Classical swine fever (CSF) marker vaccine - Trial I. Challenge studies inweaner pigs

Two commercial marker vaccines against classical swine fever virus (CSFV) a nd companion diagnostic tests were examined in 160 conventional pigs. To te st the vaccines in a "worst case scenario", group of 10 weaners were vaccin ated using a single dose of an E2 (gp55) based vaccine at days -21, -14, -1 0 or -7, and subsequently challenged at day 0. The challenge virus was CSFV 277, originating from a recent outbreak of classical swine fever (CSF) in Germany. In all groups, only 5 out of 10 pigs were challenged; the remainin g 5 pigs served as vaccinated contact controls. Also, three control groups, each consisting of 10 non-vaccinated pigs, were challenged in parallel to the vaccinated animals. CSFV could be isolated from all non-vaccinated pigs, Among these pigs 40% d isplayed a chronic course of the infection (virus positive for more than 10 days). Pigs vaccinated 21 or 14 days before challenge displayed no clinica l signs of CSFV after challenge. However, they were still able to replicate CSFV when challenged, as measured by reisolation of CSFV from leukocytes o f the directly challenged pigs. CSFV could be isolated from the leucocytes of 25% of the pigs vaccinated 21 days before challenge and 50% of the pigs vaccinated 14 days before challenge. Chronic infection was not observed, bu t transmission to one vaccinated contact pig occurred. From all pigs vaccinated 10 or 7 days before challenge, CSFV could be reiso lated. We observed a chronic course of infection in 5% of pigs vaccinated 1 0 days before challenge and in 30% of pigs vaccinated 7 days before challen ge. The mortality rate was 20% in the pigs vaccinated 10 days before challe nge, and varied between 20 and 80% in pigs vaccinated 7 days prior to chall enge. The contact animals had lower mortality (0-20%) than directly challen ged pigs, probably mirroring the delayed time point of infection. There was thus some protection against clinical illness by both marker vaccines, but not a solid protection against infection and virus shedding. The efficacy of the vaccine was best if used 3 weeks before challenge and a clear correl ation between time inter-Val from vaccination to challenge and the level of virus shedding was observed. Each vaccine had its own accompanying discrim inatory ELISA, but 18% of the virus positive pigs never scroconverted in th ese tests. (C) 2001 Elsevier Science B.V. All rights reserved.