– Dr. Michelle Arnold, Ruminant Extension Veterinarian, University of Kentucky Veterinary Diagnostic Lab
What is “BLV”? Bovine Leukemia Virus (BLV) is an “oncogenic retrovirus” common in cattle throughout the United States. “Oncogenic” means the virus can cause the infected animal to develop cancer. A “Retrovirus” is a unique type of virus that uses an enzyme to reverse its genetic code from RNA into DNA which then gets inserted into the host cell’s DNA and remains there for life. A well-known retrovirus in humans is the human immunodeficiency virus or “HIV” that causes the disease “Acquired Immune Deficiency Syndrome” or “AIDS”. Cattle infected with bovine leukemia virus have the disease known to veterinarians as “Enzootic Bovine Leukosis” or EBL, but it is most often referred to as “Leukosis”.
How common is BLV in beef cattle? Compared with dairy cattle, much less is known about BLV and beef cattle. A survey completed in 2019 of 28 cow-calf herds in the Midwest found at least one BLV-infected animal in 21 of those 28 herds and more than a third of the individual cows tested were positive. A similar study of bulls on 39 Midwest farms found nearly 50% of these operations had at least one positive bull and 45% of the 121 bulls tested were positive.
Why should BLV infection and leukosis be of concern when it is so common in cattle? Up until recently, the economic loss from leukosis was thought to be only due to death from cancer (lymphoma) or carcass condemnation at slaughter. However, like HIV in humans, now we understand the most important impact from BLV is disruption of the immune system that allows more diseases to occur, resulting in suboptimal performance and early culling. Because BLV indirectly allows other disease conditions to flourish, there has been delayed recognition of its importance to overall cattle health.
How is the virus transmitted? This is one area of knowledge that is rapidly changing with improved technology. Blood-borne transmission is the most recognized route, but it can occur in many ways. Traditionally, BLV was thought to spread primarily through biting flies and poor management procedures such as injections with used needles, surgical castration/dehorning/tattooing with bloody equipment, and rectal palpation with dirty sleeves. However, other modes of transmission are possible including, for example, blood exchange during natural service when there is penile or vaginal trauma, and calves may be infected during pregnancy, or blood exchange during calving or through colostrum or milk while nursing an infected dam. Although BLV can spread through management procedures, recent studies have shown that changing management practices does not dramatically decrease prevalence. Similarly, transmission by biting insect vectors such as horseflies was found to be important in some, but not all agricultural areas.
Perhaps as important as knowing how the virus is transmitted, is knowing which cows are most likely to transmit the virus to their herdmates. A minority of cows are referred to as “super shedders” and are thought to be responsible for most BLV transmission in a herd. A new quantitative polymerase chain reaction (qPCR) assay has recently been developed that measures the amount of virus in the blood sample called the “proviral load” or PVL (“Provirus” is the portion of viral genetic material that is found within the lymphocyte). PVL is a good indicator of infectivity; for example, a cow that is highly infectious may have a PVL = 0.5 or more, which a 0.5 means there is one BLV-infected lymphocyte out of every 2 in the sample. For more information, visit https://mycentralstar.com/diagnostic-services#1556487494253-1f97cd0c-b360 and click on BLV.
Do all cattle with leukosis develop cancer? Leukosis has 3 distinct stages: asymptomatic (no visible symptoms of disease), persistent lymphocytosis, and a cancerous stage (leukemia, lymphoma, and lymphosarcoma).
- “Persistent lymphocytosis” or “PL” is a condition diagnosed by a 3-fold increase in total lymphocyte count (a type of white blood cell) above normal, and that lasts for at least three months but with no cancer development. PL is thought to occur in approximately 1/3 of all BLV-infected cattle. Lymphocytes are one type of white blood cell vital in immune system function. Dairy cattle with PL have decreased milk production, increased culling rates and more cancers. Beef cattle are probably similar, but studies are lacking to make this claim.
- Cancers, including leukemia, lymphoma and malignant lymphosarcoma, are the most common neoplastic (cancerous) diseases identified in cattle slaughtered in the United States and the largest single reason cattle are condemned during postmortem inspection. However, less than 5% of BLV- infected cattle go on to develop cancer. Interestingly, approximately 1/3 of the cattle with lymphomas did not have excessively high lymphocytes (PL) so cattle do not have to go through a PL stage to develop cancer. Also, younger animals, generally < 2 years old, can develop sporadic forms of cancer (juvenile, thymic and cutaneous lymphoma) that are not caused by the bovine leukemia virus.
Figure 1: Lymphosarcoma in the kidney. Photo courtesy of the UKVDL
What does a cow with cancer look like? The end stage of Enzootic Bovine Leukosis is cancer, usually a malignant lymphosarcoma. Only a small fraction of BLV-infected cattle, < 5%, develop cancer after an average incubation period of 7 years after infection with the virus. The sites most affected by tumors are the heart, abomasum (true stomach), uterus, kidney (see Figure 1), spinal cord, and the area behind the eyeball. Early symptoms often include loss of appetite, weight loss, decreased milk production, and sometimes fever. The most common and usually earliest outward sign of cancer is enlarged lymph nodes which look like fist-size swellings in front of the shoulder, in the flank and other lymph node locations. Some cattle with cancer will show no sign of the disease and die suddenly.
Other symptoms depend on the organ affected by the tumor and can include:
- Tumors in the heart that can cause cardiovascular arrhythmias, jugular vein distension, rapid heart rate or heart murmurs. These tumors in the heart are often found in cases of sudden death where the cow is described as “absolutely normal” the day before she was found dead.
- Abomasal (stomach) tumors cause digestive problems resulting in loss of appetite and weight, constipation, or diarrhea, abomasal tympany (bloat), and slow gut motility.
- Retrobulbar (behind the eyeball) tumors can cause bulging eyes, blindness, and other ocular signs.
- Spinal tumors may cause rear limb weakness or paralysis or other neurologic signs due to tumor growth.
How is BLV diagnosed? Blood testing is the first step to identify BLV-positive (infected) animals. An inexpensive serum ELISA test is available at the UK Veterinary Diagnostic Laboratory to detect antibodies to the virus. Once an animal is infected and tests positive for antibodies, she will remain test positive for her lifetime. Testing can be done in animals over six months of age. Peripheral lymphocytosis can be diagnosed by a complete blood count (CBC) on a blood sample submitted to a veterinary practice or vet diagnostic lab. Tumors such as lymphosarcoma are usually diagnosed after death at necropsy but some are identified ante-mortem through a biopsy of an affected lymph node.
What should a producer do with a BLV ( +) animal? The decision on what to do with a positive animal is best determined through consultation with the herd veterinarian, based on the specific goals of the individual farm. Implementation of a BLV control program depends largely on a comparison of the cost of the disease and the cost of preventing disease. Economic losses stem directly from the inability to sell cattle for export or as bull studs, condemnation of carcass at slaughter if tumors are present, and clinical disease/death loss. Recent research has shown indirect losses through lost milk production realized in lower weaning weights, higher culling rates, and immune system dysfunction also need to be considered. Seed stock producers and especially those who export internationally are more likely to aggressively pursue BLV-free herd status. Disease control is based on testing and segregating or culling positive cattle, adding only BLV-negative cattle to the herd, and making management changes that eliminate contact with infected blood. Reducing transmission through single-use needles/sleeves and disinfecting equipment between animals and implementing an integrated pest management program will help reduce the risk of BLV and all other blood-borne diseases. There is evidence of genetic influence on susceptibility to BLV that may become another tool for removing BLV soon.
If I test my herd and a large percentage are BLV +, do I have to just sell them all and start over? The good news is that you can progressively reduce BLV transmission by removing the most infectious cattle first. A majority of antibody + cows (ELISA +) pose a low risk of transmitting BLV to their herdmates so by identifying and culling only the most infectious cattle first will greatly reduce new cases of the disease. Any cow that shows + on the inexpensive ELISA test (meaning she has antibodies) can undergo the new, more expensive qPCR test for proviral load then one can cull or segregate based off results. With this method, the number of BLV-infected cattle in the herd decreases quite rapidly to the point where the inexpensive ELISA blood test for antibodies can be used alone to make culling decisions until BLV is eradicated in the herd. Twenty countries in the world are BLV free now but they started with very low infection rates and therefore could test and remove all animals positive for BLV antibodies. The qPCR for proviral load is an opportunity for countries like the US to remove the most infectious cattle and move a step closer to being BLV-free.