BLV stands for bovine leukemia virus, a retrovirus that infects cattle by targeting a type of white blood cell called B lymphocytes. It is the most common infectious cause of cancer in cattle and the leading reason cows are condemned at slaughter in the United States. Most infected animals never show obvious signs of illness, which is exactly what makes the virus so widespread and so easy to overlook.
How BLV Works Inside a Cow’s Body
BLV belongs to the same virus family as HIV. It’s a single-stranded RNA virus that inserts its genetic material into the DNA of B lymphocytes, a key part of the immune system. Once there, the virus stays for life. There is no cure, and infected cattle remain carriers permanently.
The infection typically follows one of three paths. The majority of infected cows, roughly 60 to 70%, remain what’s called “aleukemic.” They carry the virus silently with no detectable changes in their blood counts and no outward symptoms. About 30% develop a condition called persistent lymphocytosis, where their white blood cell counts stay abnormally elevated for months or years. This signals that the virus is actively disrupting immune function, even though the cow may still appear healthy. A smaller group, between 1 and 5% of infected cattle, eventually develops malignant B-cell lymphoma (cancerous tumors in the lymph nodes and organs) after a long incubation period that can stretch years.
How the Virus Spreads
BLV travels from cow to cow through blood. Specifically, it spreads when infected lymphocytes from one animal enter the bloodstream of another. This makes everyday farm procedures surprisingly effective transmission routes. Tattooing, dehorning, ear tagging, rectal palpation, blood collection, and injections can all transfer tiny amounts of blood between animals if equipment isn’t changed or disinfected between each cow.
Reusing needles is one of the most common culprits. Even a small trace of blood on a shared needle is enough to introduce the virus to a new host. Large biting flies like horseflies can also carry infected blood from one animal to another, though this route is considered minor compared to direct blood contact from farm tools.
Cows can also pass BLV to their calves. This vertical transmission happens either during pregnancy, when the virus crosses the placenta, or after birth through infected colostrum (the first milk a calf drinks). Both routes mean that calves born to positive dams need to be treated as potentially infected from day one.
The Hidden Cost: Immune Suppression
The real damage BLV does to a herd often goes unnoticed because most infected cows don’t develop cancer. Instead, the virus quietly weakens the immune system in ways that show up as other problems. Infected cows produce less milk, and three large studies have confirmed that herd milk production drops as the percentage of BLV-positive cows rises. One estimate puts the annual loss at roughly $380 per infected milking cow.
That immune suppression has a cascade of consequences. BLV-positive cows are significantly more prone to mastitis (udder infections). Research from Hokkaido, Japan found that cows with high viral loads were 2.6 times more likely to develop subclinical mastitis than uninfected cows, with half of those high-viral-load animals developing mastitis within just 52 days after calving. The estimated loss per high-viral-load cow from mastitis alone was over $400. Infected cattle also experience reduced vaccine effectiveness, increased lameness, shorter lifespans, and greater difficulty handling heat stress.
Testing and Diagnosis
The two standard tests for BLV are a blood-based ELISA test and an older test called AGID. The ELISA is the more sensitive option, detecting antibodies in about 95% of animals that have been infected for at least 55 days. For cattle older than 8 months, both tests approach 100% specificity, meaning a positive result is highly reliable.
Testing calves is trickier. A calf that drinks colostrum from a BLV-positive cow will absorb maternal antibodies and test positive even if it isn’t actually infected. These borrowed antibodies can persist until about six months of age, so any calf that tests positive before then should be retested after reaching six months to confirm whether it truly carries the virus.
Controlling BLV on a Farm
Because there is no vaccine and no treatment, managing BLV comes down to preventing blood-to-blood contact between animals. The practical steps are straightforward but require consistent discipline across every person who handles the herd.
- Single-use needles. Never reuse needles, IV lines, or syringes that may carry blood contamination between animals. This includes oxytocin vials and other shared medicine containers that a used needle may have contacted.
- Disinfect all tools. Tattoo pliers, ear taggers, ear notchers, hoof trimming equipment, dehorning tools, and rectal ultrasound probes should all be thoroughly disinfected between animals. When dehorning, use burning-type dehorners when possible to avoid open bleeding, or disinfect cutting tools between each calf and keep animals separated until wounds dry.
- Manage outside equipment. Require that hoof trimmers, veterinarians, and anyone else bringing equipment onto the farm disinfect it before use. Don’t share tools at fairs or expositions.
- Test new arrivals. Every animal brought onto the farm should be tested on arrival, retested 60 days later, and possibly again at six months. Treat all new additions as potentially positive until proven otherwise.
- Insect control. Reduce biting fly populations as much as possible, particularly horseflies and other large species that take blood meals.
- Calf management. Calves born to positive dams should be handled as if they are positive. In herds working toward elimination, feeding colostrum only from BLV-negative cows or using heat-treated colostrum can help break the cycle.
In herds with low infection rates, the most effective strategy is to test the entire herd and remove positive animals. In herds where infection is already widespread, a phased approach that prioritizes removing cows with persistently high white blood cell counts can gradually reduce prevalence without gutting the milking string all at once.
Prevalence in the United States
BLV is remarkably common in U.S. dairy herds. Estimates suggest that the virus is present in the vast majority of dairy operations, and individual cow-level infection rates run high in many regions. BLV-induced lymphoma is the single leading cause of cattle condemnation at U.S. slaughter plants. Despite this, the United States has no mandatory national eradication program, and routine testing is not required.
This stands in contrast to several other countries. New Zealand and Australia will only import or export BLV-free milk. Many European nations have already eradicated the virus from their herds and enforce strict import restrictions based on BLV status, including rules around embryo transfers. For U.S. producers looking to export cattle, genetics, or dairy products, a herd’s BLV status can directly affect market access.
Is BLV a Risk to Humans?
This is an area of active scientific debate, and the honest answer is that nobody knows for certain yet. BLV DNA has been detected in a notable percentage of retail milk and meat samples from infected cows: one study found viral DNA in about 39% of milk samples and 32% of meat samples, while another found it in roughly half of both. Antibodies against BLV have also been found in human blood samples, suggesting that some people have been exposed to the virus.
Two large meta-analyses have found a statistical association between the presence of BLV DNA in human tissue and an increased risk of breast cancer, with odds ratios between 2.5 and 3.9. However, this remains controversial. Finding viral DNA in tissue does not prove the virus is actively infecting human cells or causing disease. Some studies have found no significant difference in BLV prevalence between cancer patients and healthy controls. No confirmed case of BLV transmission causing disease in a human has been documented.
Pasteurization inactivates the virus in milk, and thorough cooking does the same for meat. The potential concern centers on unpasteurized dairy products and undercooked beef from infected animals. For most consumers buying commercially pasteurized milk and properly cooked meat, current evidence does not point to a clear risk.