Immunisation of pigs with a major envelope protein sub-unit vaccine against porcine reproductive and respiratory syndrome virus (PRRSV) results in enhanced clinical disease following experimental challenge

Abstract

Disease exacerbation was observed in pigs challenged with virulent porcine reproductive and respiratory syndrome virus (PRRSV) following immunisation with a recombinant GP5 sub-unit PRRSV vaccine (rGP5) produced in E. coli. Eighteen animals were divided into three experimental groups: group A were immunised twice IM with rGP5, 21 days apart; group B acted as positive controls (challenged but not immunised); and group C were negative controls. Pigs in groups A and B were challenged 21 days after the second immunisation of the group A animals. Following challenge, three pigs given rGP5 exhibited more severe clinical signs than the positive controls, including respiratory distress and progressive weight-loss. Although not statistically significant, the more severe disease exhibited by group A animals may suggest previous immunisation as a contributory factor. The mechanisms of these findings remain unclear and no association could be established between the severity of disease, non-neutralising antibody concentrations and tissue viral loads.

Fig. 1

Alignment of ORF5 sequences of the porcine reproductive and respiratory syndrome virus strains used in the immunisation (Sp-6) and challenge (5710) of the pigs.

Fig. 2

Graph illustrating the mean clinical scores for each experimental group following experimental challenge with porcine reproductive and respiratory syndrome virus. Red triangles GP5, group A (vaccinated with GP5 protein and challenged); green circles NV/C, group B (challenged only); mauve crosses NV/NC, group C (not vaccinated or challenged).

Fig. 3

Graph illustrating daily weight gain of pigs in groups A (vaccinated with GP5 protein and challenged), B (challenged only), and C (not vaccinated or challenged) following experimental challenge with porcine reproductive and respiratory syndrome virus.

Fig. 4

Graph illustrating the mean rectal temperature (°C) for each experimental group from 10 days before to 18 days post-challenge with porcine reproductive and respiratory syndrome virus. Red squares, group A (vaccinated with GP5 protein and challenged); green triangles, group B (challenged only); mauve squares, group C (not vaccinated or challenged).