Bovine leukaemia virus
| Bovine leukemia virus | |
|---|---|
| Virus classification | |
| Group: | Group VI (ssRNA-RT) |
| Order: | Unassigned |
| Family: | Retroviridae |
| Subfamily: | Orthoretrovirinae |
| Genus: | Deltaretrovirus |
| Species: | Bovine leukemia virus |
Bovine leukemia virus (BLV) is a retrovirus which causes enzootic bovine leukosis in cattle. It is closely related to the human T‑lymphotropic virus type 1 (HTLV-I). BLV may integrate into the genomic DNA of B‑lymphocytes as a DNA intermediate (the provirus), or exist as unintegrated circular or linear forms. Besides structural and enzymatic genes required for virion production, BLV contains an oncogene coding for a protein called Tax and expresses microRNAs of unknown function. In cattle, most infected animals are asymptomatic; leukemia is rare (about 5% of infected animals), but lymphoproliferation is more frequent (30%).
Many potential routes of BLV transmission exist. Transmission through procedures that transmit blood between animals such as gouge dehorning, vaccination and ear tagging with instruments or needles that are not changed or disinfected between animals is a significant means of BLV spread. Rectal palpation with common sleeves poses a risk that is increased by inexperience and increased frequency of palpation. Transmission via colostrum, milk, and in utero exposure is generally considered to account for a relatively small proportion of infections. Embryo transfer and artificial insemination also account for a small number of new infections if common equipment and/or palpation sleeves are used. While transmission has been documented via blood feeding insects, the significance of this risk is unclear. Transmission relies primarily on the transfer of infected lymphocytes from one animal to the next, and BLV positive animals with lymphocytosis are more likely to provide a source for infection. Virus particles are difficult to detect and not used for transmission of infection.
In general, BLV causes only a benign mononucleosis-like disease in cattle. Only some animals later develop a B-cell leukemia called enzootic bovine leukosis. Under natural conditions the disease is transmitted mainly by milk to the calf.
The variety of organs where white blood cells occur explains the many symptoms: enlargement of superficial lymph nodes, a digestive form, a cardiac form, a nervous form, a respiratory form, and others. Lymph node enlargement is often an early clinical sign. An unexpected clinical finding is protrusion of the conjunctival membrane, due to enlargement of retro-ocular lymph nodes.
...
Wikipedia