Bovine Leukosis Virus (BLV) is a pathogen that infects cattle lymphocytes, a type of white blood cell, by inserting its genomic material into the host. BLV is a blood-borne disease that survives in white blood cells called lymphocytes. BLV is transmitted from an infected bovine to an uninfected one by transferring BLV-infected blood between these animals during the process of performing many common farm practices. The primary sign of clinical disease is tumors in the uterus, abomasum, heart, spinal canal, and/or lymph nodes. Most animals remain asymptomatic despite becoming persistently infected by the virus. Only about 5% of the cows infected with BLV ever develop a clinical disease, and most infected cows show no signs of the disease, and the infection does not appear to affect their milk production or any other functions.

BLV is very common, and many cattle are infected with the virus. A 1996 national survey found that 88.6% of the dairy herds surveyed and 43% of the cows in these herds are infected with BLV. Although infection levels were particularly high in the Southeast, herds in all areas of the country are significantly infected. In the US, the most recent surveys indicate that 89% of dairy operations and 38% of beef operations had cattle seropositive for BLV.

BLV is spread through blood and blood products that contain infected lymphocytes. Transmission of BLV from an infected bovine to an uninfected one is usually a consequence of transferring BLV-infected blood between these animals during the process of performing many common farm practices. BLV can also be transmitted through colostrum and waste milk, in utero, and equipment used between animals, including needles, syringes, OB sleeves, dehorners, tattoo pliers, ear taggers, medicine vials, hoof knives, nose tongs, rectal ultrasound equipment, tail docking equipment, ear notchers, and milking equipment.

To keep BLV out of a negative herd, it is recommended to test all incoming and returning cattle, segregate and retest in 45-60 days, manage as positive until BLV status is determined, and take measures to control the spread of BLV. It is critical to prevent horizontal spread of blood and fluids containing blood cells from infected to uninfected animals, clean and dry single-use maternity pens, remove calf from cow ASAP, and not feed colostrum from positive cows.