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:

The Answer to a Colon Cancer Mystery

Colorectal cancer is the third most common cause of cancer death in the world. Thankfully, the good bacteria in our gut take the fiber we eat and make short-chain fatty acids, like butyrate, that protect us from cancer. We take care of them, and they take care of us. If we do nothing to colon cancer cells, they grow. That’s what cancer does. But if we expose the colon cancer cells to the concentration of butyrate our good bacteria make in our gut when we eat fiber, the growth is stopped in its tracks. If, however, the butyrate stops, if we eat healthy for only one day and then turn off the fiber the next, the cancer can resume its growth. So, ideally, we have to eat a lot of fiber-rich foods—meaning whole plant foods—every day.

What about the populations, like those in modern sub-Saharan Africa, where they don’t eat a lot of fiber yet still rarely get colon cancer? Traditionallly. they used to eat a lot of fiber, but now their diet is centered around highly refined corn meal, which is low in fiber—yet they still have low colon cancer rates. Why? This was explained by the fact that while they may be lacking protective factors like fiber, they are also lacking cancer-promoting factors like animal protein and fat. But are they really lacking protective factors?

If you measure the pH of their stools, the black populations in South Africa have lower pH, which means more acidic stools, despite comparable fiber intakes. That’s a good thing and may account for the lower cancer rates. But, wait a second. Low colon pH is caused by short-chain fatty acids, which are produced by our good bacteria when they eat fiber, but they weren’t eating any more fiber, suggesting there was something else in addition to fiber in their diets that was feeding their flora. And, indeed, despite low fiber intake, the bacteria in their colon were still churning out short-chain fatty acids like crazy. But if their bacteria weren’t eating fiber, what were they eating? Resistant starch. “[T]he method of cooking and eating the maize [corn] meal as a porridge results in an increase in resistant starch, which acts in the same way as fiber in the colon,” as a prebiotic, a food for our good bacteria to produce the same cancer-preventing, short-chain fatty acids.

As I discuss in my video Resistant Starch and Colon Cancer, “[r]esistant starch is any starch…that is not digested and absorbed in the upper digestive tract [our small intestine] and, so, passes into the large bowel,” our colon, to feed our good bacteria. When you boil starches and then let them cool, some of the starch can recrystallize into a form resistant to our digestive enzymes. So, we can get resistant starch eating cooled starches, such as pasta salad, potato salad, or cold cornmeal porridge. “This may explain the striking differences in colon cancer rates.” Thus, they were feeding their good bacteria after all, but just with lots of starch rather than fiber. “Consequently, a high carbohydrate diet may act in the same way as a high fiber diet.” Because a small fraction of the carbs make it down to our colon, the more carbs we eat, the more butyrate our gut bacteria can produce.

Indeed, countries where people eat the most starch have some of the lowest colon cancer rates, so fiber may not be the only protective factor. Only about 5 percent of starch may reach the colon, compared to 100 percent of the fiber, but we eat up to ten times more starch than fiber, so it can potentially play a significant role feeding our flora.

So, the protection Africans enjoy from cancer may be two-fold: a diet high in resistant starch and low in animal products. Just eating more resistant starch isn’t enough. Meat contains or contributes to the production of presumed carcinogens, such as N-nitroso compounds. A study divided people into three groups: one was on a low-meat diet, the second was on a high-meat diet including beef, pork, and poultry, and the third group was on the same high-meat diet but with the addition of lots of resistant starch. The high-meat groups had three times more of these presumptive carcinogens and twice the ammonia in their stool than the low-meat group, and the addition of the resistant starch didn’t seem to help. This confirms that “exposure to these compounds is increased with meat intake,” and 90 percent are created in our bowel. So, it doesn’t matter if we get nitrite-free, uncured fresh meat; these nitrosamines are created from the meat as it sits in our colon. This “may help explain the higher incidence of large bowel cancer in meat-eating populations,” along with the increase in ammonia—neither of which could be helped by just adding resistant starch on top of the meat.

“[T]he deleterious effects of animal products on colonic metabolism override the potentially beneficial effects of other protective nutrients.” So, we should do a combination of less meat and more whole plant foods, along with exercise, not only for our colon, but also for general health.


This is a follow-up to my video Is the Fiber Theory Wrong?.

What exactly is butyrate? See:

For videos on optimizing your gut flora, see:

Interested in more on preventing colon cancer? See:

If you’re eating healthfully, do you need a colonoscopy? Find out in Should We All Get Colonoscopies Starting at Age 50?.

When regular starches are cooked and then cooled, some of the starch recrystallizes into resistant starch. For this reason, pasta salad can be healthier than hot pasta, and potato salad can be healthier than a baked potato. Find out more in my video Getting Starch to Take the Path of Most Resistance.

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: