Current knowledge on PRRS virus immunology is still limited but it seems clear that modified live PRRS vaccines (MLV) are a reasonable choice for the immunization of pigs. Recently, interest in intradermal vaccination in swine has increased due to research into skin and subcutaneous tissue immunology and the possibility of using needle-free injection devices (NFIDs). The use of NFIDs in the swine industry offers some advantages over conventional needle-and-syringe methods, especially due to the reduced pain and stress to pigs and the increase in the uniformity of the dosage administered to the herd. Although intradermal PRRS virus vaccination in swine has been investigated, the high variability between different virus isolates makes it advisable to assess the intradermal response for each vaccine strain.
The MLV vaccine UNISTRAIN® PRRS has recently obtained the indication for intradermal administration with Hipradermic®, a needle-free injector with connectivity developed by HIPRA for the intradermal vaccination of pigs. A study was conducted in 2-week old piglets in order to demonstrate the clinical protection and the duration of immunity after a heterologous challenge provided by the intradermal administration of UNISTRAIN® PRRS in swine with Hipradermic®.
Thirty-seven 2-week-old piglets, clinically healthy and free from virus and antibodies against PRRS, were randomly assigned to three different groups: ID vaccinated group (n=11), intramuscularly (IM) vaccinated group (n=12) and control group (CTR; n=14). Animals in the ID group were immunised intradermally with UNISTRAIN®PRRS vaccine (0.2 ml/dose; 103.5 CCID50/animal) administered with Hipradermic®. Animals in the IM group were immunised intramuscularly with UNISTRAIN® PRRS vaccine (2 ml/dose; 103.5 CCID50/animal; administered with needle and syringe). The animals in the CTR group received 2 ml of PBS using the same strategy as in the IM group. At 26 weeks of age, all piglets were challenged by the intranasal route with a heterologous pathogenic strain of European genotype I of the PRRSV (89% ORF5 homology; 106.39 CCID50/animal). Animals were examined daily after challenge over the following 35 days. Virus detection was performed by real time RT-qPCR (at 2, 5, 8, 14, 21, 28 and 35 days post-challenge) and the Area Under the Curve (AUC) of viraemia was calculated from the challenge to the end of the study. AUC and length of viraemia were analysed using a non-parametric Mann-Whitney U test (p<0.05) and the percentage of viraemic animals using a two-tailed chi-square test/Fisher’s exact test (p<0.05).
The vaccinated groups had a significantly lower serum viral load, as determined by AUC (IM=0.0x100 CCID50/ml; ID=0.0x100 CCID50/ml), when compared to non-vaccinated pigs (CTR=3.1x104 CCID50/ml). In the vaccinated groups, a significant reduction in the number of viraemic animals was observed at: 5, 8, 14, 21, 28 and 35 days after challenge.
The length of the viraemia after challenge was also statistically lower in the vaccinated groups compared to non-vaccinated pigs.
The results obtained allow us to conclude that the duration of immunity of the UNISTRAIN® PRRS vaccine was achieved 24 weeks after vaccination. In addition, UNISTRAIN® PRRS administered in piglets by the intradermal route with Hipradermic® had a comparable effect on the fast clearance of the virus to IM administration using a traditional syringe and needle. UNISTRAIN® PRRS vaccine administered ID or IM appears to be a useful tool to decrease viraemia and thus achieve a reduction in the infection pressure of the PRRS virus in swine.