Difficulty in detecting in vivo extracellular infective virus in cattle naturally infected with bovine leukemia virus

A search was made for the presence in vivo of infective bovine leukemia virus (BLV) in secretions and excretions as well as in several tissues from five bulls, three cows and one calf. Two of the bulls and three of the cows were natural tumour cases of enzootic bovine leukosis, another three of the bulls were natural cases of persistent lymphocytosis and the calf was a natural tumour case of juvenile bovine leukosis. BLV infection was confirmed for all cattle, except the juvenile leukosis case, by detection of BLV-specific agar gel immunodiffusion (AGID) antibodies. Cell-free preparations were made from homogenates of lymphocytes, lymph nodes, spleens, livers, intestines, urinary bladders, salivary glands, mammary glands, pools of prostate glands and testicles and feces as well as from plasma, urine and milk, by passing them through 5 micrometers membrane filters. BLV infectivity in these cell-free preparations was examined by syncytia infectivity assay using susceptible cell cultures of bovine or ovine origin. Infective BLV could not be isolated from any of these cell-free preparations of plasma, secretions, excretions and tissues, although it was isolated consistently from the in vitro cultures of viable lymphocytes obtained from BLV-infected cattle. There was no indication of BLV involvement in the case of juvenile bovine leukosis.