Bovine Leukemia Virus, an Immunosuppressive Retrovirus, Recently Found in 1 in 3 Humans
Bovine leukemia virus (BLV) is an immunosuppressive and cancer-causing virus from the delta-retrovirus family, whose natural host is cows and bulls. In the US it is estimated that 44% of dairy cows and 10% of beef cattle are infected with BLV.
Until recently, BLV was not thought to infect humans, but in the last few years it has emerged that at least around 1 in 3 humans are infected with BLV.
One 2003 study found that 74% of humans have antibodies to bovine leukemia virus, so BLV's prevalence in humans could be as high as that; but the authors point out that these antibodies do not necessarily prove that humans are actually infected with the virus, because the antibodies could just be a response to heat-denatured BLV antigens consumed in food.
However, irrefutable evidence that BLV infects humans comes from one US 2015 study and an Australian 2017 study which respectively found that 29% and 41% of healthy women have bovine leukemia virus DNA in their breast tissue. So this is clear evidence that at least around one-third of the human population at least is infected with the BLV retrovirus.
These US and Australian studies also respectively found that 59% and 81% of women with breast cancer had BLV in their breast tissues, thus suggesting bovine leukemia virus may play a role in breast cancer. Although a study which sequenced the genome of some breast cancers found no evidence of BLV DNA in these tumors, which argues against the link between BLV and human breast cancer. Thus at present the jury is still out regarding whether BLV can cause breast cancer.
How is Bovine Leukemia Virus Transmitted to Humans?
How do humans get infected with BLV? The Australian study says: "It is not known how humans acquire infection with BLV. It could be from other humans already infected with BLV or from consumption of unpasteurized dairy or undercooked beef products from BLV infected cattle."
Bovine leukemia virus is present in raw unpasteurized milk, so it is possible this virus passed into human populations millennia ago, in an era long before pasteurization of milk was introduced, and is now transmitted from human to human. The human practice of drinking another animals' milk, which became common after the agricultural revolution 10,000 year ago, could have first introduced this cow virus into human populations.
It is also possible that humans are even today catching BLV directly from cows by consuming unpasteurized dairy or undercooked beef. At this stage we don't know how the virus gets into humans. (A study on people who consume raw milk found no increased incidence of breast cancer, though.)
The Immunosuppressive Effects of Bovine Leukemia Virus
Bovine leukemia virus (which is also called bovine leukosis virus) is known to cause various forms of immune suppression in cows, which are as follows:
One study found that in cows, BLV induces TGF‐beta secretion from regulatory T cells. TGF‐beta in turn was found to suppress the secretion of the antiviral Th1 cytokines interferon gamma and TNF-alpha by CD4 T cells, and TGF‐beta suppresses natural killer cells.
One study found that in cows, BLV decreased IgM antibodies and decreased CD4 T-cells in the blood. And a correlation was found between BLV infection and lack of spontaneous recovery from Trichophyton verrucosum (ringworm fungus) infection, indicating an immunosuppressive effect of BLV in cows.
The paper also notes that other BLV studies have found significant association between herd BLV status and general disease incidence in the herd. In other words, cow infected with BLV were generally more prone to getting other diseases.
This study also details the B-cell and T-cell abnormalities found in cows positive for BLV. The study notes that BLV infection also negatively affects the functioning of the monocytes and macrophages of the immune system.
This article states that in cows, the primary cellular target of BLV infection is B-cells. Once infected with the virus, B-cell populations increase due to abnormal growth, resulting in a persistent lymphocytosis and eventually lymphomas (cancer of the lymphatic system) in cows.
At present we don't know if BLV causes in humans the same immunosuppressive effects in creates in cattle. But BLV is closely related to the human retrovirus virus HTLV-1, and certainly in the case of human HTLV-1 infection, case studies of opportunistic infections with cytomegalovirus and herpes zoster have been reported.
But given cows infected with BLV are generally more prone to getting other diseases, I'd like to see research on whether humans infected with BLV also become more prone to developing disease.
Edited by Hip, 10 April 2019 - 07:35 PM.
