Decades ago, concern was raised that the milk of dairy cows frequently contains a leukemia-causing virus. But they were talking about bovine leukemia, the leading cancer killer among dairy cattle. Most U.S. dairy herds are infected with the bovine leukemia virus; thus, the question of whether they’re releasing infectious virus into milk is an important public health consideration.
So, 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 bovine leukemia virus.
Just because we’re exposed doesn’t mean it’s causing human disease. How do we know bovine leukemia virus can even infect human cells? We didn’t, until 1976, when it was discovered that bovine leukemia virus can indeed infect human, chimpanzee, and monkey cells. Okay, but that still doesn’t mean it necessarily causes cancer in other species.
You can’t lock human infants in a cage, and feed them infected milk. But, you can cage infant chimpanzees, and chimps Bwah and Roger—fed the infected milk—developed leukemia and died. 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—or at least impugn the safety of steaks—but, human beings are not chimpanzees. Yes, our DNA may be 98% identical, but we may share 60% of our DNA with a banana. We need human studies.
Thanks to the pesky Nuremberg principles, we can’t do interventional trials. But, what about observational studies? Do cattle farmers have higher rates of cancer? Apparently so—leading some to suggest that milk- and egg-borne viruses may be important in the development of human leukemias and lymphomas.
But, farmers may be exposed to all sorts of potential carcinogens, such as pesticides. It’s like, yes, large animal vets have more leukemia and lymphoma, but some were also particularly lax in the use of X-ray protective equipment. So, it didn’t necessarily have anything to do with viruses.
What we needed were so-called serology studies—testing people’s blood for antibodies against the virus, which would prove human exposure—and we got ‘em. Ten different studies looking for bovine leukemia virus antibodies in cancer patients and non-cancer patients; creamery employees vs. office employees; veterinarians; unpasteurized milk drinkers; the whole gamut—and not a single study found a single individual with antibodies to BLV.
So, 1981, case closed; strong evidence BLV was not transmissible to people. But, the strength of the evidence is only as strong as the strength of the test. Bwah and Roger didn’t develop detectable antibodies either—and they died from it.
The tests back then were just not really sensitive. Clearly, the question of whether bovine leukemia virus poses a public health hazard still deserves a thorough investigation, using newer, highly sensitive molecular probes. Until then, it’s probably wise to avoid factory farmed milk. More here on the role of bovine leukemia virus and its role in breast cancer.
Sources cited in Dr. Gerger’s article:
- G C Buehring, S M Philpott, K Y Choi. Humans have antibodies reactive with Bovine leukemia virus. AIDS Res Hum Retroviruses. 2003 Dec;19(12):1105-13.
- M J Burridge. The zoonotic potential of bovine leukemia virus. Vet Res Commun. 1981 Dec;5(2):117-26.
- H M McClure, M E Keeling, R P Custer, R R Marshak, D A Abt, J F Ferrer. Erythroleukemia in two infant chimpanzees fed milk from cows naturally infected with the bovine C-type virus. Cancer Res. 1974 Oct;34(10):2745-57.
- D C Graves, J F Ferrer. In vitro transmission and propagation of the bovine leukemia virus in monolayer cell cultures. Cancer Res. 1976 Nov;36(11 Pt 1):4152-9.
- J F Ferrer, S J Kenyon, P Gupta. Milk of dairy cows frequently contains a leukemogenic virus. Science. 1981 Aug 28;213(4511):1014-6.
- K J Donham, J W Berg, R S Sawin. Epidemiologic relationships of the bovine population and human leukemia in Iowa. Am J Epidemiol. 1980 Jul;112(1):80-92.
- H M Lemon. Food-borne viruses and malignant hemopoietic diseases. Bacteriol Rev. 1964 Dec;28:490-2.
- A Blair, H M Hayes Jr. Cancer and other causes of death among U.S. veterinarians, 1966-1977. Int J Cancer. 1980 Feb 15;25(2):181-5.
- A Blair, T L Thomas. Leukemia among Nebraska farmers: a death certificate study. Am J Epidemiol. 1979 Sep;110(3):264-73.
- National Human Genome Research Institute. 2010 National DNA Day Online Chatroom Transcript
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