Bovine Lymphoma (Leukosis, BLV) Study Guide

Overview and Clinical Importance

Bovine leukemia virus (BLV) is an oncogenic deltaretrovirus closely related to human T-lymphotropic virus types 1 and 2. BLV is the causative agent of enzootic bovine leukosis (EBL), the most common neoplastic disease in cattle worldwide. The virus integrates into B-lymphocyte DNA, creating a lifelong infection.

BLV infection follows a predictable progression: approximately 70% of infected cattle remain asymptomatic, about 30% develop persistent lymphocytosis (PL), and fewer than 5% develop lymphosarcoma (also called lymphoma), typically in cattle aged 4-8 years. The economic impact is substantial, with losses from reduced milk production, premature culling, carcass condemnation, and trade restrictions.

Etiology and Classification

Bovine Leukemia Virus Characteristics

BLV is an exogenous C-type oncogenic retrovirus belonging to the genus Deltaretrovirus, family Retroviridae. Key viral characteristics include: stable genome with approximately 97% nucleotide sequence homology among strains worldwide; integration into host B-lymphocyte DNA as a provirus; primary target cells are CD5+ IgM+ B-lymphocytes; the virus does not cause chronic viremia; and infection results in persistent antibody production.

Classification of Bovine Lymphosarcoma

Transmission and Epidemiology

Routes of BLV Transmission

BLV transmission requires transfer of infected lymphocytes. The virus is extremely unstable outside cells and does not cause chronic viremia. Key transmission routes include:

Horizontal transmission (primary route): Iatrogenic procedures including contaminated needles and syringes, rectal palpation sleeves, dehorning equipment, ear taggers, tattoo pliers, hoof knives, and surgical instruments; biting flies (Tabanidae, Stomoxys calcitrans); and animal-to-animal contact through nasal and ocular secretions.

Vertical transmission (10-20% of cases): Transplacental infection from infected dam to fetus; ingestion of infected colostrum or milk (though maternal antibodies may be protective).

Natural breeding: BLV is NOT transmitted in normal semen from healthy BLV-positive bulls; however, if seminal vesiculitis or reproductive disease is present, infected lymphocytes may contaminate semen.

Global Prevalence and Epidemiology

Clinical Signs and Presentation

Enzootic Bovine Leukosis (Adult Form)

Clinical signs depend on tumor location. The most commonly affected organs include the heart (66%), abomasum (61%), uterus (38%), kidney (32%), epidural space (26%), and retrobulbar area. Most cattle present with nonspecific signs including anorexia (34%), weight loss (16%), and fever (14%).

Clinical Signs by Organ System

Sporadic Bovine Leukosis Forms

Juvenile (Calf) Form

Affects calves less than 6 months old. Characterized by sudden onset of diffuse generalized lymphadenopathy with peripheral lymph nodes enlarged 10-30 times normal size. Additional signs include weight loss, fever, tachycardia, dyspnea, bloat, and posterior paresis. Profound lymphocytosis (greater than 50,000/mcL) is common. This form is rapidly fatal, with death occurring 2-8 weeks after onset.

Thymic (Adolescent) Form

Affects cattle 6-24 months old, more common in beef breeds. Presents with large, firm swelling in the ventral neck region (cervical or intrathoracic thymus). Clinical signs include weight loss, reduced appetite, mild bloat, dysphagia, jugular distension, brisket edema, and tachycardia. Death is usually due to acute cardiac and respiratory failure secondary to ruminal tympany.

Cutaneous Form

Affects cattle 1-3 years old. Characterized by discrete, alopecic intradermal plaques or nodules (1-5 cm diameter) on neck, back, rump, and thighs. Lesions may be painful and can ulcerate. The condition may initially regress but typically recurs as generalized lymphosarcoma with a fatal prognosis. This form is of T-cell origin, resembling mycosis fungoides in humans.

Diagnosis

Diagnostic Testing for BLV Infection

Since BLV infection is lifelong and triggers persistent antibody production, serological testing is the gold standard for diagnosis. A positive serological test indicates current infection (not just exposure).

Diagnosis of Lymphosarcoma

A positive BLV test does NOT mean the animal has lymphosarcoma. Definitive diagnosis of lymphosarcoma requires cytology or histopathology demonstrating neoplastic lymphocytes.

Antemortem Diagnostic Test Sensitivities

Gross Pathology and Necropsy Findings

At necropsy, lymphosarcoma is characterized by white, nodular, raised masses varying in size from 0.5 cm to several centimeters. Tumors infiltrate multiple organs and lymph nodes are typically massively enlarged with multifocal white nodules on cut surface. Common findings include: heart with multifocal white epicardial, myocardial, and endocardial nodules (particularly right atrium); thickened abomasal wall with ulcerated mucosa; enlarged sublumbar lymph nodes palpable per rectum; white extradural masses compressing spinal cord; and retrobulbar soft tissue masses.

Treatment and Prognosis

There is NO treatment for BLV infection or lymphosarcoma. The prognosis for cattle with clinical lymphosarcoma is grave, and euthanasia is recommended to prevent suffering. Parenteral corticosteroids may transiently decrease severity of clinical signs but do not alter disease progression.

Prevention and Control

There is NO vaccine available for BLV. Prevention relies entirely on management practices to reduce transmission. Eradication has been successful in many European countries through testing and slaughter programs.

Key Prevention Strategies

• Single-use needles: Never reuse needles between animals for injections or blood collection
• Change rectal sleeves: Use new palpation sleeve for each animal during rectal exams
• Equipment disinfection: Clean and disinfect dehorning equipment, ear taggers, tattoo pliers between animals
• Fly control: Implement biting fly control programs (ear tags, dust bags, sprays)
• Colostrum management: Feed pasteurized colostrum or colostrum replacer; do not feed colostrum from BLV-positive dams
• Calf separation: Early separation of calves from infected dams in dairy operations
• Testing new additions: Test all incoming cattle; isolate and retest at 45-90 days before introduction
• Herd segregation: Separate BLV-positive and BLV-negative animals into different management groups

Eradication Program Components

A successful BLV eradication program consists of: testing the entire herd; culling or segregating all positive animals; retesting herd in 30-60 days and repeating culling/segregation; PCR testing calves (to avoid maternal antibody interference); continuing testing until entire herd tests negative; and maintaining testing every 6 months once negative status is achieved.

Exam Focus: Testing without biosecurity is like bailing a leaky boat - both are essential for eradication. The combination of TESTING + MANAGEMENT CHANGES is required for success.

"SINGLE NEEDLE, SINGLE SLEEVE, SINGLE USE"

Remember the "Triple S" rule: Single-use needles, Single-use sleeves, Sanitize equipment between animals.