TISSUE CULTURE-PROPAGATED ORF VIRUS-VACCINE PROTECTS LAMBS FROM ORF VIRUS CHALLENGE

Twenty, eight-day-old specific pathogen-free (SPF) lambs were vaccinat ed by a single scarification approximately 4 cm in length on the inner right thigh with a double-pronged applicator. The titre of live virus in the vaccine was 10(7.2) TCID50/ml and the estimated dose per lamb was 0.04 ml. Three months and six months later 10 of the vaccinated la mbs and five age-matched unvaccinated control specific pathogen free l ambs were challenged by a single scarification with virulent virus on the inner left thigh in the same way. After the vaccination all 20 lam bs developed lesions characteristic of orf virus infection that had la rgely resolved four weeks later, when they all had reciprocal ELISA an tibody titres greater than or equal to 3200 that persisted in all but one of them until they were challenged. After the challenge, the devel opment of lesions in the vaccinated and unvaccinated sheep was compare d daily for four weeks by means of a clinical scoring system. Both gro ups of vaccinated lambs had significantly lower (P < 0.01) total clini cal scores after challenge at three months and six months than the unv accinated lambs.