Bovine Leukosis Virus
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Bovine leukosis virus, more commonly referred to as BLV, is a silent problem in Southeastern cattle herds. This destructive pathogen can go unnoticed for a long time and quietly rob cattle herds of their productivity. BLV is a member of the same family of viruses as feline leukemia virus (FLV) and a distant cousin of Human Immunodeficiency Virus (HIV). As such, there is interest in the disease as it may relate to human health. To date, there is not conclusive evidence that the virus poses any detrimental threat to handlers nor people consuming products from infected animals. A 2015 study did attempt to link certain types of breast cancer in women to BLV. Further research is being conducted on this front.
According to a small-scale survey study conducted in 1997, approximately 40% of beef cattle responding operations tested positive for BLV infection and of the cattle associated with these operations about 10% tested positive for the virus. The numbers gathered in this study were too small to generate national averages, however. This may seem low in perspective to other infectious pathogens of cattle. Unfortunately, infection rates are significantly higher in the Southeastern and South-Central regions of the country. According to data from the same study mentioned before, 60% of farms and ranches in our area potentially have infected cattle and approximately 20% of individual cows are affected.
BLV positive cattle for the most part will not be clinically affected by disease resultant of their infection. For beef cattle, we as a research community have not accurately quantified total economic losses from infection. In the dairy industry, BLV losses have been documented in the literature. With a 50% infection rate, it is estimated that a milking herd will generate $6400 less per 100 head than a herd without BLV. A cow infected with BLV will produce 3% less milk than her uninfected cohort. Losses aren’t likely that dramatic for the average infected beef cow, but my suspicion is they do exist in the form of lowered immune responses, fertility rates, and environmental resilience. On-going research is being conducted within the joint NCDA and NCSU Beef Cattle Research System to provide data to producers affected by this disease.
Bovine Leukosis Virus infects the cow’s white blood cells and incorporates its DNA into the DNA of the cell. This allows it to evade the cow’s immune system rather effectively and go about its business without interruption. 30% of infected cattle develop a persistent leukocytosis, or an abnormal elevation in white blood cell (lymphocytes) counts in the blood. This is often called leukemia, although it is a different non-lethal form than what would be seen in human blood cancers. Leukemic cattle do not often have clinical disease, but it could noticed by your veterinarian if the cow were being evaluated for other unrelated disease.
Although most cattle do not develop clinical disease, some do. Up to 5% of infected cattle (and not just the leukemic cattle) will develop lymphosarcoma, a malignant tumor-forming cancer that can cause premature losses of productive cows or bulls. This typically is a condition of mature cattle (5 years or more), but I have seen animals as young as 15-18 months develop lymphosarcoma. This disease can lead to carcasses being condemned at slaughter or deaths on the farm. There is also a juvenile form of the disease as well, but it is relatively uncommon. Tumors usually form in lymph node centers, the heart, the stomach, the uterus, or the spinal cord of infected cows. Where the tumors develop determines the course of disease experienced; tumors in the spinal cord lead to down cows while tumors in the stomach lead to progressive weight loss. In my experience, tumors hardly ever come alone, therefore the typical cow presents with a variety of symptoms. The typical presentation I see is a cow getting slow or falling behind the herd, progressively losing weight, or “the creeper” – a down cow that pulls herself around and acts like she has a lot of life but can’t get up. For the record, the infection doesn’t always read the book and can show up in unexpected places like the brain, the eye, or other organs in the body. Lymphosarcoma often can seem very fast-onset as tumors can dramatically increase in size during periods of extreme stress like calving, environmental adversity (heat stress, cold stress), or concurrent disease.
In 2009, a single case of lymphosarcoma in a dairy cow was estimated to cost a producer $400-600. Loss of a beef cow and the calf she would produce in today’s market comes to about $1300 by my figuring, plus the maintenance costs she has incurred between weaning her last calf and time of death. This does not factor in any veterinary or disposal costs, should those also be incurred. In the low margin business that is the cow-calf business, these costs should be considered significant. If you have 50 cows with 20% prevalence, you could expect to lose 1 cow every 2 years. If you have 50 cows with 80% prevalence because you reuse needles, you could expect to lose 2 cows per year.
BLV spreads by blood contact between animals. Most often, this would occur by our doing with contaminated processing equipment such as taggers, reused needles, palpation sleeves, or castration knives. Biting insects, particularly horn flies and horse flies, also transmit the virus, and these vectors are probably responsible for the higher prevalence of infection in Southern cattle operations. There is evidence that BLV can be spread from an infected cow to her calf through colostrum, although not as effectively as through blood contamination. Non-commercially sourced semen can also spread the infection between animals. Please note that all modern commercial studs now require bulls be BLV negative to prevent transmission of the disease through purchased semen.
There is no cure for BLV infection. Once a cow is infected, she will harbor the virus for life and potentially spread the virus to other animals. Calves that nurse from a positive dam can get some protection from antibodies present in the colostrum against infection, but often will be exposed and infected later in life after colostral protection is gone. There is also no vaccine for BLV. Good biosecurity protocols are a producer’s best bet for dealing with this virus. Here are some practical tips:
- Test new purchases prior to arrival to farm. If not possible, quarantine additions for 45-60 days and test at end of quarantine. Testing costs around $2 per head and can be accomplished with a single blood sample.
- Consider annual testing of animals in the herd. Animals may be infected early in life but not show up positive on a blood test for several years. To accurately identify carriers in the herd, you must retest previously negative animals annually.
- Use only disposable needles and do not reuse syringes that have any blood contamination.
- Ask your veterinarian or AI technician to change sleeves and to disinfect equipment between cows. Hold yourself to the same standards.
- Do not share equipment or animals between farms.
- Develop effective insect control programs. Discuss management plans with neighbors as well.
- Develop a strategic plan with your veterinarian on how to practically deal with positive animals.
- Do not use positive cows as embryo transfer recipients.
- Segregate positive cows from negative cows as stringently as possible on farm.
- Perform veterinary procedures on BLV positive cows last.
Consider what BLV infection means for your farm. For purebred breeders, control may be a more significant concern than for commercial calf operations. Control and eradication of BLV is difficult and can be frustrating. However, given the economic and welfare drain that having BLV in the herd presents, I believe producers should have a conscious and proactive plan for dealing with the infection rate within the herd. While eradication may not be the goal, limiting spread within the herd should be a top priority for all managers.