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 such as tattooing ears, dehorning, rectal palpation, or injecting animals with a vaccine or medication. Insect vectors such as Tabanid spp. and other large biting flies may also transmit the virus. Transmission of BLV may also occur trans-placentally from an infected dam to her fetus or immediately post-partum when the newborn calf ingests BLV-infected colostrum produced by its dam. Once infected, cattle develop a persistent (lifelong) antibody response.
There are three main clinical outcomes in cattle infected with BLV. Most animals remain asymptomatic, i.e. show no outward signs of disease (approximately two-thirds of infected animals). Less than 5% of BLV-infected cattle develop lymphosarcoma. The remainder of BLV-infected cattle develop persistent lymphocytosis.
Lymphosarcoma is most commonly observed in 4 to 8 year old animals; animals less than 2 years of age rarely show signs of disease. Clinical signs are highly variable because they depend largely on the organ system (e.g. alimentary, cutaneous, ophthalmic, neurologic, reproductive, reticuloendothelial, multicentric) invaded by these neoplastic lymphocytes. Consequently, signs of infection may include, but are not limited to, peripheral and/or internal lymph node enlargement (lymphadenopathy), labored breathing (dyspnea), bloat, jugular vein distention, increased heart rate (tachycardia), brisket edema, weight loss, decreased milk production, fever, loss of appetite, infertility, rear limb weakness or paralysis, protruding eyeball (exophthalmia), gastrointestinal obstructions and/or ulcers with digested blood (melena), and increased blood lymphocytes counts.
Because no vaccine is available for BLV, virus specific antibodies found in serum or milk are a good indicator of exposure and a practical method for disease screening. Agar gel immunodiffusion (AGID), enzyme-linked immunosorbent assay (ELISA), and polymerase chain reaction (PCR) tests are all used to identify BLV-positive animals.
Eliminating the transfer of blood from infected animals to naive animals is the cornerstone of preventing spread of this virus between animals. To this end, cautery or other bloodless methods of dehorning should be used and equipment used for procedures such as castration, tattooing, or ear tagging should be adequately cleaned and disinfected between animals. Single use, disposable needles should always be used for blood collection and intramuscular injections. Handling facilities that become contaminated with blood should be cleaned. Control of biting flies should also help minimize spread of BLV between animals.
There is no curative treatment for viral infection or for lymphosarcoma in cattle. Consequently, producers must rely on management practices to eradicate BLV from their herd. These management practices have been used to either (1) identify BLV-infected cattle and slaughter positive reactors, or (2) detect and isolate BLV-infected cattle, then manage infected and non-infected cattle in separate herds, or (3) test animals for BLV status and then take appropriate biosafety and management measures to minimize exposure of non-infected animals to the infectious agent.
Bovine leukemia virus (BLV) is an oncogenic retrovirus that infects lymphocytes (white blood cells). Proliferation of these virus-infected lymphocytes results in either a persistent lymphocytosis, a benign form of the disease, or neoplastic tumors that invade many different organ systems (referred to as enzootic bovine leucosis or lymphosarcoma).
Both beef and dairy cattle are the natural hosts for this virus. In the U.S., the most recent surveys indicate that 89% of dairy operations and 38% of beef operations had cattle seropositive for BLV. Currently, there are no Federal regulations specific to curbing spread of bovine leukemia virus in the U.S. cattle population.
The economic consequences of BLV circulating in a cattle herd can be sizable, in terms of both direct and indirect costs incurred in the treatment and control of this disease. Direct costs include those associated with clinical lymphosarcoma (lost production in cattle with clinical manifestation of disease), loss of production in sub-clinically infected animals (e.g. one survey found that dairy herds with test-positive cows produced 218 kg less milk per cow per year than those with no test-positive cows), veterinary costs associated with treatment and diagnosis of lymphosarcoma, and replacement costs associated with death of affected animals or premature culling due to reduced longevity. Indirect costs include those associated with loss of revenue due to restrictions of export of cattle and cattle products such as semen and embryos and cost to society for maintenance of regulatory agencies and research.