Bovine leukemia virus is a retrovirus that causes lymphosarcoma (cancer) in cattle. It can cause economic losses to producers in the form of death of cattle, loss of milk production, cost incurred in treatment and diagnosis, and premature replacement costs for cattle dying or culled.

Less than 5% of BLV-positive cattle actually develop tumors. Most positive cows show no symptoms, have competent immune systems and are as productive as negative cows. Clinical signs rarely develop before the age of two and are most common between three and six years of age. The signs are usually non-specific such as weight loss, decreased appetite, and decreased production. Peripheral lymph node enlargement is common such as lymph nodes located in the flank or in front of shoulder. The main target organs are the abomasum, heart, uterus, behind the eyeball, and area around spinal cord. The clinical signs will change according to the organ that is involved. For example, if the spinal canal is affected it can cause a down cow or a cow that is weak in the hind end that progresses to being down due to the pressure of the tumor on the spinal cord.

The virus is spread through infected white blood cells. Infection from cow to cow is usually due to re-use of common needles, syringes, dehorners, ear tattoo instruments, castrating equipment, blood collection needles, etc. Rectal palpation using a common sleeve may also spread it but there have been conflicting results in studies that have tested this theory. The role of insects is also controversial in how much it plays a part in spreading the disease. Transmission is also speculated in cows that are in close contact through nasal secretions. Transmission from cow to calf can occur from transfer of virus across placenta but occurs less than 10% of pregnancies in infected cattle. Cows that produce a BLV-positive calf may or may not produce an infected calf in the next pregnancy. Embryo transfer is not a major source of transmission. Embryos from BLV-positive donors do not infect BLV-negative recipients nor produce infected fetuses. However there is risk for the embryo if put into BLV-positive recipient. Milk and colostrum can also transfer the disease to the calf although the rate of transmission through this route may be low.

To control the disease in herds that have a low prevalence would be to bleed the herd and cull positive animals. Unfortunately in some herds, especially dairies, prevalence is much higher. Other control measures would be use of single needles and syringes, disinfection of all common instruments, individual rectal sleeves, and insect control. For dairies, calves should be fed colostrum from non-infected cows. In a perfect world the positive cows would be kept separate from the negative cows.