Have Millions of Americans Been Infected with a Cow Cancer Virus?
by Michael Greger, MD
Lost in the recent media flurry over Mad Cow disease, a provocative study was released in the latest issue of AIDS Research and Human Retroviruses. Researchers at the University of California, Berkeley found that a significant proportion of the American public may be harboring antibodies to Bovine Leukemia Virus, which they may have been exposed to through the consumption of beef or dairy products.
Bovine Leukemia Virus (BLV) is a retrovirus that causes leukemia in cattle. BLV infected cells (lymphocyets) circulate through the blood of cattle and are present in both beef and milk. As soon as BLV was isolated in 1969 there were efforts to determine whether humans were infected with the virus. Using the best tests available at the time, researchers weren't able to detect antibodies to the virus in the bloodstream of any human tested and so the USDA had concluded that, "BLV is not transmissible to humans."
Compared to modern testing techniques, though, the tests they were using in the 1970's were extremely insensitive. Realizing that back then, researchers were even missing up to 70% of the positive bovine cases, Dr. Gertrude Case Buehring anf collegues at the University of California School of Public Health in Berkeley wondered whether the newer, more advanced testing methods would be able to detect antibodies against BLV in human blood after all. So she took blood samples from 257 people, mostly women, and tested them. Late last month her results were published.
The study was designed to see if any humans at all had antibodies to BLV. Dr. Buehring's group found antibodies reactive specifically against BLV in 191 out of the 257 people studied. An amazing 74% of the human subjects tested positive. Extrapolating this finding to the general population, hundreds of millions of Americans may have been similarly exposed to Bovine Leukemia Virus.
How did these people get exposed to this cattle virus? Less than 10% had any direct contact with live bovines. Therefore, the most likely explanation was due to their direct contact with dead ones (through beef) or through live cow secretions (dairy products).
What exactly does it mean that people have antibodies against the virus? Typically, the presence of antibodies to a particular virus indicates either past or present infection with that virus. Dr. Buehring concedes, though, that the presence of antibodies may just be an immune response to consuming dead virus, killed by the sufficient cooking or pasteurization of infected meat or milk . At this point we don't have enough data to distinguish between the two possibilities--active human infection from Bovine Leukemia Virus in unpasteurized milk or undercooked meat, or merely exposure to killed BLV.
If people can acquire active BLV infections, what consequences might that have for their health? Public health advocates often point to a study done 30 years ago in which 6 baby chimpanzees were fed unpasteurized milk from BLV infected cows. The two infant chimps fed the most infected milk were dead within a year of leukemia.
Of course just because other primates become infected and die of leukemia drinking infected milk, this doesn't mean that humans will. Unfortunately, studies on human cell lines shows that indeed human cells can also become infected with the BLV virus. This still doesn't necessarily mean that BLV can cause human disease, though. Researchers needed to start studying human populations to see if they could detect any association between BLV and human cancer rates.
Researchers found a disturbing trend. They found that geographically, the areas of highest cattle BLV infection did indeed seem to have significantly higher human leukemia rates. For example, in a study in Iowa, the counties reporting the highest rates of cattle BLV infection seemed to also have the highest rates of human acute lymphoid leukemia (ALL), which typically strikes children. Other studies have found that milk consumption, for example, was associated with the development of malignant lymphomas.
BLV currently infects the majority of dairy and beef herds in the United States and Canada. In fact, an estimated 89% of the dairy herds in the U.S. harbor infected animals. Europe, on the other hand, has very few infected animals. This may be, in part, because some of the ways in which young calves are treated in this country have been deemed too cruel and banned in Europe. Instruments used in these procedures in the U.S.--ear taggers, nose tongs, hoof knives, tattoo pliers, the needles on hormone implant guns--may be inadequately sterilized and therefore may spread the blood-borne virus.
Take, for example, dehorning. The saws and gougers used in dehorning young calves cause so much bleeding and are so hard to clean that they are particularly likely to drive potentially infected blood into the next animal in line. Dehorning is done in part to minimize injuries to handlers and to reduce bruising during transport. To minimize the risk of transmission of blood-borne pathogens like BLV, the industry is moving away from gougers and more towards red-hot electric irons or caustic paste to burn off the horn button.
A modern Dairy Management textbook explains the use of the electric iron: "lie the calf on its side and put your knee on the neck... The dehorner has to be left on the button for approximately 5-20 seconds. The time will seem longer, because of the combined unpleasantness of burning hair and a struggling calf...dehorning may be complete... when you hear a squeaking sound as the dehorner is twisted. It is the sound of the dehorner tip rubbing against the bone of the skull."
Surgical castration--in which the lower third of the calf's scrotum is cut off and the testicles are grabbed and literally just ripped out--is another opportunity for blood-stained instruments to spread infection. Both dehorning and castration are done in calves just 2-3 weeks old without any anesthetic. The USDA notes that although "Animal welfarists criticize the failure to administer anesthetics... not using anesthetics for those relatively simple procedures greatly reduces the complications caused by anesthetics...."
Other examples of practices that can spread BLV infection--done to modify the animal to fit the system, rather than adapt the system to accommodate the animal--are tail docking, which reduces the "chance of the farmer getting swatted in the face," and "extra" teat removal to "improve udder appearance."[19 ]
Tail docking is becoming more widely accepted by dairy farmers. Although one dairy textbook notes that "Some people object to the docking of tails for humane reasons. In fact, the practice of docking tails is banned in Great Britain. Others see no difference between docking cattle compared to docking the tails of pigs and sheep or the dehorning of cattle."
Removing "extra" teats is another unnecessary procedure that can expose calves to infection. Up to half of all female calves are born with "extra" teats on their udders. Dairy practitioners are taught that "Extra teats should be removed for cosmetic reasons... " The Holstein Foundation explains "Extra teats on an udder are unsightly..." The "extra" teats may also interfere with proper placement of the milking machinery.
A Dairy textbook explains the procedure: "extra teats detract from an udder's general appearance. Grasp the teat between your thumb and forefinger. Even in small calves, the nerve supply to their teats are well developed. Make sure the calf is well restrained before you proceed. Pull the teat outwards and take a generous bite with the scissors." Slicing these nipples off of young calves is also done without anesthetic.
By continuing these unnecessary practices, we may be increase the risk of spreading this disease throughout the U.S. cattle herd and by extension, into the human population. The December study in AIDS Research and Human Retroviruses concludes, "The long-held assumption that BLV is not a public health hazard was based on the failure of experiments in the 1970s to detect human antibodies to BLV. That assumption is no longer tenable in view of our demonstration of humans seropositive for BLV-reactive antibodies."
REFERENCES (Bovine Leukemia Virus): Please note many of these links no longer work due to elapased time.
1 AIDS Research and Human Retroviruses 19(2003):1105.
2 Science 213(1981):1014.
4 Journal of Virology Methods 104(2002):33.
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6 Cancer Research 36(1976):4152.
7 American Journal of Epidemiology 112(1980):80.
8 British Journal of Cancer 61(1990):454.
9 Veterinary Research 25(1994):521.
10 Working with Dairy Cattle. http://www.holsteinfoundation.org/pdffile/f1325.pdf
11 DairyBiz February 1999. http://www.moomilk.com/archive/a_health_29.htm
15 Practical Techniques for Dairy Farmers. 3rd Ed. University of Minnesota, 2000. http://www.ansci.umn.edu/practical-techniques/book.htm
16 Practical Techniques for Dairy Farmers. 3rd Ed. University of Minnesota, 2000. http://www.ansci.umn.edu/practical-techniques/book.htm
17 Arave, CW and JL Albright. "Animal Welfare Issues: Dairy." AWIC Newsletter 9(1998):3-10. The Animal Welfare Information Center is part of the USDA Agriculture Research Service.
18 Practical Techniques for Dairy Farmers. 3rd Ed. University of Minnesota, 2000. http://www.ansci.umn.edu/practical-techniques/book.htm
19 Dairy Connection December 1999. http://www.ag.ndsu.nodak.edu/aginfo/dairy/dairyext/dc9-4.htm
20 Practical Techniques for Dairy Farmers. 3rd Ed. University of Minnesota, 2000. http://www.ansci.umn.edu/practical-techniques/book.htm
21 Management of Dairy Heifers. http://www.das.psu.edu/dcn/CALFMGT/pdf/385b.pdf .
22 Dairy Connection December 1999. http://www.ag.ndsu.nodak.edu/aginfo/dairy/dairyext/dc9-4.htm
23 Practical Techniques for Dairy Farmers. 3rd Ed. University of Minnesota, 2000. http://www.ansci.umn.edu/practical-techniques/book.htm
24 Working with Dairy Cattle. http://www.holsteinfoundation.org/pdffile/f1325.pdf
25 AIDS Research and Human Retroviruses 19(2003):1105.
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