Bovine leukemia virus
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Bovine leukemia virus (BLV) is a bovine virus closely related to HTLV-I, a human tumor virus.
BLV is a retrovirus which integrates a DNA intermediate as a provirus into the DNA of B-lymphocytes of blood and milk. It contains an oncogene coding for a protein called Tax. Nevertheless in its natural host the cattle leukemia is rare. Because the oncogenic properties of the virus were discovered early, a search for evidence of pathogenecity humans started soon after discovery. Mostly farm workers drinking raw milk were tested for disease, especially for leukemia. But neither leukemia nor other signs of infection could be detected. So many in many states it was not tried to get rid of this infection**.
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 as long as common equipment and/or palpation sleeves are not used. While transmission has been documented via blood feeding insects, the significance of this risk is unclear. The bottom line appears to be that transmissino relies primarily on the transfer of infected lymphocytes from one animal to the next and that BLV positive animals with lymphocytosis are more likely to provide a source for 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. Infected lymphocytes transmit the disease too. So for artificial infection infected cells are used or the more stable and even heat resistant DNA. Virus particles are difficult to detect and not used for transmissing of infection. It is possible that a natural virus reservoir exists in the water buffalo.
In Europe attempts were made to eradicate the virus by culling infected animals. The first country considered to be free of infection was Denmark**. Soon the United Kingdom followed. Like the North American states, those of the Eastern block in Europe did not try to get rid of the virus. But the Eastern Europe states started to become leukosis free after the political changes at the end of the last century.
Natural infection of animals other than cattle and buffalo are rare, although many animals are susceptible to artificial infection. After artificial infection of sheep most animals succumb to leukemia. Rabbits get a fatal AIDS like disease similar to rabbit-snuffles, different from the benign human snuffles. But it is not known whether this naturally occurring rabbit disease is linked to BLV infection.
[edit] Research
Because of the close relationship between BLV and HTLV-I the research on BLV is important. One can use the experience with BLV for understanding HTLV-I induced diseases like ATL the adult T-cell leukemia and HMS/TSP like neurological disorders.
[edit] External links
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