Sera and cerebrospinal fluids from normal uninfected sheep contain a visna virus inhibiting factor

Abstract

VISNA is a slow virus disease of sheep characterised by a long subclinical period followed by a slowly progressing clinical illness¹. The persistent visna infection is not preceded by an acute clinical phase or by rapid virus replication. This is puzzling because visna virus replicates rapidly in sheep cell cultures^2,3. In an attempt to resolve this paradox we inoculated visna virus into sheep cell cultures maintained in fluids with 90% sheep serum but otherwise containing the same nutrients as control cultures maintained without serum. We found that viral growth was much slower in cultures with 90% serum than in control cultures. Further experiments showed that normal sheep sera inactivate visna virus rapidly at 37 °C. We conclude that sheep sera have a visna virus inhibiting factor (VIF) and hypothesise that VIF inhibits early spread of visna virus in inoculated sheep and may be involved in initiating the persistent infection.