Safeguarding public and animal health
Enzootic bovine leukosis
What is enzootic bovine leukosis (EBL)?
EBL is a contagious disease of cattle caused by the retrovirus, bovine leukaemia virus (BLV). The disease causes a fatal malignant cancer in a small percentage of infected animals and can be responsible for major economic losses.
Cattle are the main susceptible species, though infections have very rarely been described in buffaloes and sheep.
There has never been any evidence of the transmission of BLV to humans, by any route. The Advisory Committee for Dangerous Pathogens does not list it as an agent capable of human infection.
As with other retrovirus infections, infected animals remain infected for life. It has serious implications for animal health and welfare, and economic losses including international trade.
Geographical location
EBL is widespread in many parts of the world, including North and South America and Eastern Europe. The disease was last diagnosed in the UK in 1996 and we are designated as an EBL-free country.
Transmission
Transmission most commonly occurs following direct contact between infected and uninfected cattle, either vertically from cow to calf, or horizontally between cattle in close contact.
The most likely source of infection is through contact with imported cattle. Transmission via semen and embryo transfer is considered theoretically possible, but low risk.
Clinical signs
Most animals infected with the virus (about 70%) show no clinical signs. Where they do occur, the signs usually appear in animals between four and eight years old. The clinical signs are very varied and related to the organ systems involved. They may include:
- weight loss with or without reduced appetite
- anaemia
- decreased milk yield
- enlarged lymph nodes
- partial paralysis of the hind legs
- fever
- abnormal breathing
- bulging eyes
- diarrhoea
- constipation
Progression of infection to clinical disease is invariably fatal.
Post-mortem features
The disease principally involves leukaemia and lymphosarcoma. This is in the form of a malignant mixed lymphoid/connective tissue tumour presenting as firm white tumour masses in any organs but most commonly in lymph nodes.
Other organs involved may include the gastro-intestinal tract, heart, spleen, liver, kidney, lung, brain and uterus.
Diagnosis
Laboratory tests on blood, milk and tissues are required to confirm the disease. Most commonly, diagnosis is based on serology by looking for antibody to the virus in the blood of the animal. Antibodies can be detected 3-16 weeks after infection.
Tests include agar gel immunodiffusion test (AGIDT), enzyme-linked immunosorbent assay (ELISA), and polymerase chain reaction (PCR).
Calves under six months can be serologically positive due to persistence of maternal antibodies.
Epidemiological features
Natural transmission depends on the transfer of infected cells, for example during parturition, close contact, and via colostrum and milk. Transmission can also occur artificially, especially by blood-contaminated needles, surgical equipment or gloves used for rectal examinations. In areas of the world where prevalence is high, biting flies may play a role in transmission.
Disease control
There is no treatment or vaccine available. The disease can only be eradicated by the testing and elimination of reactors.
To control the spread of the disease, movement restrictions, an epidemiological investigation and additional testing may also be applied.
Good biosecurity also plays a very important part in preventing the spread of infections and EBL is no exception. See the Defra leaflet on biosecurity for further guidance.
EBL is a notifiable disease and therefore you must contact your local
Animal Health Office if you suspect signs of this disease.