Long-term protection against bovine leukaemia virus replication in cattle and sheep.

In this report, we have evaluated the ability of two different types of live attenuated bovine leukaemia virus (BLV) variants (BLV DX and BLV 6073) to protect cattle and sheep against a heterologous wild-type BLV challenge. Four months after challenge, the protection of the vaccinated animals was effective in contrast to unvaccinated controls. However, long-term protection (18 months after challenge) was observed only in six out of seven animals, one of the vaccinated cattle being infected 12 months after challenge. A second prospective approach investigated the injection of naked plasmid DNA. Two sheep were injected with plasmid DNA encoding the BLV envelope proteins; the challenge virus infection was delayed but could not be completely abrogated. Our results demonstrate that vaccines based on live attenuated viruses and naked DNA injections are able to delay BLV infection, although complete protection cannot be achieved. In addition, our data cast light onto the need to perform long-term vaccination trials because challenge superinfection can occur even after apparent protection for 12 months.