Bovine Leukemia Virus
BLV is quietly spreading within the U.S. cattle industry — already embedded in both dairy and beef herds nationwide.
The virus affects roughly 90% of dairy herds and 40% of beef herds, though experts agree those numbers are deeply underestimated.
BLV is a retrovirus that attacks white blood cells — a cousin to human HIV — that weakens a cow’s immune system and fertility.
Even more concerning, multiple studies have found BLV DNA in human breast-cancer tissue.
The research is still developing, but the implication is clear: eradicating BLV is not only an animal-health priority — it could also be a public-health safeguard.
Photo: Michigan State University Extension. “The International All Things Bovine Leukemia Virus Conference.” MSU Extension Dairy, 2025, https://www.canr.msu.edu/events/2025blv
Used for educational purposes. Not affiliated with or endorsed by MSU.
Bovine Leukemia Virus (BLV) is a contagious cattle virus that infects a cow’s white blood cells. Most BLV-positive cattle look completely healthy, which is why the virus can spread quietly for years without being noticed.
BLV is a lifelong infection: there is no cure and no commercial vaccine.
It spreads through infected white blood cells found in blood, milk, colostrum, reproductive fluids (including semen), and any tool or equipment contaminated with those cells.
While only a small percentage of cattle ever develop cancer from BLV, the virus can still affect herd health by reducing immune function, fertility, and long-term productivity.
BLV transmission occurs when infected lymphocytes (white blood cells) move from one animal to another.
Any fluid or tissue that contains WBCs can transmit the virus. That includes:
- blood (needles, tattoo/dehorning/tagging tools, surgical instruments, palpation sleeves)
- reproductive fluids (natural service, semen)
- colostrum and milk
- fetal and uterine fluids
- contaminated equipment
- vectors such as biting flies
Cow-to-calf transmission is one of the most important pathways. Infection can occur:
- in utero (across the placenta)
- at birth through exposure to maternal fluids
- through colostrum and milk postpartum
Because most infected animals show no symptoms, BLV can spread for years in both open and closed herds.
BLV’s impact in dairies is better studied and more dramatic because dairies test more often and track performance closely.
1. Milk Production Loss
Large dairy studies show BLV-positive cows often produce:
- 2–5 lbs less milk per day
- lower lifetime production
- higher somatic cell counts (SCC)
- more frequent mastitis events
This adds up to major economic loss at the herd level.
2. Reproduction Problems
Just like in beef, BLV-positive dairy cows have:
- more open days
- more services per conception
- higher cull rates for reproductive failure
3. Immunity & Health Costs
BLV weakens immune function, leading to:
- more frequent treatment for disease
- poorer vaccine response
- higher culling rates for chronic health problems
4. Herd Longevity
BLV-positive dairy cows tend to:
- leave the herd earlier
- experience more chronic health issues
- have shorter productive careers
In high-prevalence herds, BLV quietly erodes profit through lost milk, lost fertility, and increased culling.
Most BLV-positive beef cows look completely normal, which is why the virus often goes unnoticed.
However, BLV can still affect beef operations in several important ways:
1. Reproduction
BLV-positive beef cows have higher rates of:
- open or late-breeding cows
- early embryonic loss
- weaker heats and subtle estrus expression
Reduced fertility is one of the earliest and most consistent signs of herd-level BLV impact.
2. Immune Function
Because BLV infects white blood cells, many beef cattle experience:
- weaker immune response to illness
- more frequent or more severe cases of pneumonia or scours
- slower recovery times
- higher treatment costs
3. Calf Performance
Calves born to BLV-positive dams may experience:
- higher likelihood of becoming BLV-positive themselves
- weaker passive immunity
- reduced early growth in some herds
- overall increased susceptibility to disease
4. Longevity
BLV-positive beef animals may leave the herd earlier due to:
- chronic “poor doing”
- repeated illness
- unexplained weight loss or poor condition
Only a small percentage (1–5%) develop cancer (lymphosarcoma), but subclinical losses are far more common and costly.
No — BLV has not been proven to infect humans or cause human disease.
Some studies have found BLV DNA in human breast-cancer tissues, while others have found no association at all. Scientists don’t yet agree on what these findings mean.
What we can say with confidence based on available studies:
- There is no proven harm
- There is no proven safety
- More research is needed
BLV is considered a cattle-specific virus, and cooked beef and pasteurized milk are safe. The uncertainty is why awareness and transparency matter.
Testing for BLV is very easy and inexpensive.
Two common test types:
1. ELISA (milk or blood)
- Detects antibodies
- Tells you if the cow has been exposed
- Cost: usually $4–$8
- Can be done on milk or serum
2. qPCR (whole blood)
- Detects the virus’s DNA
- Confirms if the cow is actually infected
- Measures Proviral Load (how contagious the cow is)
- Cost: usually $6–$15
The most accurate test is qPCR on whole blood using a purple-top EDTA vial.
You do NOT need a veterinarian to collect blood — most producers do it themselves. You simply mail the vial to the lab.
Explore the BLV Awareness Report!
It condenses 60+ scientific studies into one clear, easy-to-read resource.
𝘋𝘪𝘴𝘤𝘭𝘢𝘪𝘮𝘦𝘳: 𝘚𝘰𝘮𝘦 𝘴𝘵𝘶𝘥𝘺 𝘭𝘪𝘯𝘬𝘴 𝘪𝘯 𝘵𝘩𝘦 𝘉𝘓𝘝 𝘈𝘸𝘢𝘳𝘦𝘯𝘦𝘴𝘴 𝘙𝘦𝘱𝘰𝘳𝘵 𝘮𝘢𝘺 𝘤𝘩𝘢𝘯𝘨𝘦 𝘰𝘳 𝘮𝘰𝘷𝘦 𝘢𝘴 𝘫𝘰𝘶𝘳𝘯𝘢𝘭𝘴 𝘶𝘱𝘥𝘢𝘵𝘦 𝘵𝘩𝘦𝘪𝘳 𝘸𝘦𝘣𝘴𝘪𝘵𝘦𝘴. 𝘐𝘧 𝘢 𝘭𝘪𝘯𝘬 𝘣𝘦𝘤𝘰𝘮𝘦𝘴 𝘪𝘯𝘢𝘤𝘵𝘪𝘷𝘦, 𝘵𝘩𝘦 𝘤𝘪𝘵𝘢𝘵𝘪𝘰𝘯 𝘪𝘯𝘧𝘰𝘳𝘮𝘢𝘵𝘪𝘰𝘯 𝘸𝘪𝘭𝘭 𝘴𝘵𝘪𝘭𝘭 𝘢𝘭𝘭𝘰𝘸 𝘺𝘰𝘶 𝘵𝘰 𝘭𝘰𝘤𝘢𝘵𝘦 𝘵𝘩𝘦 𝘰𝘳𝘪𝘨𝘪𝘯𝘢𝘭 𝘴𝘵𝘶𝘥𝘺.