words, they may not have breast cancer yet.

It can take decades before a breast tumor can be picked up on mammography. So, even though people may be harboring this virus in their breast and feeling perfectly fine, the cancer may still be on its way. That’s how other cancer-causing deltaretroviruses appear to work. These viruses can make proteins that interfere with our DNA repair mechanisms. Infected cells are then more susceptible to carcinogens and slowly accumulate mutations over time. “Therefore, evidence of BLV in normal breast tissues prior to premalignant and malignant changes would be expected.” This pattern is what we see with cervical cancer, “in which the causative virus (HPV) is found not only in the malignant [cancerous] tissue, but also in premalignant dysplastic areas [the precancerous tissue] and in normal tissue adjacent to the malignant tumor.”

If BLV, a retrovirus, is really causing thousands of cases of breast cancer every year, wouldn’t some of the anti-retroviral therapies like some of the AIDS drugs be able to counter it? Perhaps, but it’s best not to get infected in the first place.

However, the agriculture industry appeared to be more concerned about consumer confidence in U.S. dairy than consumer cancer. Indeed, the “U.S. dairy industry face[d] a brewing public-relations brouhaha,” and it became “concerned about the possibility of eventual mandatory control of these diseases in dairy cattle along with public perception and an impact on the consumption of dairy products.” What would control look like? BLV is a blood-borne virus, but how is it spread? Is Bessie sharing dirty needles? In a sense, yes: “[B]lood (and BLV virus) is readily spread from animal to animal with blood contaminated needles and/or syringes, obstetrical sleeves, saw or gouge dehorners, tattoo pliers, ear taggers, hoof knives, nose tongs,” and other instruments that aren’t disinfected between animals. So, for example, when farmers are gouging or sawing at the cows’ heads during dehorning, “they are likely to drive blood into the next animal during the subsequent dehorning process.” Or, when they’re sticking their arms into cows’ rectums for artificial insemination, it’s not uncommon for there to be rectal bleeding—then they just go from one cow to the next.

More than 20 countries have successfully eradicated BLV from their herds by changing their practices, whereas it remains an epidemic in the United States in part because we’re not cleaning and disinfecting blood-contaminated equipment for things like “supernumerary teat removal,” which is done because “the presence of extra teats detracts from the beauty of the cow.” Supernumerary teats are removed by pulling them from the udder and cutting them off with a pair of scissors. Those scissors had better be clean—otherwise they could spread BLV from calf to calf and ultimately to someone’s breakfast, lunch, or dinner. Of course, we could just not slice off their teats at all, but then how would we “improve udder appearance?”


Up to 37 percent of breast cancer cases are attributable to exposure to bovine leukemia virus? See my video The Role of Bovine Leukemia Virus in Breast Cancer and its prequel, Is Bovine Leukemia Virus in Milk Infectious?.

The meat and dairy industries’ intransigence in the face of a human health threat reminds me of the antibiotics and steroids issues—continuing to place the public at risk to save a few bucks. See, for example, Antibiotics: Agribusinesses’ Pound of Flesh and Zeranol Use in Meat and Breast Cancer.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Cow Cancer Virus Implicated in Breast Cancer

Up to 20 percent of all cancers in general are linked to infections, particularly viruses, and the list of potentially carcinogenic infectious agents is growing. It would be great if we could find a virus that contributed to breast cancer risk, because then we might have new ways to prevent and treat it. Currently, the dietary link between breast cancer and consumption of meat and dairy is considered a saturated fat effect, but there is a cancer-causing bovine virus that infects the mammary gland cells of cows. The infectious virus is then released into the milk supply. Since most U.S. dairy herds are infected, scientists posit that Americans are often exposed to this bovine leukemia virus (BLV), which I discuss in my video The Role of Bovine Leukemia Virus in Breast Cancer.

We didn’t have proof of this until 2003, 34 years after the virus was first identified. Early on, our best available tests failed to find antibodies to BLV in human blood. When our immune system is exposed to a virus, it creates antibodies to attack it. No antibodies, no exposure. “This led to the prevailing opinion that…the virus is not a public health hazard.” Though those tests “were state of the art at that time, they are extremely insensitive compared to more modern techniques.” As a result, researchers decided to re-examine the issue now that we have better tests. They took blood from about 250 people simply to address the question: “Do any humans have antibodies to BLV?” The answer? Yes, 191 of them did––74 percent. That shouldn’t have come as a surprise, however: By then, nearly 90 percent of American dairy herds were infected, and, according to the latest national survey, 100 percent of the big factory dairy farms were infected, as determined by testing the milk coming from those operations. Given this, why isn’t there an epidemic of cancer of the udder? Dairy cattle are slaughtered so young that there isn’t a lot of time for them to develop gross tumors, but that’s how most women may be getting infected. Although pasteurization should knock out the virus, who hasn’t eaten a rare, pink-in-the-middle burger at some point?

The bottom line is that the “long-held assumption that BLV is not a public health hazard…is no longer tenable…” This whole field of investigation needs to be reopened, with the next step determining whether humans are actually infected. “The presence of antibodies to particular viruses in human sera is generally interpreted as an indicator of a present or past infection with the virus.” But, theoretically, we might have developed antibodies to the dead viruses we ate, viruses that had been killed by cooking or pasteurization. Just because three-quarters of us have been exposed doesn’t mean we were actively infected by the virus.

How do we prove this? We would need to find the retrovirus actively stitched into our own DNA. Well, millions of women have had breast surgery, so why not just look at the tissue? Researchers finally did just that and published their findings in the Centers for Disease Control and Protection’s emerging infectious diseases journal: Forty-four percent of samples tested positive for BLV, proving for the first time that humans can be infected with bovine leukemia virus. The final step? Determine whether the virus is actually contributing to disease. In other words, are the bovine leukemia viruses we’re finding in human breast tissue cancer-causing or just “harmless passengers”?

One way to make that determination is to see whether the virus is more often present in those with breast cancer. No one had ever looked for the virus in breast tissue from people with cancer…until now. The “[p]resence of BLV-DNA in breast tissues was strongly associated with diagnosed and histologically confirmed breast cancer…” As many as 37 percent of human breast cancer cases may be attributable to exposure to bovine leukemia virus.


For some historical background leading up to these shocking findings, see my video Is Bovine Leukemia Virus in Milk Infectious?.

I couldn’t wait to read the meat and dairy industry journals to see how they’d try to spin this. Find out what I discovered in my final video in this series Industry Response to Bovine Leukemia Virus in Breast Cancer.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Should You Be Concerned about Bovine Leukemia Virus in Milk?

Decades ago, concern was raised that the milk of dairy cows frequently contains a leukemia-causing virus—more specifically, bovine leukemia virus (BLV), the leading cancer killer among dairy cattle. Most U.S. dairy herds are infected with the cancer virus. “Thus the question of whether dairy cows naturally infected with BLV release infectious virus into milk is an important public health consideration” and the subject of my video Is Bovine Leukemia Virus in Milk Infectious?.

Researchers at the University of Pennsylvania decided to put it to the test. And indeed, infectious virus was demonstrated in the milk of 17 of the 24 cows tested, indicating that “humans are often orally exposed to BLV.” Just because we’re exposed to it doesn’t mean it’s causing human disease, though. How do we know BLV can even infect human cells? We didn’t until 1976 when it was discovered that BLV can indeed infect human, chimpanzee, and rhesus monkey cells. Nevertheless, that still doesn’t mean BLV necessarily causes cancer in other species.

Researchers can’t lock human infants in a cage and feed them infected milk, but they can cage infant chimpanzees. Chimps Bois and Roger were fed infected milk, developed leukemia, and died. Until then, we didn’t even know chimps could get leukemia. The fact that BLV-infected milk appeared to transmit or induce leukemia in our closest living relatives certainly did raise the stakes, but human beings are not chimpanzees. Yes, our DNA may be 98 percent identical, but we may share 60 percent of our DNA with a banana. We need human studies.

We can’t do interventional trials in this case, thanks to those pesky Nuremberg principles, but what about observational studies? Do cattle farmers have higher rates of cancer? Apparently so. This finding led some to suggest that “milk- and egg-borne viruses may be highly important in the pathogenesis [or development] of human leukemia and lymphoma,” but farmers may be exposed to all sorts of potential carcinogens, such as pesticides. Large animal veterinarians may also have more leukemia and lymphoma, but some are also “particularly lax in the use of X-ray protective equipment,” so it didn’t necessarily have anything to do with viruses.

We needed so-called serology studies, testing people’s blood for antibodies against the virus, which would prove human exposure, and we got them. Ten different studies looked for BLV antibodies in cancer patients and non-cancer patients, creamery employees versus office employees, veterinarians, unpasteurized milk drinkers, and more. “Not one of these studies found a single individual with antibodies to BLV…” As a result, in 1981, the case was closed: “Therefore, there is strong serological evidence to indicate that BLV is not transmissible to man.” However, the strength of the evidence is only as strong as the strength of the test. Chimpanzees Bois and Roger didn’t develop detectable antibodies either, and they died from BLV.

The tests available a handful of decades ago were not really sensitive. “Clearly, the question of whether BLV poses a public health hazard deserves thorough investigation” using highly sensitive molecular probes. It would take a few decades for us to get such an examination, and I discuss those landmark findings in my videos The Role of Bovine Leukemia in Breast Cancer and Industry Response to Bovine Leukemia Virus in Breast Cancer.


Thankfully, feline leukemia virus does not appear to be transmissible. See Pets and Human Lymphoma.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: