Bovine leukemia virus (BLV) control generally centers on preventing blood transfer between cattle. Practices such as using clean needles, changing rectal palpation sleeves and properly disinfecting surgical equipment to reduce the risk of moving infected lymphocytes from one animal to another are commonplace.
Despite these efforts, BLV remains widespread in many regions, leading researchers to continue investigating whether other routes of transmission could help explain the virus’ persistence.
A new study published in Transboundary and Emerging Diseases offers fresh evidence supporting one long-standing theory: Blood-feeding insects may contribute to the mechanical spread of BLV. Researchers at the National Veterinary Research Institute in Poland detected BLV DNA in Culicoides biting midges and a stable fly collected on cattle farms, with viral sequences matching those found in infected cattle on the same operations. While the findings stop short of proving insects transmit the virus, they add another piece to a puzzle researchers have been trying to solve for decades.
Look Beyond Traditional Transmission Routes
Unlike many viruses, BLV is highly cell-associated, meaning transmission depends primarily on the movement of infected lymphocytes rather than free virus particles. Even small amounts of infected blood can spread the disease, making biosecurity efforts to minimize opportunities for blood transfer during routine management procedures very important.
Blood-feeding insects have remained part of the conversation because interrupted feeding could theoretically move infected blood from one animal to another. Stable flies, horn flies and horse flies may be capable of mechanically transferring BLV, but evidence under natural farm conditions has been limited. The role of Culicoides biting midges has been especially unclear despite their abundance around cattle and their importance as vectors of several livestock viruses. Their proven ability to transmit diseases such as bluetongue virus has made them a logical candidate for investigation.
BLV DNA Found in Biting Midges and Stable Flies
To investigate the question, researchers collected more than 2,400 biting midges and 146 stable flies from four cattle farms in northeastern Poland. The insects were tested for both bovine DNA and BLV DNA, and positive samples were genetically sequenced to compare them with BLV strains circulating in cattle on the same farms.
The team detected BLV DNA in 20 pools of biting midges and one stable fly. More importantly, every BLV-positive insect sample also contained bovine DNA, indicating the insects had recently fed on cattle. Further, genetic sequencing showed the viral strains recovered from insects were identical or nearly identical to those found in infected cattle on the same farms.
A strong association between recent blood meals and BLV detection was also reported. Nearly two-thirds of blood-fed midge pools tested positive for BLV DNA, while positive results were uncommon in insects that were not visibly blood-fed. Detection rates among blood-fed midges were nearly four times higher than those observed across all tested midges.
Taken together, these findings strengthen the argument that the insects acquired BLV while feeding on infected cattle rather than through environmental contamination.
It Does Not Prove Insect Transmission
Despite the promising findings, the results should not be overinterpreted.
Finding viral DNA inside an insect does not mean the insect can infect another animal. The study detected BLV genetic material but did not demonstrate that the virus survives, replicates or reaches the insect’s salivary glands in a way that would enable biological transmission.
Instead, the evidence supports the possibility of mechanical transmission, in which infected blood is physically carried from one animal to another during interrupted feeding, much like blood-contaminated equipment can spread the virus.
“Our study demonstrates the carriage of BLV proviral DNA by field-caught insects but does not demonstrate replication of BLV in the insect,” says researcher Aneta Pluta. “These findings should not be interpreted as proof of the biological vector competence of Culicoides for BLV.”
What it Means for BLV Control
The findings are unlikely to change how we approach BLV control today. Preventing blood transfer between animals remains the foundation of disease management, and the study does not suggest that insect transmission replaces those well-established risk factors.
Instead, the research raises new questions about whether biting insects may contribute to BLV circulation under certain conditions, particularly in herds with infected animals, high insect populations and opportunities for interrupted feeding.
Pluta and colleagues acknowledge that additional work is needed to determine whether insects merely carry infected blood or play a measurable role in moving BLV between cattle under field conditions. Future studies examining infectivity, transmission efficiency and the impact of insect control on BLV incidence will be needed before management recommendations change.
For now, biting insects remain one possible piece of the BLV puzzle. This study doesn’t complete the picture, but it provides some of the strongest field evidence to date that these insects may contribute to the mechanical movement of BLV-positive blood within cattle herds.
