Slow Diseases Caused by Conventional Viruses in Animals
Visna is a disease of sheep caused by visna virus, a lentivirus. It is a single-stranded RNA virus with an RNA-dependent DNA polymerase in the virion. Integration of the DNA provirus into the host cell DNA appears to be responsible for persistence of the virus within the host. Clinically, the condition manifests after a long incubation period with a prolonged progressive course. It has an insidious onset with paresis, progressing to total paralysis and death. The viruses are present in the saliva, blood, and CSF of the infected animal. High levels of serum antibodies are present in the serum, but these are not protective.