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 occurs almost exclusively via infected cells, i.e. through direct (injuries) or indirect (instruments, used syringes) blood contact, rarely also 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 2023.

Enzootic bovine leukosis is caused by the bovine leukosis virus (BLV), which belongs to the family Retroviridae, genus Deltaretrovirus. It is therefore an enveloped, 80-120 nm RNA virus. Like some other viruses from the Retroviridae family, BLV is also oncogenic and is therefore categorised as an oncornavirus. B lymphocytes are the target cells of the virus. BLV is spread worldwide. The clinical picture of bovine leucosis was already described in the late 19th century, but the pathogen was not identified until 1969.

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

Transmission

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

Cattle, water buffalo and capybara are naturally infected with BLV. Following experimental infections, sheep can develop clinical symptoms of the tumorous form. Other animal species such as deer, goats, pigs, buffalo, rabbits, cats and dogs have reacted to experimental infection by producing antibodies.

Symptoms

Most infections with BLV are subclinical. After an incubation period of 6 months to 3 years, 30-70 % of infected cattle develop persistent lymphocytosis, which is characterised by an increase in circulating B lymphocytes. In animals over 2 years old, for example, lymphocyte counts below approx. 7,000 per µl are normal and above approx. 10,000 per µl are considered leucotic. Only in 0.1-10 % of all infected animals and 10-30 % of cattle with persistent lymphocytosis does tumour formation (lymphosarcoma) occur, often after years of preleukosis. This results in three clinical forms 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
• Consequences of tumour formation: Difficulty breathing, difficulty swallowing, exophthalmos, lameness, tympania
• General weakness
• Digestive disorders
• Decrease in milk yield
• Fever
• Neurological manifestation with corresponding symptoms

Clinically inapparent infected animals form the pathogen reservoir of BLV and are a risk with regard to the transmission of the disease, as they must be regarded as potential virus shedders for the rest of their lives. The long incubation period and the slow course of the disease are factors that favour the maintenance of the infection in a herd of cattle. Furthermore, economic losses occur in herds with clinically inapparent BLV infections due to the decline in milk yield, reduced conception rates and various secondary infections.

The frequency of tumour formation increases with age. In addition, some cattle breeds have a genetic predisposition to enzootic bovine leukosis. For example, cattle of the Holstein-Friesian breed are statistically significantly more likely to develop tumours than Brown Swiss or Simmental cattle.

Control/prevention

Enzootic bovine leukosis is a notifiable animal disease. Monitoring is carried out in Austria in accordance with the Cattle Health Monitoring Ordinance, Federal Law Gazette II No. 334/2013, which is based on the Animal Health Act (TGG), Federal Law Gazette I No. 133/1999 as amended. Monitoring is anchored at EU level by Delegated Regulation (EU) 2020/689. Samples are taken and analysed according to a risk-based sampling plan. All reactants must be handed over to slaughterhouses for slaughter within the culling period on the basis of the aforementioned requirements.

Austria has been officially recognised as free of enzootic bovine leukosis since 1999.

Vaccination

Vaccination against enzootic bovine leukosis is prohibited in Austria. To date, there is also no authorised vaccine.

Cattle infected with BLV usually produce specific antibodies that can be detected serologically (ELISA technique or AGID) and in milk (ELISA only) from 3-16 weeks post infection. ELISA tests are generally much more sensitive than the AGID.

Maternal antibodies are detectable up to 6-7 months after birth and offer protection against infection. 2-6 weeks before and 1-2 weeks after birth, cows can have serum antibodies that are not detectable in the AGID. During this period, the antibodies move from the mother's bloodstream into the colostrum. Therefore, when using the AGID test, a negative result is not conclusive and should be repeated. However, the AGID test can be carried out with colostrum during this phase.

The real-time PCR acts as a rapid and sensitive method for confirming a serological diagnosis from blood or organs or as a method for acute infection diagnostics before antibodies have formed.

Sample material:

• Blood (EDTA/serum)
• Milk, colostrum

• tumour-altered organs, animal body, lymph nodes, spleen

Possible detection methods:

• ELISA (serum, milk): Antibodies against BLV
• Agar gel immunodiffusion (AGID) (serum): Antibodies against BLV
• Molecular biological detection using PCR (EDTA blood, lymph nodes, altered organs, spleen)
• Virus isolation and cultivation (EDTA blood)