Many people think that cancer is only an adult disease. Nothing could be further from the truth. Cancer is the second leading cause of death among U.S. children.115 Only accidents claim more young lives. Leukemia, a cancer of the blood and blood forming tissues, is the leading cause of childhood cancer deaths in our country, as stated in Figure 19: Cancer in Children.116
Why is this of special concern in a chapter on diseases in animals? Because there is another disease that is rampant in the American cattle population. This disease is bovine leukemia virus (BLV). The possible connection of leukemia in children with this cattle disease is explained in Figure 20: Potential Danger of the Bovine Leukemia Virus.117
Notice, as with BSE, there is a direct connection to cattle. Bovine leukemia virus (also called the Bovine Leukosis Virus or BLV) refers, of course, to a form of leukemia occurring in cattle. Note how prevalent this virus is in American dairy herds. Beef cattle usually have a lower rate of infection. Nonetheless, at any time, roughly 20 percent of the U.S. cattle population is infected.118 In addition to leukemia, this virus can cause bovine lymphosarcoma, a cancer of the lymph tissues in cattle.
Note that the leukemia-infected cows themselves are not the only ones at risk. The virus passes into the milk.119 This milk appears to have the potential to cause disease in animals of other species who drink the infected beverage. BLV-contaminated milk that is unpasteurized, when fed to sheep and chimpanzees, has been linked to serious diseases, as shown in Figure 21: BLV Induces Tumors.120, 121
Still other species have been shown to be susceptible to BLV: white tail deer, pig, the domestic rabbit, and cat.122 Ferrer and associates cited evidence years ago that in the laboratory, human cells also become infected with the bovine leukemia virus.123 Pasteurization kills the virus,124, 125 but we have already seen that milk can become tainted due to contamination after the pasteurization process or because of mechanical problems with pasteurization equipment. Outbreaks of human infections from pasteurized milk due to germs like E. Coli, salmonella, and Yersinia all provide examples of pasteurization's shortcomings. In short, although pasteurization is a useful procedure that has increased milk safety, it by no means sterilizes dairy products. Even after pasteurization, milk is actually teeming with viruses and bacteria. Chapter 11 entitled "Milk, Friend or Foe?" has more information on this subject.
Ultimately we ask ourselves, is BLV a threat to human health? The answer so far is that we do not know. Although a single study showed more cases of human leukemias in areas that had more cattle,126 other studies reviewed by Dr. Reginald Johnson of the U.S. Animal and Plant Health Inspection Service found no relationship between BLV and human disease. It must be pointed out, however, that bovine leukemia virus is very similar to the human T-cell leukemia virus Type I (HTLV-1).127 Both are in the family of retroviruses that also include HIV. The ability of this family of viruses to infect cells and then cause disease years later is of particular concern. Consequently, the propensity for human risk seems to be present although I am currently unaware of any definite human threat.
In short, BLV may or may not be causing human health problems. However, the BSE story provides an eloquent example of how an infectious disease may appear at first to present no human risk only to turn out later to be a serious threat. The fact that BSE and BLV can both infect a wide variety of species is worthy of a final note. It seems prudent to avoid any germ that can cross species barriers and cause life-threatening illnesses in unrelated creatures.
