Bovine Leukosis: A cancerous disease caused by the Bovine Leukosis Virus (BLV).
Targeted Cells: Infects beef and dairy cattle, particularly affecting lymphocytes, which are a type of white blood cell responsible for immune responses.
Disease Progression
Healthy B-cells begin to become infected by BLV.
Disease Stages:
Aleukemic: The presence of the virus without significant clinical signs.
Persistent Lymphocytosis: Approximately 30-40% of infected cattle develop an abnormal increase in lymphocytes in the blood, commonly referred to as lymphocytosis. This condition differs from most forms of human leukemia.
Tumors: About 1-5% of infected cattle will progress to develop malignant tumors known as lymphosarcomas.
Clinical Signs and Symptoms
Common signs in infected cattle include:
Protruding eyeballs.
Weight loss.
Enlarged lymph nodes.
Gastrointestinal obstructions.
Paralysis in hind limbs.
Infertility, often due to tumors in the uterus.
Common Sites for Tumors
The most prevalent sites for tumor development include:
Uterus.
Right atrium of the heart.
Abomasum (fourth stomach compartment).
Lymph nodes.
Other sites can include:
Kidneys.
Brain.
Eyes.
Transmission
Modes of Transmission:
Primarily through the blood of infected cattle.
To a lesser extent, may also be transmitted via saliva, semen, and milk.
Management Practices: Practices that lead to direct exposure of blood from infected to uninfected cows (such as shared needles, blood-contaminated syringes) can increase infection prevalence.
Secondary Transmission Routes
Significant causes of transmission include:
Use of common needles.
Blood-contaminated syringes and drug vials.
Rectal palpation can also contribute to the spread.
Lesser contribution from colostrum feeding, cross-placental transmission, and possibly flies.
Diagnosis and Culling Decisions
Diagnosis: Infected cattle can be diagnosed by testing serum for virus-specific antibodies.
Interpretation of Results: In young calves under six months, positive results may be due to maternal antibodies acquired through colostrum.
Culling Considerations:
Testing serum for infected cows is not useful for culling decisions unless clinical signs of lymphosarcoma and elevated lymphocyte counts are present.
High prevalence of BLV in herds makes routine testing and culling economically unfeasible due to the predominantly benign nature of the infection in many cows.
Prevalence Studies
The USDA conducted studies on BLV in dairy cattle as part of the National Animal Health Monitoring System (NAHMS) in 1996 and 2007.
Key findings include:
Over 1,000 dairy operations participated in the 1996 study, with at least one BLV infected cow found in 89% of farms.
The infection prevalence within herds varied significantly, but an average of 40% of cows were found infected in the Midwest.
Results indicated that larger herds were more likely to test positive for infection.
Management Practices to Reduce BLV Prevalence
Recommended management practices include:
Utilize separate needles and properly discard syringes contaminated with blood.
Maintain a clean needle in multiuse drug vials.
Identify and manage BLV positive cows more carefully.
Change palpation sleeves after examining a BLV positive cow to avoid cross-contamination.
Feed colostrum and milk exclusively from BLV negative cows or use milk replacer.
Consider the use of pasteurized colostrum.
Use electric or gas burning equipment instead of gouging tools for dehorning.
Regularly clean all tattoo and ear tag equipment before each use.
When feasible, separate BLV positive from negative animals, though this can be challenging in practice.
Economic Impact of Bovine Leukosis
Cost Implications: BLV can lead to significant economic losses in herds, detailed as follows:
Increased heifer replacement costs.
Loss of revenue from condemned carcasses of cull cows.
Reduced fertility and decreased milk production.
Specifically regarding herd prevalence:
The average annual cost in a herd with 50% prevalence is nearly $6,400 per 100 milking cows, equating to approximately $64.00 per cow per year.