Bovine leukemia 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 closely related to the human T-lymphotropic virus type 1 HTLV-I. The natural host of BLV is cattle. BLV integrates into the genomic DNA of B-lymphocytes as a DNA intermediate (the provirus). 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 its natural host (the cattle) leukemia is rare (about 5% of infected animals) but lymphoproliferation is rather frequent (30%). Because the oncogenic properties of the virus were discovered early, a search for evidence of pathogenicity in humans started soon after discovery. Mostly farm workers drinking raw milk were tested for disease, especially for leukemia. It was discovered in 2003 that some humans carry antibodies reactive to BLV. In 2014, researchers discovered the presence of BLV positive cells in the human breast tissue. A case-controlled study was published in 2015 which suggests a possible association between breast cancer, and BLV. However, more recently, another case-control study conducted on Chinese patients did not find any association between BLV and breast cancer. In addition an exhaustive analysis of 51 whole genomes of breast cancers do not show any trace of BLV DNA and thus excludes a clonal insertion of BLV in breast tumor cells and strongly argues against an association between BLV and breast cancer.
High prevalence of virus was found from testing by USDA. "As part of the 2007 dairy study, bulk tank milk was collected from 534 operations with 30 or more dairy cows and tested with an Enzyme Linked-Immunosorbent Assay (ELISA) for the presence of antibodies against BLV. Results showed that 83.9 percent of U.S. dairy operations were positive for BLV (table 1)." BLV infection can be detected by ELISA or PCR.
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. A quote from a USDA fact sheet, "The high individual animal prevalence of BLV reported in the Dairy 1996 study suggests that testing and culling seropositive animals may not be a cost effective method to control the disease. Instead, preventing disease transmission by implementing preventive practices would likely be more cost-effective."
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 Pasteurella, different from the benign human snuffles. It is not known whether this naturally occurring rabbit disease is linked to BLV infection. "Although several species can be infected by inoculation of the virus, natural infection occurs only in cattle (Bos taurus and Bos indicus), water buffaloes, and capybaras. Sheep are very susceptible to experimental inoculation and develop tumours more often and at a younger age than cattle. A persistent antibody response can also be detected after experimental infection in deer, rabbits, rats, guinea-pigs, cats, dogs, sheep, rhesus monkeys, chimpanzees, antelopes, pigs, goats and buffaloes."
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