Visna virus: A slow virus with an RNA dependent DNA polymerase

VISNA virus is the cause of a progressive neurological disease of sheep which results in paralysis and death^1,2. After inoculation, the virus multiplies within the host, but illness may not be manifest for months or years³. Visna virus shares some physical, chemical and biological properties with oncogenic RNA viruses⁴. The similarities include virion and core size, site of maturation, growth properties, nucleic acid composition⁵, and sensitivity to physical and chemical agents. The inhibition of visna virus replication by low concentrations of actinomycin D (ref. 5) indicates a requirement for DNA synthesis for the growth of this agent. An RNA dependent DNA polymerase has been discovered in many oncogenic RNA viruses^6,7. This communication demonstrates the presence of similar enzyme activity in visna, a virus without known oncogenic potential.