Prevalence of bovine leukemia virus (BLV) appears to be changing in the U.S. A recent study of cattle in eastern Kansas found that 42 out of 44 herds had at least one cow test positive for BLV. The same study found 55% of the cattle tested were positive for BLV. This is in contrast to the 1997 National Animal Health Monitoring System report in which BLV was found in 38.7% of the beef cattle operations and 11.5% of all cows tested were positive for BLV.
BLV is a retrovirus capable of causing cancer in cattle. The disease that is caused by the virus may be referred to as enzootic bovine leukosis (EBL), malignant lymphoma or lymphosarcoma. Most cattle infected with the virus remain asymptomatic or show no clinical signs of the disease. However, BLV is responsible for production losses due to increased veterinary costs, reproduction inefficiency, decreased milk production, death and carcass condemnation at slaughter.
Cattle become infected with the virus when blood or body fluids are transferred between animals. Lymphocytes, a particular white blood cell, are the specific cells that are infected with the virus. Many different routes of transmission of BLV have been proposed, but more research is needed to fully understand BLV transmission.
Cattle that are infected with BLV have three possible outcomes. The most common outcome is the animal appears normal. Another 30% of the cattle will have an elevated lymphocyte count that is referred to as persistent lymphocytosis. The last outcome is cancer—however, less than 5% of the cattle with BLV will ever develop lymphosarcoma.
Common symptoms of lymphosarcoma include appetite and weight loss, fever, eye problems, digestive problems, problems walking, hind limb paralysis and enlarged lymph nodes. Most cattle are three years old or older before tumors develop.
Brian Freking, Oklahoma State University Extension Southeast District livestock specialist
Currently, no treatments exist for cattle that are infected with BLV. Eradicating the disease requires testing and culling infected cattle until no positive cases are found for two years. This may not be economically feasible in a highly infected herd. A less intense approach being studied is to eliminate cows with high viral loads. It is thought that these cattle may be super shedders.
BLV will continue to be a problem in the U.S. until a vaccine is developed or there is an economic incentive to eradicate the disease. Until that time, producers should follow proper biosecurity to do all they can to prevent the spread of the virus.— Barry Whitworth, DVM, Oklahoma State University Extension specialist
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