Inhibition of virus replication in the tonsils of pigs previously vaccinated with a Chinese strain vaccine and challenged oronasally with a virulent strain of classical swine fever virus.

After an oronasal (O.N.) infection with classical swine fever (C.S.F.) virus, virus multiplication can be detected in the tonsils from Day 2 post infection (p.i.) till death. The course of viral replication during the first 10 days after O.N. challenge exposure of pigs, previously vaccinated with a Chinese strain vaccine in the presence or absence of maternal antibodies, was studied using direct immunofluorescence techniques on cryostat sections and virus isolations. When piglets were challenged O.N. in the presence of maternal antibodies, virus replication in the tonsils still occurred. The multiplication period and the localization of the virus, however, were directly correlated to the maternal antibody levels. The maternal antibody level also seems responsible for the efficacy of the vaccination to prevent challenge virus replication in the tonsils: vaccination in the presence of low maternal antibody titers completely inhibited virus replication; vaccination in the presence of high maternal antibody titers only reduced the multiplication period of the O.N.-administered virulent virus. In both cases, animals were challenged 1 week post vaccination. Vaccination of seronegative animals resulted in an almost complete inhibition of the virus replication in the tonsils during a full fattening period: cryostat sections revealed a limited virus replication in three out of 20 animals. In one of these animals, virus replication was probably so negligible that virus isolation remained negative.