Delayed-onset enzootic bovine leukosis possibly caused by superinfection with bovine leukemia virus mutated in the pol gene

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leucosis (EBL), to which animals are most susceptible at 4–8 years of age. In this study, we examined tumor cells associated with EBL in an 18-year-old cow to reveal that the cells carried at least two different copies of the virus, one of which was predicted to encode a reverse transcriptase (RT) lacking ribonuclease H activity and no integrase. Such a deficient enzyme may exhibit a dominant negative effect on the wild-type RT and cause insufficient viral replication, resulting in delayed tumor development in this cow.