Enzootic bovine leukosis (ERL) is a viral disease of cattle caused by the bovine leukosis virus (BLV) that can lead to tumour development in various organs. According to current knowledge, there is no risk of infection for humans.

The bovine leukosis virus is widespread worldwide. In Europe, due to successful monitoring and control programmes, there are only isolated cases of the disease.

Enzootic bovine leukosis occurs mainly in cattle, buffalo, water buffalo and occasionally in sheep.

Transmission is mainly by direct contact and iatrogenic (caused or aggravated by medical procedures), rarely intrauterine. Blood-sucking insects are possible vectors.

Between the infection and the appearance of tumors are usually months to years

Infections with the bovine leukosis virus are usually subclinical. In 0.1-10 % of infected animals, usually from the age of 3 years, the course becomes severe with the development of tumours. The clinical symptoms depend on the organ system in which the tumours develop.

It no therapy

Enzootic bovine leucosis is a notifiable animal disease according to the Animal Diseases Act. There is currently no effective vaccine. The bovine leukosis virus can be inactivated by commercially available disinfectants.

Austria has been officially free of enzootic bovine leucosis (ERL) since July 1999. Risk-based monitoring of all cattle farms via milk and blood samples is carried out annually. The Austrian cattle herds were also officially free of enzootic bovine leukosis in 2021.

Enzootic bovine leukosis is caused by bovine leukosis virus (BLV), which belongs to the family Retroviridae, subfamily Orthoretroviridae and genus Deltaretrovius. Thus, it is enveloped, 80-100 nm RNA virus. Like some other viruses of the Retroviridae family, BLV is oncogenic and is thus classified as an oncornavirus. B lymphocytes are the target cells of the virus. BLV is distributed worldwide with the exception of Europe, Australia and New Zealand. The clinical picture of bovine leukosis was described as early as the late 19th century, but the pathogen was not identified until 1969.

Differential diagnoses include sporadic leukosis and multifocal lymphomas of as yet unknown origin.

Transmission

Transmission is mainly horizontal through infected lymphocytes, e.g. iatrogenic during surgical interventions or herd vaccinations. The virus is passed from animal to animal by close contact. Less frequently, transmission of the virus occurs in utero. Blood-sucking insects can serve as vectors. BLV-infected cells are also shed in colostrum and milk. However, infection of calves by this route is of minor importance.

Infections with BLV occur naturally in cattle, water buffalo and capybara. Following experimental infections, sheep may develop clinical signs of the tumorous form of progression. Other species such as deer, goats, pigs, buffalo, rabbits, cats, and dogs responded to experimental infection by producing antibodies.

Symptomatology

Most infections with BLV are subclinical. 30-70 % of infected cattle develop persistent lymphocytosis after an incubation period of 6 months to 3 years, characterized by an increase in circulating B lymphocytes of up to 80 %. In 0.1-10 % of all infected animals or 10-30 % of cattle with persistent lymphocytosis, tumour formation (lymphosarcoma) occurs, often after years of preleukosis. This results in three clinical courses of the disease:

• Clinically inapparent infection
• Persistent lymphocytosis (preleukosis)
• Tumorous form: leads to the death of the animal

The clinical symptoms are strongly dependent on the localisation of the tumours and are therefore varied. The following non-specific symptoms may develop:

• Weight loss, inappetence
• Enlargement of the lymph nodes
• Consequential symptoms of tumor formation: Shortness of breath, difficulty swallowing, exophthalmos, lameness, tympany.
• General weakness
• Digestive disorders
• Decrease in milk yield
• Fever
• Neurological manifestation with corresponding symptoms

Clinically inapparently infected animals form the pathogen reservoir of BLV and are a danger with regard to the transmission of the disease, as they must be regarded as potential virus excretors for life. The long incubation period as well as the slow course of the disease are favourable factors for the maintenance of the infection in a cattle herd. Furthermore, economic losses occur in herds with clinically inapparent BLV infections due to decreased milk yield, decreased conception rates, and various secondary infections.

The incidence of tumor formation increases with age. In addition, some breeds of cattle have a genetic predisposition to enzootic bovine leukosis. For example, cattle of the Holstein-Friesian breed are statistically significantly more likely to contract the tumorous form than Brown or Simmental cattle.

Control/Prevention

Enzootic bovine leucosis is a notifiable animal disease. In Austria, surveillance is carried out according to the Rindergesundheits-Überwachungs-Verordnung, BGBl. II No. 334/2013, which is based on the Tiergesundheitsgesetz (TGG), BGBl. I No. 133/1999 as amended. Monitoring is anchored at EU level by the requirements of Directive 64/432/EEC. Samples are taken and examined according to a risk-based sampling plan. All reagents are to be delivered to slaughterhouses for slaughter within the culling period on the basis of the aforementioned specifications.

Austria has been officially free of enzootic bovine leucosis since 1999.

Vaccination

Vaccination against enzootic bovine leucosis is prohibited in Austria. To date there is also no approved vaccine.

Cattle infected with BLV usually produce specific antibodies that are detectable serologically (ELISA technique or AGID) from 3-16 weeks post infection. ELISA methods for testing blood or milk samples for antibodies to BLV in blood or milk have been published and various commercial ELISA tests are commercially available. ELISA tests are usually much more sensitive than AGID.

Maternal antibodies are detectable up to 6-7 months after birth and provide protection against infection. At 2-6 weeks prepartum and 1-2 weeks postpartum, cows may have serum antibodies that are not detectable in the AGID. The antibodies move from the dam's bloodstream to the colostrum during this period. Therefore, when using the AGID test, a negative result is not conclusive and should be repeated. However, the AGID test can be performed with colostrum during this period.

Real-time PCR functions as a rapid and sensitive method for confirming a serological diagnosis from blood or organs or as a method for acute infection diagnosis before antibody formation has occurred.

Sample material:

• Blood (EDTA/serum)
• milk, colostrum

Possible detection methods:

• ELISA (serum, milk): antibodies against BLV.
• Agar gel immunodiffusion (AGID) (serum, colostrum): antibodies against BLV
• Molecular biological detection by PCR (EDTA blood)
• Virus isolation and culture (EDTA blood)