Do Poultry Viruses Cause Human Cancers?

The incidence of cancers has been rising for the last half century, and the question is why? Up to 20 percent of all cancers are caused by infectious agents, chiefly viruses. We’ve known this was possible for a century, when a cancer-causing virus was discovered in chickens. The idea was considered such heresy that Dr. Peyton Rous, the man who made this landmark discovery, wouldn’t get his Nobel Prize until 55 years later.

If there are cancer-causing chicken viruses, might they have any effect on people who handle or eat poultry? Concern has been raised about the potential infectivity of cancer-causing farm animal viruses for decades. The first question was whether there was any evidence of human exposure, and, indeed, people do have antibodies to these cancer-causing chicken viruses in their bloodstream. This indicates that the virus is no stranger to our immune systems. Is there any evidence, though, that the virus itself can get into our blood? There wasn’t any such evidence…until 2001.

As I explain in my video The Role of Poultry Viruses in Human Cancers, there is a cancer-causing herpesvirus in poultry, but does it pose a public health hazard? Researchers used DNA fingerprinting techniques to test the blood of 202 people and found that 20 percent, or one in five individuals, had viral DNA in their bloodstream. Testing positive for avian herpesvirus doesn’t mean these diseases can necessarily infect human cells, however. But, as it turns out, they can indeed.

But do they cause human disease? How can that be figured out? Since we can’t just inject people, researchers looked at poultry workers, which is the way we figured out how other farm animal diseases, such as brucellosis and anthrax, jumped to humans. In fact, studying workers is also how we discovered the carcinogenic nature of things like asbestos and benzene. If the poultry workers, who are exposed day in and day out, don’t have higher cancer rates, then presumably the viruses are harmless. Unfortunately, they do have higher rates. In fact, those with high exposure to cancer-causing poultry viruses have “increased risk of dying from several cancers.”

As such, “the relative ease” with which some of the viruses can infect human cells, as well as infect and cause tumors in primates in laboratories, “may be of public health significance, particularly because of the…increased risk of cancer in meat workers” and the evidence that we may become infected with these viruses. However, even if poultry workers are at risk, it doesn’t mean people who merely eat chicken or eggs are. For example, workers who kill chickens were found to be six times more likely to die from brain cancer compared to workers who do not kill poultry, but the slaughterers have live birds flapping in their faces. The “intensity of exposure to these viruses in the general population cannot be expected to be as high as those experienced by poultry workers…[but] the general population is nevertheless widely exposed” to the viruses simply because we eat so many chickens and eggs.

This is supported by data showing that it’s not only the factory farm workers who are at higher risk for brain tumors, but also butchers and meat cutters who have no exposure to live birds, particularly those who don’t wear gloves and frequently have cuts on their hands. These workers are at higher risk for other cancers, as well.

Those who handle meat for a living also have higher rates of non-cancer mortality, such as increased death from heart disease and other health concerns outlined at 3:32 in my video. Some of the poultry viruses not only cause cancer in chickens, but also atherosclerosis. Indeed, that cancer-causing poultry herpesvirus also triggers the buildup of cholesterol crystals in chickens. But, what about in people? “Because chickens infected with Marek disease virus, a herpesvirus, develop atherosclerotic lesions after infection, [researchers] looked for the presence of herpesvirus or parts thereof in human artery wall tissue…” Evidence of the virus was found, though any role they play in human heart disease remains speculative.

“Considerable attention has been paid to substances present in animal food before and after cooking as risk factors for human diseases such as heart disease, diabetes, and various cancers…[and] exposures have included heme [iron], fat or cholesterol, dioxins,” and the cooked meat carcinogens. We didn’t think, however, about the animal viruses, which “are important not only for supermarket workers and other workers in the meat and poultry industries, but also because the general population is exposed.” Indeed, the study that found chicken virus DNA circulating in people’s bloodstreams also found about the same rates in office workers as they did in chicken slaughterhouse workers, which you can see at 4:42 in my video.


Other viruses may actually play a role in the obesity epidemic. See, for example, Infectobesity: Adenovirus 36 and Childhood Obesity

For other potential microbiological hazards in poultry, check out:

And, for potential chemical hazards in poultry, see:

In health,
Michael Greger, M.D.

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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: