BLV: Bovine Leucosis Virus

Dairy Pipeline: October 1997

by Tom L. Bailey
Dairy Production Medicine
Virginia-Maryland Regional College of Vet Medicine
Virginia Tech

BLV or bovine leucosis is a disease that Virginia dairymen need to be more familiar with and take steps to decrease the prevalence of in their herds. BLV is a viral infection of the lymphocytes (white blood cells) of cattle that may exist in the cow without causing clincial disease or any outward signs of a problem. At other times, cows will have decreased production, and tumors in the heart, abomasum, spinal cord, uterus, and lymph nodes. Herd surveys have indicated that many dairies experience over 80% of the cows with positive titers for the virus. This does not mean that 80% of the cows will demonstrate clinical signs of the disease, only that 80% of the cows have come in contact with the virus and have mounted an immune response. Typically in a herd with this degree of infection, approximately 2 to 5% of the cows will experience clinical disease, usually at an age of greater than 3 years. Direct costs of the disease are associated with those that develop clinical disease, resulting in death, culling, and decreased milk production. Indirect costs are increased replacement animals. Also, cattle sales are increased to achieve BLV negative status and semen and embryo sales are reduced, as well as export of cattle. Most BLV infections are transmitted via blood (lymphocytes), with colostrum and milk as potential transfers, and to a lesser degree in utero, semen, embryos, saliva, nasal secretions, urine and feces. Therefore the principle goal should be aimed at the transfer of blood between cows and between calves. A single needle for injections in an individual animal is a primary focus. Never remove the needle from a cow and place this possibly infected needle in a bottle of medication or vaccine, because the contents of the bottle may now be infected with virus. We have instituted a single palpation sleeve for each cow examined, decreasing the chance of potential transfer between cows by rectal palpation. Dehorning and tattoo instruments, tagging pliers, cutting instruments for extra teats or other surgical procedures that may result in the transfer of blood need to be disinfected or sterilized between animals. Vectors such as flies or ticks can potentially transmit the disease but their significance is controversial, however steps should be taken to decrease exposure to such vectors. Cows can be tested to identify positive animals, however, whole herd testing is expensive and will depend on the individual herd. Once positive cows have been identified, colostrum and milk from non-infected animals can be fed to calves to decrease the potential transfer from dam. Animals entering your dairy should be BLV negative. Many steps can be taken to decrease the incidence of BLV in dairy herds. The primary route of transmission is by blood and thought should be given to any procedure that results in the transfer of blood or blood products between cows or calves. Consult with your veterinarian for more information on BLV and to establish a control program to decrease the incidence of the disease in your dairy.

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