Louping-ill encephalomyelitis in the sheep: I. The relationship of viraemia and the antibody response to susceptibility

Abstract

Of 33 six-month-old Scottish Blackface sheep inoculated subcutaneously with a high dose of louping-ill virus, 22 developed fatal neurological disease at between 7 and 12 days after inoculation and 11 survived without obvious symptoms of encephalomyelitis. Virus was demonstrated in plasma from 90 per cent. of these sheep at 24 hours after inoculation. Circulating virus was detected in all except one animal, which was moribund on day 7. Both mean and maximum levels of viraemia tended to be higher in those that succumbed. Viraemia terminated much more rapidly in the survivors. This was very definitely associated with the earlier appearance of both neutralizing and haemagglutination inhibiting antibodies in sheep that survived. The immune response in louping-ill seems to be essentially protective. Initial activity was due to IgM, which was progressively replaced by IgG. Neutralizing antibody and virus circulated concurrently for several days. Sera from all except 3 animals contained pre-existing heat-stable neutralizing substances, which had no obvious influence on the outcome of infection. No symptoms were seen in any of 8 control sheep.