Bovine Leukemia Virus in Cows: Transmission, Testing, and Herd Control
- Bovine leukemia virus, or BLV, is a lifelong retrovirus infection of cattle that spreads mainly when infected blood cells move from one animal to another.
- Most infected cows never look sick, but about 29% develop persistent lymphocytosis and fewer than 5% go on to develop lymphosarcoma.
- Your vet may screen with serum or milk ELISA, confirm or clarify status with AGID or PCR, and help build a herd plan based on prevalence and goals.
- There is no treatment that clears BLV from an infected cow. Control focuses on testing, reducing blood transfer, and separating or culling positive animals when practical.
- Common herd-control steps include single-use needles, changing rectal sleeves, disinfecting dehorning and tattoo equipment, avoiding bloody milk for calves, and improving fly control.
What Is Bovine Leukemia Virus in Cows?
Bovine leukemia virus, or BLV, is a contagious retrovirus of cattle linked to enzootic bovine leukosis. After infection, the virus becomes lifelong. Many cows appear normal for years, which is one reason BLV can spread quietly through a herd.
BLV is not the same thing as cancer in every infected cow. Most infected cattle stay outwardly healthy. Merck Veterinary Manual notes that about 29% of infected cattle develop persistent lymphocytosis, while fewer than 5% develop lymphosarcoma, a cancer of lymphoid tissue. When tumors do form, signs depend on which organs are affected.
For pet parents and producers, the biggest challenge is often herd impact rather than dramatic early symptoms. BLV can affect culling decisions, slaughter value, replacement planning, and biosecurity routines. Your vet can help interpret what a positive test means for an individual cow and for the herd as a whole.
Symptoms of Bovine Leukemia Virus in Cows
- No visible signs
- Persistent lymphocytosis on bloodwork
- Enlarged lymph nodes
- Weight loss and declining body condition
- Reduced milk production
- Difficulty breathing, bloat, or jugular distention
- Rear limb weakness or paralysis
- Cutaneous plaques or firm skin masses
Many cows with BLV have no obvious symptoms, so herd testing matters even when animals look healthy. When signs do appear, they often reflect lymphosarcoma, not the early infection itself.
See your vet promptly if a cow has enlarged lymph nodes, unexplained weight loss, falling milk production, skin plaques, or repeated poor-doing. See your vet immediately for breathing trouble, severe bloat, marked weakness, or rear limb paralysis, because those signs can point to advanced disease or another urgent condition.
What Causes Bovine Leukemia Virus in Cows?
BLV spreads mainly through the transfer of infected lymphocytes in blood. That means the virus is most often moved during routine herd procedures that accidentally transfer tiny amounts of blood from one cow to another. Common risks include reusing needles, using the same palpation sleeve between cows, and sharing dehorning, tattooing, castration, implanting, or blood-collection equipment without proper cleaning and disinfection.
Transmission can also happen around calving and calf care. Merck notes that feeding bloody milk should never be done, and some herds consider milk replacer for calves as part of control planning. Colostrum decisions are more nuanced, because colostral antibodies may offer protection even in higher-prevalence herds. This is a good area to review with your vet before changing calf protocols.
Other routes appear less important but still matter in some settings. Fly control may help reduce risk, especially where biting flies are heavy. Natural breeding is usually a minor route unless breeding is traumatic or blood contamination is involved. Blood transfusions and biologic products containing blood are high-risk ways to spread BLV if donors are not screened.
How Is Bovine Leukemia Virus in Cows Diagnosed?
Diagnosis usually starts with herd screening, not symptoms alone. USDA APHIS states that AGID, ELISA, and PCR are all used to identify BLV-positive cattle. In practice, many herds begin with ELISA on serum or milk because it is practical and relatively low-cost for screening.
A positive antibody test shows exposure and infection status, but it does not prove a cow has lymphosarcoma. If your vet is concerned about cancer, they may also recommend a physical exam, complete blood count to look for persistent lymphocytosis, ultrasound or other imaging in selected cases, and biopsy or histopathology of suspicious masses or enlarged lymph nodes.
Testing costs vary by lab and region. Current U.S. veterinary diagnostic lab listings show BLV ELISA around $5.00 at Iowa State University and $7.00 at Cornell's Animal Health Diagnostic Center, before farm-call, sample collection, shipping, accession, and veterinary interpretation fees. For that reason, herd plans are usually built around both disease goals and budget.
Because BLV control is a herd issue, your vet may recommend testing all adults, testing replacement animals before entry, or using repeated screening over time. The best schedule depends on herd prevalence, whether the herd is dairy or beef, and whether the goal is risk reduction, segregation, or long-term eradication.
Treatment Options for Bovine Leukemia Virus in Cows
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Targeted testing of high-risk or newly purchased cattle
- Single-use needles for blood draws and IM injections
- Changing rectal sleeves between cows
- Cleaning and disinfecting dehorning, tattoo, castration, and implant equipment
- Avoiding bloody milk for calves
- Basic fly-control improvements
- Record-keeping to identify known positive animals
Recommended Standard Treatment
- Whole-herd or adult-herd screening with ELISA and selected confirmatory testing
- Veterinary herd-risk assessment and written biosecurity plan
- Segregation of positive and negative groups when facilities allow
- Testing replacements before entry
- Calf-management changes to reduce blood and milk-associated exposure
- Routine retesting at intervals recommended by your vet
- Culling decisions based on production, disease status, and herd goals
Advanced / Critical Care
- Repeated whole-herd testing with rapid follow-up on positives
- Aggressive test-and-cull or test-and-separate strategy
- Dedicated equipment and staff flow for negative groups
- Enhanced maternity, calf, and colostrum management protocols
- Consultation with herd veterinarian, diagnostic lab, and production advisors
- Necropsy or biopsy workups for suspect lymphosarcoma cases
- Replacement planning to rebuild a lower-prevalence herd
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Bovine Leukemia Virus in Cows
Bring these questions to your vet appointment to get the most out of your visit.
- Which BLV test makes the most sense for my herd right now: milk ELISA, serum ELISA, AGID, or PCR?
- Based on our herd size and setup, should our goal be risk reduction, segregation, or long-term eradication?
- Which routine procedures on our farm are most likely to transfer blood between cows?
- How often should we retest the herd, and which age groups should be included each round?
- What should we do with BLV-positive cows that are still producing well?
- How should we manage calves, colostrum, waste milk, and any bloody milk to lower transmission risk?
- Do we have enough facility separation to manage positive and negative groups safely?
- If a cow has enlarged lymph nodes or weight loss, what tests do we need to check for lymphosarcoma or other causes?
How to Prevent Bovine Leukemia Virus in Cows
Prevention centers on one big idea: stop blood transfer between cattle. Merck Veterinary Manual describes this as the cornerstone of BLV prevention. Practical steps include using single-use needles, changing rectal sleeves between cows, and thoroughly cleaning and disinfecting any equipment that may contact blood or tissue.
Procedure choices matter. Bloodless dehorning methods can help reduce exposure. Equipment used for dehorning, castration, tattooing, implanting, and blood collection should be cleaned between animals. Handling areas, maternity pens, and calving spaces should also be cleaned when contaminated with blood or body fluids.
Calf management is another key piece. Do not feed bloody milk to calves. Depending on herd prevalence and your goals, your vet may discuss colostrum sourcing, milk replacer use, and testing of replacement animals before they join the herd. Artificial insemination and careful donor screening for any blood products can also reduce risk.
There is no single prevention plan that fits every farm. A high-prevalence dairy herd may need a gradual control program, while a lower-prevalence herd may be able to pursue test-and-remove more aggressively. Your vet can help match the plan to your facilities, labor, and long-term herd goals.