EARLY DETECTION OF BOVINE LEUKEMIA-VIRUS BY USING AN ENZYME-LINKED ASSAY FOR POLYMERASE CHAIN REACTION-AMPLIFIED PROVIRAL DNA IN EXPERIMENTALLY INFECTED CATTLE

Bovine leukemia virus is the causative agent of bovine leukosis and has been described in many countries throughout the world. We describe here a sensitive and readily applicable assay for the detection of bovine leukemia proviral DNA. Detection relies on initial amplification of proviral DNA by using polymerase chain reaction (PCR) followed by an enzyme-linked assay (PCR-ELA). Amplification is carried out by using one biotinylated primer and a second primer containing the GCN4 protein binding site. DNA is detected by a colorimetric assay after it is coupled to GCN4-coated plates and subsequently incubated with horseradish-streptavidin peroxidase and the appropriate substrate to produce a chromogenic reaction. It was possible to detect proviral DNA for all of eight bovine leukemia virus-infected calves by 2 weeks postinfection. Use of the more conventional agar gel immunodiffusion assay failed to reveal the presence of the virus in any of the animals up to 4 weeks postinfection. The PCR-ELA detected as little as 0.1 to 0.2 ng of amplified DNA per well, which compares very favorably with ethidium bromide staining of gels, by which 1 to 2 ng per lane was detected. This method lends itself to mass screening, is carried out in a similar way to an enzyme-linked immunosorbent assay, and does not require gel electrophoresis or the use of radioactive gene probes.