Host-Virus Interaction in Visna Infected Sheep

Twenty-four sheep infected intracerebrally with 3 × 105 TCID50 of visna virus have been observed over a period of 4 years in order to study some aspects of the host-virus interaction in this slow infection. All the sheep responded with neutralizing antibodies to the virus but viral activity was not inhibited. Viremia was demonstrated from 2 months to 4 years after inoculation in samples of whole blood with neutralizing antibody titers of 1:128 to 1:512 in the serum. Visna virus was grown from the cerebrospinal fluid of some sheep during the whole period of 4 years and from samples with both normal and elevated cell counts. Twelve of the 24 sheep died of paralytic visna at different times after inoculation. Eight of the 15 clinical cases began 30 to 43 months after injection, 5 began about a year, and 2 within the first 6 months after injection. They all had typical visna lesions in the central nervous system. Visna virus was grown from the choroid plexus, spleen, salivary gland and lungs in most cases and from the kidney in three cases. Three of the remaining 12 sheep now show clinical signs of visna but 9 are in good condition. The possibility that antigen-antibody reaction on the surface of infected glial cells plays a role in the demyelinating process is discussed.