Enzootic bovine leukosis is caused by Bovine Leukemia Virus (BLV), which belongs to the family Retroviridae, subfamily Orthoretroviridae and genus Deltaretrovius. Thus, it is an 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 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. Experimental infections of sheep may lead to 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.
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 death of the animal
The clinical symptoms are strongly dependent on the localization of the tumors and are therefore varied. The following non-specific symptoms may develop:
- Weight loss, inappetence
- Enlargement of the lymph nodes
- Consequential symptoms of tumor formation: Respiratory distress, difficulty swallowing, exophthalmos, lameness, tympany
- General weakness
- Digestive disorders
- Decrease in milk yield
- 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 significantly more likely to contract the tumorous form than Brown or Simmental cattle.
Enzootic bovine leukosis 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 leukosis since 1999.
Vaccination against enzootic bovine leukosis is prohibited in Austria. To date there is also no approved vaccine.