Oral transmission of porcine reproductive and respiratory syndrome virus by muscle of experimentally infected pigs.

The current study was performed to determine if porcine reproductive and respiratory syndrome virus (PRRSV) could be transmitted to pigs by feeding muscle tissue obtained from recently infected pigs. Muscle obtained from pigs infected with either a European strain (EU donor pigs) or American strain (US donor pigs) of PRRSV was fed to PRRSV-free receiver pigs. The donor pigs were slaughtered 11 days post-infection (dpi). PRRSV was detected by conventional virus isolation in muscle at 11 dpi from 7 of 12 EU donor pigs and 5 of 12 US donor pigs. In contrast to conventional virus isolation, all muscle samples from infected pigs were positive for viral nucleic acid by PCR, except for muscle from one animal infected with the American strain of PRRSV. Five hundred grams of raw semimembranosus muscle from each of the donor pigs was fed over a 2 days period (250 g per day) to each of two receiver pigs (48 receiver pigs). The receiver pigs were housed separately in five groups. One of the five groups was fed muscle obtained from US donor pigs that was also spiked with the American strain of PRRSV. Sentinel pigs were placed in-contact with the group of receiver pigs fed spiked muscle. All receiver pigs became viraemic by 6 days post-feeding (dpf). There was evidence of horizontal transmission with sentinel pigs, in-contact with receiver pigs, becoming viraemic. The study demonstrates that PRRSV could be infectious through the oral route via the feeding of meat obtained from recently infected pigs.

Safety and protective efficacy of porcine reproductive and respiratory syndrome recombinant virus vaccines in young pigs.

Three porcine reproductive and respiratory syndrome virus (PRRSV) recombinants, generated by mutagenesis of an infectious cDNA clone of the Lelystad virus (LV) isolate, were tested for their safety and protective efficacy as potential PRRSV vaccines in pigs. Recombinant vABV688 contains two amino acid substitutions in the minor structural protein GP(2) resulting in improved growth on cell line CL2621; in recombinant vABV707 the region encoding the ectodomain of the major unglycosylated membrane protein M has been replaced by that of the murine lactate dehydrogenase-elevating arterivirus; recombinant vABV746 lacks the six C-terminal amino acids of the nucleocapsid protein N. First, we determined the safety of these recombinant viruses by monitoring the stability of the introduced mutations in 8-week-old pigs. We showed that the introduced genomic mutations were maintained throughout the viraemic period. Second, the protective efficacy of immunization with the recombinant viruses against challenge with a homologous and a heterologous PRRSV strain was determined in two pigs and compared with the efficacy of vABV437, a virus derived from the parental LV cDNA. The viraemia in pigs immunized with the recombinant viruses was reduced compared to pigs immunized with vABV437. In addition, the length of viraemia was reduced in the sentinel pigs that were introduced into the groups immunized with vABV746, vABV688, and vABV707, however, all of the sentinel pigs became infected. Pigs immunized with vABV707 and vABV437 were protected against challenge with homologous virus LV-Ter Huurne and transmission of the latter virus. None of the immunized pigs were protected against heterologous challenge with the virulent US isolate SDSU#73, but the vABV707- and vABV746-immunized pigs were protected against transmission of this virus from challenged pigs. In conclusion, the obtained viral recombinants are interesting candidates to be further explored for their use as vaccines against PRRSV.

Virological kinetics and immunological responses to a porcine reproductive and respiratory syndrome virus infection of pigs at different ages.

The objective of this study was to measure the effect of two variables (pig age and virus strain) on selected responses (clinical signs, viraemia, virus excretion and seroconversion) of pigs following exposure to porcine reproductive and respiratory syndrome (PRRS) virus. Therefore, young (6 till 8 weeks old) and old (6 months old) pigs were infected with 3 different PRRSV strains, i.e. LV ter Huurne (LVTH), LV4.2.1 and SDSU#73. Regardless of the strain used for exposure, young pigs were more susceptible to infection as shown by a higher number of viraemic and virus excreting pigs. Strain differences were also evident. LV ter Huurne induced virus excretion in a higher number of pigs and with a higher virus titre, whereas SDSU#73 induced most severe clinical signs. LV4.2.1 induced viraemia and virus excretion in a low number of pigs. The kinetics of the antibody response differed per virus strain. The results presented here are useful in developing a less expensive standardised infection model, consisting of young pigs intranasally infected with a virulent PRRSV strain, to study the efficacy of new vaccine strains.