by George Looby, DVM
The cause(s) of breast cancer in humans continues to elude researchers in their ongoing quest for answers to the many possible factors that may contribute to its development. This devastating disease has been recognized for many years with many of the world’s best and brightest minds searching for answers that will lead not just to a cure but more importantly to the cause. Here the question arises as to whether it is one disease or does it have many contributing factors which, when interacting, give rise to the condition that clinicians are trained to recognize.
Many of the well-recognized conditions that may contribute to the development of cancers of any type include alcoholism, tobacco use, obesity, diet and a sedentary life style. On the second tier of possible contributing causes is one that many readers may relate to, Bovine Leucosis.
Researchers in the Division of Infectious Diseases and Vaccinology at the U C Berkeley’s School of Public Health have come up with some very interesting findings. Lead investigator Gertrude Buehring and her co-workers decided to analyze breast tissue for the presence of Bovine Leukemia Virus (BLV). To date investigators have been able to attribute less than 10 percent of cancer cases to hereditary causes, leaving the remaining 90 percent unknown.
What led the research team to study BLV as a possible contributing factor in breast cancer can be traced to studies done in mice many years ago. It has long been known that the virus causing breast tumors in mice can be transmitted to their offspring via milk which prompted the team to take a look at milk to determine if it might be in any way implicated in the human breast cancer puzzle. The human drinks far more cow’s milk in a lifetime than it does human milk, which led the investigators to take a critical look at it.
Bovine Leukosis is a retrovirus found in both beef and dairy cattle that can cause malignant lymphoma and lymphosarcoma in 1 percent to 5 percent of infected animals in a herd. Clinical signs of the disease included enlarged lymph nodes, weight loss, poor appetite and decreased milk production. There is no treatment and no vaccine is available but a good diagnostic blood test is available to monitor the level of infection in a herd to allow owners to make appropriate management decisions.
To get an idea of the prevalence of this disease a USDA study was conducted in 2007 where bulk tank samples were taken from 534 operations each with more than 30 milking cows and tested using the Enzyme Linked Immunosorbent Assay (ELISA) test. This test has long been used to test for the presence of BLV antibiodies. This 2007 study showed that 83.9 percent of tested herds were positive for BLV and in those herds milking over 500 cows 100 percent of those herds were BLV positive. An old bacteriology professor might have suggested that the causative organism of Bovine Leukemia is ubiquitous in nature.
Given that the number of clinically affected animals in a herd may be small and a high percentage of the total herd positive on the blood test culling does not represent a viable option in most operations. Preventative measures can be implemented to control the spread of the disease using good sanitation as the best available method. The use of individual needles when doing injections of any kind is strongly recommended. Thoroughly sanitizing dehorning equipment and tattoo pliers between uses is advocated, no operator can be sure of what virus might be lurking on any given instrument.
Pasteurization temperatures destroy the BLV but not all dairy products are subject to that process and there is still a large volume of raw milk consumed in this country every day despite repeated warnings to the contrary. There are cracks in almost any preventative program that allow unwanted invaders to do their dirty work warnings to the contrary.
The way in which humans become infected with the BLV is not known but the incidence of the disease is higher in countries with high milk consumption although in one study the findings could find no relationship between breast cancer and the drinking of cow’s milk. Transmission between cow and calf occurs naturally. There is the possibility that given the long relationship between cattle and humans, over 2000 years according to some estimates, that the virus has long been established in the human population and reenters the population when certain triggering mechanisms are in place.
Dr. Buehring suggests that with little or no pressure from regulatory or consumer groups there has been little incentive for the cattle industry to institute measures to control the spread of the virus. Viruses have already been established as probable causes of various diseases and vaccines developed to control them. Examples include Hepatitis B virus, which can cause liver cancer, and the human papilloma virus, which can lead to cervical and anal cancers. Findings of BLV in breast tissue of humans does not provide conclusive evidence that it is a cause of breast cancer in humans either in whole or in part but additional work should provide supporting evidence if indeed it is.
Breast tissue to conduct the study was obtained from a supply bank maintained by the National Cancer Institute. Samples of breast tissue from 239 women were analyzed some from known breast cancer patients, others from cancer free patients and still others that were classified as precancerous. The samples were analyzed to determine the presence of DNA of BLV. It is important to note that samples were also subjected to other testing by other laboratories to serve as checks on the investigators at Berkeley.
The message that research reveals is that if a patient tests positive for BLV that person has a 3.1 times greater probability of having cancer than if it were not there. Researchers tend to be very conservative when discussing the results of their work so Dr. Buehring takes great pains to emphasize that this work does not prove that the presence of BLV DNA is the cause of breast cancer but should serve to stimulate further investigations along those lines.
All investigations of this importance require that the results of the findings be confirmed by at least one other group of investigators working independently of the original group. If indeed the work of Dr. Buehring and her co-workers can be repeated by another group their findings could have a major impact on one of the most important diseases of our time.