Virulence of vABV 414, the virus derived from the infectious cDNA clone of the Lelystad virus, for third trimester pregnant gilts

Abstract - The Lelystad virus (LV) is the causative agent of the porcine reproductive and respiratory syndrome (PRRS), whose most obvious features are late gestation reproductive problems in sows and respiratory disease in pigs. Recently, an infectious cDNA clone of LV was constructed which enables the study of replication, the function of viral proteins and the molecular characterisation of the virus. Furthermore, it can be used for the development of viral vectors and vaccines. This study was performed to establish the virulence characteristics of vABV 414, the virus derived from this infectious cDNA clone, as compared to LV and its CL2621 cell line adapted derivative LV 4.2.1, in third trimester pregnant gilts. Twenty-four synchronised and pregnant gilts were housed under identical conditions and randomly allocated to the following four study groups: Group 1: Lelystad virus; Group 2: control; Group 3: vABV 414; Group 4: LV 4.2.1. All virus stocks were grown and titrated on porcine alveolar macrophages (PAMs). In the virus groups, gilts were intramuscularly injected with 2 mL of the virus batch on or near day 85 of gestation (virus titer 10⁵ TCID50/mL). The control group was injected with cell culture supernatant. The serological response in the gilts (at the day of farrowing and at the end of the study, 28 days post-farrowing) and the piglets (days 2, 14, and 28 after birth) was determined by ELISA or by the immunoperoxidase monolayer assay (IPMA). The virus was isolated from serum samples or from body fluid samples on PAMs at these study points, where appropriate. The following virulence criteria were recorded: clinical parameters in gilts (body temperature, general health status), litter quality at farrowing until 2 days post-farrowing and litter performance from 3 until 28 days post-farrowing. No adverse effects were recorded after intramuscular application of the inocula. A non-significant rise in body temperature was noted in all virus groups 2 to 3 days after inoculation. No PRRS antibodies were present in any of the gilts before inoculation. In the control group, no sero-conversion was noted and no virus was isolated from any of the sera of the gilts and their offspring at the mentioned sampling points. On the day of farrowing, all virus-inoculated principal gilts were positive for PRRSV antibodies and remained positive throughout the study. vABV 414 gave rise to higher antibody levels (ELISA) than the other viruses. This was reflected by a higher maternal antibody level in the piglets born to the vABV 414 inoculated sows from day 1 to day 14 during the post-farrowing observation period. No PRRSV was isolated from the sera of the gilts for any of the indicated study-points. The virus was, however, isolated from the sera or body fluids of the piglets in all virus groups. Thirty percent, 14%, and 13% of the piglets born to sows from groups 1, 3, and 4 respectively, were virus-positive at birth. In all virus groups, persistently infected piglets were identified during the 28-day observation period. Seventeen percent, 24%, and 5% of the piglets born to gilts from groups 1, 3, and 4, respectively, were virus-positive at weaning, 28 days post-farrowing. In groups 1 and 3, the percentages of stillborn piglets, including the piglets that died within 48 hours after birth, were 48% and 34% respectively, and significantly higher than those in the control group (3.5%; P < 0.05). No significant difference was observed for this parameter between group 4 (24%) and the control group. No mummified foetuses were seen in any of the groups. In conclusion, vABV 414, LV, and LV4.2.1 cross the placenta and may give rise to in-utero infections of foetuses. vABV 414, the virus that is derived from the infectious cDNA clone of LV, shows obvious virulence for pregnant gilts and their litters. Its effects on reproductive performance are similar to the wild type strain. Although not statistically proven, the effect of LV4.2.1. on reproductive performance seems to be a little less vicious. Poor litter quality and early performances seem to be correlated with the presence of PRRSV in piglets.