The cows presented with high white blood cell counts, referred to as persistent lymphocytosis (PL) which is associated with the development of bovine leukemia/lymphosarcoma.

[citation needed] The first animal studied was an eight-year-old Holstein cow (R-29), her white blood cell count was elevated and her physical condition was steadily declining; after delivering a calf she weakened and became severely emaciated.

[citation needed] When the isolated BIV was inoculated into colostrum deprived young calves, they developed elevated leukocyte counts.

These calves, however, did not decline as R-29 did, which led researchers to believe that the isolated BIV was not the causative agent of the bovine leukemia/lymphosarcoma.

It was put into storage and went unstudied until the discovery that acquired immunodeficiency syndrome (AIDS) was caused by HIV.

The attached virus enters the cell by one of two ways, receptor mediated endocytosis or viral envelope-cell membrane fusion.

Other accessory genes include vif (viral infectivity factor), tat (transcription activator), and rev (protein expression regulator).

In primate lentiviruses there is usually an ORF for nef (negative factor); this is not present in BIV.

BIV has a structure like all retroviruses, and contains two copies of its positive sense single stranded RNA genome.

Some of the spread is attributed to reuse of contaminated needles used in vaccinations, communal sharing of colostrum by calves, and failure to completely sterilize instruments after invasive treatments.