Enzootic bovine leukosis

What it is

Enzootic bovine leukosis (EBL) is caused by bovine leukaemia virus (BLV), a Deltaretrovirus that integrates its genetic material (provirus) into the DNA of B lymphocytes and remains in the animal for life. Infection has three possible outcomes:

• Infection without signs: most infected animals only have antibodies and never become ill.
• Persistent lymphocytosis: a benign, sustained increase in blood B lymphocytes, in about a third of infected animals.
• Lymphoma (lymphosarcoma): malignant B-cell tumours in lymph nodes and organs, in less than 5% of infected animals, usually in adult cows.

The virus is transmitted with infected blood cells: shared needles, rectal palpation sleeves or dehorning, tattooing or ear-tagging equipment, blood-feeding insects such as horse flies, colostrum and milk, and occasionally in utero. It must be distinguished from sporadic bovine leukosis (juvenile, thymic and cutaneous forms), which is not caused by BLV.

Many European countries are officially free of EBL after eradication programmes, while in dairy cattle in other regions, such as North America, infection is very common.

Who it affects

Cattle, especially adult dairy cows. Prevalence rises with age and is higher in large herds and in those that take no measures to prevent blood-borne transmission. Lymphoma is rare in young animals.

Anatomy involved

• Heart: one of the most common sites of lymphoma, with a predilection for the wall of the right atrium. Tumour infiltration causes right-sided heart failure (distended jugular veins, brisket oedema), arrhythmias and pericardial effusion.
• Abomasum: lymphoma thickens the wall diffusely or nodularly; it can ulcerate and bleed (dark faeces, melaena) or impair abomasal emptying.
• Uterus: lymphoma forms masses or thickening of the uterine wall that can be palpated per rectum, together with enlargement of the pelvic lymph nodes.
• Other common sites are the superficial and internal lymph nodes, the retrobulbar tissue (exophthalmos) and the epidural space of the vertebral canal (pelvic limb paresis).

Clinical signs

The vast majority of infected animals show no signs. In lymphoma, signs depend on the organs affected: enlarged lymph nodes, weight loss and reduced milk yield, exophthalmos, pelvic limb paresis, right-sided heart failure, melaena or digestive disturbances, and masses palpable per rectum. Lymphoma usually runs a progressive, fatal course.

Diagnosis

• Serology: ELISA on serum or milk, including bulk-tank milk for herd status, and agar gel immunodiffusion. A seropositive animal is infected, but this does not prove that a tumour is a BLV lymphoma.
• PCR: detects the provirus and, quantitatively, measures the proviral load, useful to identify the animals that transmit most. It is also useful in calves with colostral antibodies.
• Haematology: persistent lymphocytosis.
• Confirmation of lymphoma: cytology or biopsy of lymph nodes or masses, ultrasonography (for example of the heart or abomasum) and necropsy.

Treatment and control

Lymphoma has no practical treatment in cattle: affected animals are culled for welfare and the carcass may be condemned. Control aims to break blood-borne and colostral transmission:

• One needle per animal, a new sleeve for each rectal palpation and disinfection of equipment.
• Colostrum from negative cows or heat-treated, and insect control.
• Test and cull positive animals when prevalence is low, or segregate them when it is high, prioritising removal of animals with the highest proviral load.
• Purchase of negative animals.

There is no vaccine in general use.

Key studies

• Ruggiero et al. (2019), field trial in 3 United States dairy herds in which cows with the highest proviral load and lymphocyte count were identified and preferentially culled or segregated. After 2–2.5 years, the incidence of new infections fell from 13.8 to 2.2 and the combined prevalence from 62.0% to 20.7%. There were few herds and no control group.
• Shrestha et al. (2024), 3-year study in 10 dairy herds that took proviral load into account in culling decisions. Within-herd prevalence fell significantly in 4 herds and incidence in 9. Proviral load remained fairly stable, so a single test per cow may suffice. There was no control group.
• Nekouei et al. (2016), historical cohort of 4052 cows from 348 Canadian herds (1858 seropositive and 2194 seronegative). Positive cows consistently had a higher probability of being culled or dying, and those that lived 2 or 3 lactations produced between 1171 and 2554 kg less lifetime milk than negative cows. It is an observational study.
• Saeedi-Moghaddam et al. (2024), systematic review and meta-analysis of 17 studies, 12 of them meta-analysed, on BLV and human breast cancer. It found a statistical association (OR 2.12; 95% CI 1.77–2.54), with substantial heterogeneity between studies. It rests on observational studies and does not show a causal relationship; the possible zoonotic role of BLV remains unconfirmed.