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Bovine Leukosis Virus, also sometimes referred to as “Bovine Leukemia Virus” or BLV has been recognized as a cancer-causing virus in cattle for over 4 decades. Despite widespread distribution in the United States, the virus usually has a minor economic impact on commercial dairies and has thus been ignored while attention is focused on more obvious issues such as lameness, mastitis, breeding problems, and infectious diseases such as BVD (Bovine Virus Diarrhea) or Johne’s disease (Mycobacterium avium subsp. paratuberculosis). However, with a recent report that BLV could be associated with breast cancer in women, we may no longer have the luxury of treating BLV as a virus confined just to animals. Now we may be forced to deal with it as a public health concern.
What is BLV?
BLV is a member of the retrovirus family of viruses, a close relative to cancer-causing viruses in other species (eg. Avian Leukosis, Feline Leukemia) and a distant cousin to the Human Immunodeficiency Virus. (HIV). The virus is carried in the white blood cells of infected cattle, and once a cow or calf has become infected she will remain infected for the rest of her life.
How is BLV Spread?
A single drop of blood from an infected cow contains enough virus to infect a susceptible herdmate. Since infected cows carry the virus in their white blood cells, contact with blood or other body fluids that contain white blood cells can spread the virus. For instance, biting insects can transmit virus to an uninfected cow by biting her after feeding on an infected cow. People have also been responsible for spreading the infection on some dairies. Common husbandry practices such as surgical dehorning, castration, and tattooing have the potential to spread virus if instruments contaminated with blood are not properly disinfected between animals. Using the same needle on several animals, as during herd vaccination, can carry a small drop of blood from an infected cow to a susceptible cow. The practice of not changing rectal exam sleeves between pregnancy examinations on infected and uninfected cows has also been shown to spread the infection. White blood cells laden with BLV are also found in nasal secretions, colostrum, milk, and semen, so these can also transmit the virus from infected to susceptible individuals.
What Happens When a Cow Is Infected with BLV?
When the virus infects a susceptible cow she will remain infected with the virus, and potentially infectious to other cattle, for the rest of her life. There is no treatment to eliminate the virus from an infected cow and there is no vaccine to prevent infection. Within weeks after becoming infected, the cow will produce antibodies which can be detected in blood or milk. However, in the majority of cases no other clinical effects will be observed; the infected cow will remain outwardly normal for the rest of her life. A very small percentage (generally 2%-3%) of BLV-infected cows will be stricken with cancer caused by the virus after a long incubation period of several years. The cancer caused by BLV is referred to as “Enzootic Bovine Lymphosarcoma”, which is a cancerous tumor affecting lymph nodes, the abomasum, heart, uterus, spinal cord, or other internal organs of the cow. It does not usually result in cancerous cells in the bloodstream, so is not truly a “leukemia” as seen in people or other animals. Those few unfortunate cows that go on to develop lymphosarcoma will have clinical signs that depend on the site of tumor development. In the beginning there may be vague signs such as reduced appetite, a drop in milk production, weight loss, and reduced activity. Other signs specific to the different tumor locations include:
- digested blood in the stool due to abomasal bleeding
- edema and other signs of heart failure
- hind leg paralysis
The disease is ultimately fatal, and if the cow is culled she will be rejected for human consumption at the processing plant.
How Do We Know If a Cow Is Infected, and How Widespread Is BLV?
Since all infected cows produce antibodies in their blood, infection is easily diagnosed by an ELISA test—a rapid technology for detecting antibodies. It has recently been shown that antibodies are also found in the milk of infected cows, so cows can easily be screened by a milk ELISA available through some DHIA organizations. The test has excellent sensitivity (detects 98% of infected cows) and specificity (less than 1% false-positives from uninfected cows). Remember, the test only tells if a cow is infected with a virus, not if she has cancer. The vast majority (about 97%) of cows infected with BLV never develop lymphosarcoma.
In US dairies, BLV is virtually everywhere. Studies showed that at least 89% of all US dairies have infected cows, with an even higher percentage in the Southeast.1 On an individual cow level, approximately half of all US dairy cows are infected with the virus.
What Is the Economic Impact of BLV?
Infection with BLV can impact the bottom line of a dairy in several ways. For producers who wish to sell genetic stock for export or bulls for stud, BLV testing is required and BLV+ animals are rejected even if they are clinically healthy. In commercial herds not interested in selling genetic stock, there can still be financial losses associated with reduced production and culling or death of cows due to lymphosarcoma. Even though only a small percentage of BLV+ cows go on to develop cancer, these losses can begin to add up, particularly in herds with a high (greater than 50%) prevalence of infection.
What about cows that are infected with the virus, but don’t have cancer and are outwardly healthy? Does BLV in those outwardly healthy cows subject them to reduced resistance against other diseases, or does it affect their production? There is conflicting information on this subject. Many studies showed that BLV infection has no effect on milk production, breeding efficiency, or resistance to common diseases. However, some recent studies did show a possible 2-3% reduction in milk production in herds with BLV, with production affected more in the herds with the highest prevalence.2 In summary, producers with a high prevalence of BLV infection in their herd (greater than 50%) might expect to see some limited production and culling losses associated with BLV, but probably not as significant as mastitis, lameness, or Johne’s disease.
What About People?
Until very recently, BLV has been regarded as a “cows only” problem. However, sweeping changes in the way we think about BLV could be in store based on some recent research results from the University of California-Berkeley published in September 2015.3 In this study, BLV was found in mammary tissue samples from women with breast cancer at a significantly higher rate than from non-cancerous breast tissue samples. The association between breast cancer and BLV virus was very strong, and the authors determined that although breast cancer has many causes including inherited susceptibility, 37% of breast cancer cases could be attributed to BLV exposure. The authors acknowledge that it is not known how these women may have become infected. We know that BLV is present in cows’ milk, but the virus is destroyed by the pasteurization process and previous studies did not show an association between raw milk consumption and breast cancer. Thus the route of exposure remains a mystery.
It is important for the U.S. dairy industry to take notice of these results. Many producers have chosen to live with BLV in their herds, and have not taken steps to control it because the impact on production is minimal. We now need to take a hard look at how future studies involving BLV infection and humans could affect consumer confidence in the U.S. dairy industry.
How Can I Control BLV in My Herd?
The first step in controlling BLV is to determine whether one’s herd is infected, and, if so, how extensively. Producers should discuss a testing strategy with their veterinarian, but testing all of the cows with a serum or milk ELISA will likely give a good assessment of the herd’s status.
If the herd is one of the very few in the United States that is not infected with BLV, then the objective is to remain free of infection and the recommendations are straightforward. Maintaining a closed herd will prevent introduction of BLV through addition of BLV+ purchases. If purchasing herd replacements is necessary, they should be screened by an ELISA test and confirmed negative for BLV before being admitted to the premises.
However, most producers will not be so lucky and will be amongst the 89% of infected herds. The specific control program will depend on the goals of the producer and may also be dictated by the extent of the problem. For instance, a herd with only a few infected animals might wish to adopt a test-and-cull strategy to achieve BLV-free status. Unfortunately, for most herds more than 25% of the cows will be infected and eradication will not be a realistic objective in the near future. For these herds, reasons for implementing a control program might include:
- Raise BLV-negative heifers for domestic or international sale.
- Reduce prevalence of BLV infection to decrease the chances of losing cows to lymphosarcoma.
- Reduce BLV prevalence to minimize any losses due to decreased milk production
- Reduce the infection prevalence to a level that will make eradication more realistic if it becomes mandated in the future due to a link with human disease
Regardless of the goal, the foundation of a control strategy is to reduce transmission of BLV to susceptible calves and keep them separated from likely-infected adult cows. As BLV-negative heifers mature and enter the herd, they will be replacing infected cows and lowering the overall prevalence.
Controlling the spread of BLV infection requires using our understanding of the various ways the virus is transmitted and taking steps to block that transmission from happening. Steps you can take to prevent BLV transmission begin right when the calf is born and include:
- Separate the calf from its dam as soon as possible after birth, before it has a chance to stand and attempt to nurse.
- Freeze or heat-treat colostrum (low temperature pasteurization, 145oF for 30 minutes) to destroy BLV without damaging important antibodies necessary for calf health.
- House calves apart from adult cattle.
- Dehorning, tattooing, and other instruments should be washed thoroughly with a disinfectant soap such as chlorhexidine between each use to remove BLV-contaminated blood
- Use separate sterile hypodermic needles for each animal.
- Use a clean rectal exam sleeve for each animal, or at least change the sleeve after use on known-positive animals if they have been identified by testing. BLV can be transmitted by a rectal exam sleeve even if there is no blood visible on the sleeve.
Steps such as these have been shown to result in marked improvement in BLV prevalence in a herd, for example from 50% of the animals infected to 15%, within 2-3 years. A more aggressive approach would include segregating BLV-positive animals into separate housing, milking them separately, etc. However this approach would not be practical on many farms. By taking the above steps now and reducing the BLV prevalence to a manageable level, total eradication of BLV from the herd will be more easily achieved should public health concerns make eradication necessary.
Bovine Leukosis Virus is widespread in United States dairies. It causes the cancerous condition lymphosarcoma in a small percentage of infected cattle, and may cause some financial loss through reduced milk production. The virus has recently been linked to breast cancer in women, which should alert the dairy industry to begin taking steps to reduce prevalence of this virus in dairies. Taking steps to reduce prevalence of BLV now will reduce the economic impact on farms, and will also serve to make future eradication more realistic if it becomes necessary.
Raymond W. Sweeney, VMD
Professor of Medicine, Chief Section of Medicine and Ophthalmology
University of Pennsylvania School of Veterinary Medicine
1. USDA-APHIS, 1997. High prevalence of BLV in U.S. Dairy Herds. NAHMS Dairy ’96. https://www.aphis.usda.gov/animal_health/nahms/dairy/downloads/dairy96/D…
2. Ott, SL et al. 2003. Association between bovine-leukosis virus seroprevalence and herd-level productivity on US dairy farms. Preventive Veterinary Medicine; 61:249-262.
3. Buehring, GC et al. 2015. Exposure to bovine leukemia virus is associated with breast cancer; a case control study. PLoS ONE 10(9): e0134304. doi: 10.1371/journal.pone.0134304