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Nature of the disease
Enzootic Bovine Leukosis is contagious disease of cattle caused by the retrovirus, bovine leukaemia virus (BLV). The disease causes fatal malignant cancer in a small percentage of infected animals and can be responsible for major economic losses.
OIE List B disease
Susceptible species
Cattle although infections have rarely been described in buffaloes, sheep and capybaras.
Enzootic Bovine Leukosis is ubiquitous and outbreaks or serological evidences have been recorded in a number of Pacific countries or territories (Australia, French Polynesia, Guam, New Zealand, Northern Mariana's, Palau, Tonga)
Clinical signs 
Most infected animals (about 70% ) do not develop the disease. The clinical signs usually appear in animals between 4 and 8 years old. When present, the clinical signs observed are related to the organ systems involved and include (non exhaustive list):
  • Weight loss with or without reduced appetite,
  • Anaemia
  • Decreased milk yield,
  • Enlarged external and enlarged internal lymph nodes,
  • Partial paralysis of the hind legs,
  • Fever,
  • Abnormal breathing,
  • Bulging eyes,
  • Diarrhoea,
  • Constipation

Animals that developed tumours will generally progressively loose condition and die.

A part from the enzootic leukosis described above, other lymphoma are described in Cattle as Sporadic Bovine Leukosis, however only those involving the BLV are considered as Enzootic Bovine Leukosis, they are:

  • Calf form which affects animals less than 6 months old. It is characterised by a general lymphadenopathy and widespread tumour metastasis.
  • Thymic form in animals of 6-8 months old with a thymic tumour and sometimes extension into the thorax.
  • Skin leucosis is a non fatal form. Young adults develop superficial cutaneous tumour that disappear spontaneously after a few weeks.
Post-mortem findings 
Firm white tumour masses in any organs and more commonly in lymph nodes. Organs most frequently involved are the abomasum, right auricle of the heart, spleen, intestine, liver, kidney, omasum, lung, and uterus.
Differential diagnosis
Due to the diversity of location and clinical forms of the disease the differential diagnosis is extremely variable. 
Specimens required for diagnosis
Experimental diagnosis can be attempted by identification of the virus from blood samples in EDTA. Techniques involve culture of the virus for isolation, PCR and sheep inoculation.

More commonly diagnosis is based on serological reactions from serums. Antibodies can first be detected 3-16 weeks after infection. Tests include Agar immunodiffusion and Milk ELISA. Calves under 6 months can be false positive due to persistence of maternal antibodies. 

Insects may be involved in the transmission of the bovine leukosis virus (BLV). Natural transmission depends on the transfer of infected cells, for example during parturition. Artificial transmission occurs, especially by blood-contaminated needles, surgical equipment, gloves used for rectal examinations. Lateral transmission in the absence of these contributory factors is usually slow. In regions where blood-sucking insects occur in large numbers, especially tabanids, these may transmit the virus mechanically.
Risk of introduction
Enzootic Bovine Leukosis can be introduced through importation of infected live animals, semen and embryos and in a less extent contaminated material and arthropod vectors. 
Control / vaccines
The disease can be eradicated by testing and elimination of reactors. However the cost of such programme must be balanced with the economical benefit of it, especially at national level, zoning is an economic alternative where a particular zone concentrates farms with international trade opportunities. Sometimes eradication in large commercial herds with control of entering cattle is more realistic. 

To control the spread of the disease, zoning and testing should be applied, control of insects and ticks is also necessary. There is no economic treatment and no vaccine. 

  • Bovine Leukosis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 521-522
  • Department for Environment, Food and Rural Affairs (DEFRA), 2002, last consulted on the 12/08/2003
  • Enzootic Bovine Leukosis, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 954-965
  • Office International des Epizooties, 2002