[Slow virus infections of the nervous system].

The term "slow infection" was coined originally in the veterinary literature to describe several trans­ missible diseases of sheep. 1 Two of these diseases, scrapie and visna, have become the prototypes of slow infection of the central nervous system (CNS). After inoculation of sheep with tissues from an in­ fected sheep, a latent period of one or more years ensues, during which the sheep appears well. This is followed by the insidious onset of neurological signs that progress without fever for one to six months and usually lead to death. Scrapie is characterized prima­ rily by ataxia, and visna by progressive paralysis. Pathologically, the diseases are quite distinct. The lesions in scrapie are confined to the CNS, where there is a proliferation of astrocytes and degeneration of neurons with cytoplasmic vacuolization. By contrast, the CNS lesions of visna are characterized by inflammation and demyelination. The agents re­ sponsible for these two slow infections also differ greatly. The scrapie agent has been transmitted to a wide variety of other animals, but the agent does not cause cytopathic changes in cell culture, and no virus particles have been definitely identified in infectious tissues by electron microscopy. Infectivity of tissue is remarkably stable upon exposure to physical and chemical treatments that inactivate classic viruses. Finally, animals naturally or experimentally infected with scrapie fail to develop any evidence of an im­ mune response against the agent. 2 Recently a pro­ tein has been associated with infectivity, and a fibril that could represent the infectious agent has been identified by electron microscopy. 3,4 In contrast, visna virus is an enveloped RNA retrovirus. Although the agent can be transmitted only to sheep, it can be grown in cell cultures of a varietyof species in which it causes acute cytopathic changes. Furthermore, in