Bovine leukemia virus (BLV)

Also known as

Bovine lymphosarcoma, lymphsarcoma, enzootic bovine leukosis

Description

Bovine leukemia virus (BLV) infects lymphocytes (white blood cells). Proliferation of these virus-infected lymphocytes in the body can affect cattle in several different ways.

There are three main clinical outcomes in cattle infected with BLV. Approximately 2/3 of infected individuals show no outward signs of disease (they have a benign form of the disease but may act as carriers). Less than 5% of BLV-infected cattle actually develop lymphosarcoma (cancer, with malignant tumors in lymphatic tissue, caused by growth of abnormal lymphocytes, invading many different organ systems). The remainder of BLV-infected cattle develop persistent lymphocytosis (abnormally high white blood count).

Lymphosarcoma is a viral-induced form of cancer. Infection may occur before birth or via colostrum if the dam is infected, but usually doesn’t show up in young calves. Actual disease generally does not become apparent until months or years later. In affected animals it is eventually fatal.  

Some family lines seem more susceptible; the disease is more common in dairy cattle than beef breeds. White blood cell count may show evidence of disease before an animal shows signs. A high white count with many immature cells is typical of leukemia.

Once infected, the animal produces an antibody response (and has antibodies present in the blood for the rest of its life, showing it has been exposed) but not all exposed animals develop persistent lymphocytosis (excess amount of lymphocytes in the blood) or lymphosarcoma (cancer).

Lumps under the skin are indication of enlarged lymph nodes. Internal swollen nodes can’t be seen but if there are some in the chest or abdomen they may interfere with lung or digestive function and mimic other diseases.

Enlarged internal nodes may not be discovered except by accident during exploratory surgery or by rectal palpation. Signs vary, depending on which body system is affected by the swollen nodes.

Often the only sign is unexplained weight loss or decline in milk production. Sometimes calves are affected--with enlargement of various lymph nodes, infiltration of the bone marrow by certain types of white blood cells, and anemia.

Both beef and dairy cattle are natural hosts for this virus. Surveys in the U.S. indicate that 89% of dairy operations and 38% of beef operations had some cattle seropositive for BLV but prevalence of actual disease is very low; not all cattle exposed to the virus develop lymphosarcoma.

There are economic consequences of BLV circulating in a cattle herd, however. 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.

One survey found that dairy herds with test-positive cows produced 480 pounds less milk per cow per year than those with no test-positive cows. In addition, veterinary costs associated with treatment and diagnosis of lymphosarcoma, and replacement costs associated with death of affected animals or premature culling can be high.

Indirect costs include those associated with loss of revenue due to restrictions on export of cattle, as semen and embryos.

Lymphosarcoma is most commonly seen in 4 to 8-year-old animals; animals less than 2 years of age rarely show signs. Clinical signs are also highly variable, depending on the organ system (digestive tract, skin, eyes, nerves, reproductive tract, circulatory and immune systems, etc.) invaded by the abnormal (cancerous) lymphocytes.

Signs of infection may thus include, but are not limited to, peripheral (just under the skin) and/or , labored breathing, bloat, , weight loss, decreased milk production, fever, .

Signs

• Lumps under the skin
• Weight loss
• Loss of appetite
• Drop in milk production
• Labored breathing
• Fever
• Bloat
• Anemia
• Jugular vein distention
• increased heart rate
• Brisket edema
• Internal lymph node enlargement
• Infertility
• Rear limb weakness or paralysis
• Protruding eyeballs
• Gastrointestinal obstructions and/or ulcers
• Increased white blood counts

Causes

Since the disease can be passed from one animal to another via infected blood it can be spread by biting flies that feed on multiple animals.

The disease can be transferred from one animal to another by infected blood; thus it can be spread by human handling when surgical instruments, hypodermic needles or dehorning tools are not disinfected between animals.

Prevention

With blood testing, some European countries tried to eliminate the disease with test and slaughter programs but prevention in individual herds is difficult. Any animal with obvious signs should be culled. Blood tests and blood counts can identify animals that might be in early stages, and provide a tool for culling and for checking prospective replacement animals.

Control of horse flies and other biting flies may reduce incidence of transmission between animals. Since surgical instruments, dehorners, ear tattoo pliers, castrating tools and hypodermic needles may spread the virus from one animal to another, care should be taken when doing these procedures.

Cautery or other bloodless methods of dehorning should be used. Any equipment used for procedures such as castration, tattooing, or ear tagging should be adequately cleaned and disinfected between animals. Single use, disposable needles should be used for blood collection and injections. Handling facilities that become contaminated with blood should be cleaned.

Studies have shown that infection can be reduced from 80% down to 4% in dairy heifers being dehorned, just by changing dehorning methods. The virus can be transmitted by rectal examination of cattle--especially in dairy herds if the same obstetrical sleeve is used when palpating multiple cows. Changing sleeves between animals may limit spread of disease.

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. Several tests can be used to identify BLV-positive animals. Infected cattle can then be slaughtered or isolated from non-infected cattle in separate herds. Steps can then be taken to practice appropriate biosafety and management measures to minimize exposure of non-infected animals.

Treatment

There is no treatment that works against this virus.