Vet. Res. 31 (2000) 97-97

Efficacy of an American and a European serotype PRRSV vaccine after challenge with American and European wild-type strains of the virus

G.G. Labarque^a, H.J. Nauwynck^a, P.A.M. van Woensel^b, N. Visser^b and M.B. Pensaert<sup>a

a</sup>  Laboratory of Virology, Faculty of Veterinary Medicine, University of Gent, Salisburylaan 133, 9820 Merelbeke, Belgium
^b  Intervet International BV, Wim de Körverstraat 35, 5831 AN Boxmeer, The Netherlands

Abstract - Porcine reproductive and respiratory syndrome virus (PRRSV) isolates can be classified into two distinct serotypes, the European and the North American serotype. Because of this distinction, it is of interest to know the degree of cross-protection between these two serotypes. In the present study, the efficacy of an attenuated PRRSV vaccine based on a European serotype was compared with that of a live vaccine based on an American serotype by challenge with homologous and heterologous wild-type strains. The two experiments were performed in six groups of seven pigs each. In each of the two experiments, one group was vaccinated intramuscularly with 10^5.3 TCID ₅₀ of a modified live vaccine strain of the European serotype, one group was vaccinated intramuscularly with 10^5.2 TCID ₅₀ of a modified live vaccine strain of the American serotype and one group served as the unvaccinated control group. Four weeks after immunisation, all the groups were intranasally challenged. In experiment 1, the challenge was performed with 10^5.0 TCID ₅₀ of a wild-type American virus strain (US5) which was isolated from an outbreak of reproductive disorders in Iowa. In experiment 2, the challenge was performed with 10^5.0 TCID ₅₀ of a wild-type European virus strain (I ₂) which was isolated in the Netherlands. After challenge, body temperature and clinical signs were recorded daily until 14 days post-challenge (PC). Pigs were weighed at challenge and 7, 14 and 21 days PC. Blood for virological examination was collected at challenge and 3, 7, 10, 14 and 21 days PC. Respiratory signs were not observed in any of the groups after challenge. In both experiments, the body temperatures of pigs that underwent a heterologous challenge were significantly higher than those of pigs that underwent a homologous challenge and than those of the unvaccinated control pigs. In experiment 1, no significant differences in weight gain were observed between the different groups. In experiment 2, weight gains after the homologous challenge were significantly higher than those after the heterologous challenge and than those of the unvaccinated control pigs. Compared to the controls, the reduction of viraemia after the homologous challenge in experiment 1 and 2 was 86% and 85% respectively. After the heterologous challenge the reduction of viraemia was only 18% and 11% respectively. Since clinical signs remained absent among the unvaccinated control pigs, it was not possible to compare the efficacy of both vaccines based on clinical signs. Other parameters such as body temperature and weight gain were indicative but not sufficient to evaluate the efficacy of the vaccines. The efficacy of PRRSV vaccines could be assessed by determining the reduction in viraemia after challenge with a wild-type virus strain. The efficacy was significantly better in pigs that underwent a homologous challenge compared to pigs that underwent a heterologous challenge as well in duration as quantity of viraemia. It is clear from these studies that the genomic and antigenic differences between the North American and the European strains affect the extent of cross-protection and that vaccines preferably should include the prevaling serotype.

Corresponding author: M.B. Pensaerta Tel.: (32) 9 264 73 66; fax: (32) 9 264 74 95;
    e-mail: maurice.pensaert@rug.ac.be

© INRA, EDP Sciences 2000