Bovine Leukosis

Bovine leukosis (BL), also known as bovine lymphoma or enzootic bovine leukosis, is a viral disease that primarily affects cattle. It is caused by the bovine leukosis virus (BLV), a retrovirus belonging to the genus Deltaretrovirus. Most cattle infected with BLV are asymptomatic and therefore easily overlooked. However, the sick cows are infected with the virus for life, and the abnormal immune function of the body causes frequent abortions of pregnant cows, and the lactation volume and meat quality of dairy cows continue to decline, causing huge economic losses to the cattle breeding industry. BioVenic, an experienced veterinary diagnostics CRO, has been actively involved in the development of diagnostics solutions and vaccines to combat this disease.

Etiology

BLV belongs to the family Retroviridae, a member of the genus Deltaretrovirus. Its particles are spherical or short rod-shaped, with a double-layer capsule and a fibrous structure. The virus has poor resistance to the outside world and immediately loses its ability to infect an environment above 60℃.

Fig. 1 Schematic diagram of BLV structure.¹

The Economic Impact of BLV Infection

The economic loss caused by BLV-induced lymphoma is known to be the major impact of BLV in cattle. The beef cattle eliminated due to lymphoma undoubtedly brought huge economic losses to farmers.

Infection of cows with BLV can result in reduced milk production and a shortened cow lifespan. The impact of BLV infection on milk production is difficult to measure, and cows with reduced production are usually quickly culled, causing certain economic losses.

Distribution

More than 20 countries, including Denmark, Finland, Switzerland, and the Netherlands, have announced the virus has been eradicated. However, in Eastern Europe, BLV infection and transmission are still very common, and the virus has a high prevalence in Belarus, Ukraine, Croatia, and other countries. Occasionally, cattle disease is reported in parts of Italy, Portugal, Greece, and Romania. In other parts of the world, the virus is more prevalent. It has a high prevalence in dairy and beef cattle in Canada, the United States, South America, Korea, etc.

Fig. 2 Distribution map of bovine leukosis published by OIE.¹

Sustainable Animals

The naturally susceptible hosts of BLV are cattle, including dairy cows and beef cattle. Sheep, goats, alpacas, rabbits, rats, and chickens can also be artificially infected.

Transmission

BLV has the following two modes of transmission.

• Horizontal Transmission
  
  • Infected cattle or their secretions as the medium: respiratory secretions, vaginal secretions, milk, urine, feces, etc., resulting in the spread of BLV.
  • Parasitic insect vectors: BLV can be transmitted through the bite of blood-sucking insects.
  • Using medical devices as a medium: iatrogenic transmission is most common in large-scale farms. Cross-transmission of BLV between infected and healthy cattle is usually due to repeated use of syringes, needles, and straight examination gloves by veterinarians and breeders, or the use of non-sterile designers, castrators, ear punches, etc.
• Vertical Transmission
  
  • BLV is transmitted from parent to calf during the perinatal period through the placenta, birth canal, or lactation.

Pathogenesis

The pathogenesis of BLV begins with the virus entering the host's body through mucosal surfaces or abrasions, infecting susceptible B-lymphocytes. The virus then integrates its genetic material into the host's lymphocyte DNA, leading to lifelong infection. The integrated viral DNA, known as a provirus, replicates along with the host's cells during division, causing uncontrolled lymphocyte proliferation. By integrating the provirus and producing viral proteins, infected lymphocytes evade the host's immune surveillance and form neoplastic cells. Over time, these transformed cells lead to tumor development in various organs, such as lymph nodes, abomasum, and heart. BLV infection can manifest in different clinical forms, including persistent lymphocytosis (PL) or benign and malignant lymphomas. Additionally, immunosuppression caused by BLV makes cattle more susceptible to secondary infections, further impacting their health and productivity.

Zoonotic Risk

While some scholars have identified BLV nucleic acid in human breast tissue, it is crucial to note that BLV does not present a significant zoonotic risk. This indicates that the virus is not directly harmful to humans.

Signs and Symptoms

Although the clinical symptoms of BL are different, most of the affected cattle show loss of appetite, rapid weight loss, growth retardation, fatigue, difficulty in walking, damage to the abomasum, and dysfunction of the forestomach.

The clinical symptoms of the 6-month-old calf showed continuous fever, shortness of breath, and enlarged and symmetrical lymph nodes.

Adult cattle aged 4 to 8 have obvious loss of appetite, sweating, bloated rumen, dyspnea, diarrhea, and watery or muddy excrement. Papules and redness appear on the epidermis of some affected cattle, which will disappear for a while but will reappear later. Diarrhea, distended abdomen, and exophthalmos are also present in some cattle.

Diagnosis

There are currently a variety of BLV detection methods to choose from. These methods can be divided into two types, one is immunological diagnosis based on BLV-specific antibodies or antigens, and the other is molecular biology diagnosis based on viral nucleic acid.

• Immunodiagnostics Development

We develop advanced immunological diagnostic kits for our customers, including agar gel immunodiffusion (AGID) assays and ELISA for the detection of BLV-specific antibodies and antigens.

• Molecular Diagnostics Development

The nucleic acid-based PCR diagnostic kits we develop for our customers include standard PCR, nested PCR, qPCR, and blood-based direct PCR, which can greatly accelerate the prevalence detection of BLV.

Treatment

Currently, there is no specific treatment or vaccine available for BLV-infected animals. Cattle found to be BLV-positive should be culled.

Prevention and Control

To effectively prevent and control the spread of BLV, comprehensive measures must be implemented to minimize horizontal and vertical transmission. As a leading company in the biological industry, BioVenic advocates for the adoption of evidence-based strategies to safeguard cattle health and productivity. Here are the key measures recommended for preventing and controlling BLV.

• Eliminating Blood Transfer and Vector Control
  
  • Use bloodless methods for dehorning, such as cautery, to minimize the risk of blood transmission.
  • Implement single-use, disposable needles for blood collection and intramuscular injections to avoid cross-contamination.
  • Maintain clean handling facilities and promptly cleaned areas contaminated.
• Management Practices to Prevent BLV Transmission
  
  • Keep maternity pens clean and dry, and separate calves from infected cows promptly.
  • In situations where BLV prevalence exceeds 60%, contemplate freezing colostrum before use to inactivate the virus.
  • Avoid feeding waste milk, as it may contribute to transmission.
  • Separate BLV-positive and negative cows, if feasible, to minimize exposure within the herd.
• Hygienic Procedures and Equipment Sanitization
  
  • Disinfect equipment that has come in contact with blood or body tissues to prevent cross-contamination.
  • Use individual sterile needles for transdermal injection or blood collection to avoid sharing contaminated equipment.
  • Disinfect dehorners thoroughly before and after use to reduce the risk of transmission.
  • Replace examination gloves and sleeves between animals to prevent potential contamination.
• Preventing Vertical Transmission
  
  • Heat-treat or pasteurize colostrum before feeding to reduce in-utero transmission.
  • Use BLV-seronegative recipients for embryo transfer to minimize the risk of transmitting the virus.

BLV has long been an animal disease of major concern due to the huge losses it brings to the agricultural economy. BioVenic strives to provide veterinarians and animal health researchers with cutting-edge diagnostic kits, and innovative vaccine development services. If you need our technical support, please feel free to contact us.

References

1. Marawan, M. A., et al. "Bovine leukaemia virus: current epidemiological circumstance and future prospective." Viruses. 2021; 13 (11): 2167.